Updated: 2021/Apr/14


dhcpd.conf(5)                 File Formats Manual                dhcpd.conf(5)



NAME
       dhcpd.conf - dhcpd configuration file

DESCRIPTION
       The dhcpd.conf file contains configuration information for dhcpd, the
       Internet Systems Consortium DHCP Server.

       The dhcpd.conf file is a free-form ASCII text file.  It is parsed by
       the recursive-descent parser built into dhcpd.  The file may contain
       extra tabs and newlines for formatting purposes.  Keywords in the file
       are case-insensitive.  Comments may be placed anywhere within the file
       (except within quotes).  Comments begin with the # character and end at
       the end of the line.

       The file essentially consists of a list of statements.  Statements fall
       into two broad categories - parameters and declarations.

       Parameter statements either say how to do something (e.g., how long a
       lease to offer), whether to do something (e.g., should dhcpd provide
       addresses to unknown clients), or what parameters to provide to the
       client (e.g., use gateway 220.177.244.7).

       Declarations are used to describe the topology of the network, to
       describe clients on the network, to provide addresses that can be
       assigned to clients, or to apply a group of parameters to a group of
       declarations.  In any group of parameters and declarations, all
       parameters must be specified before any declarations which depend on
       those parameters may be specified.

       Declarations about network topology include the shared-network and the
       subnet declarations.  If clients on a subnet are to be assigned
       addresses dynamically, a range declaration must appear within the
       subnet declaration.  For clients with statically assigned addresses, or
       for installations where only known clients will be served, each such
       client must have a host declaration.  If parameters are to be applied
       to a group of declarations which are not related strictly on a per-
       subnet basis, the group declaration can be used.

       For every subnet which will be served, and for every subnet to which
       the dhcp server is connected, there must be one subnet declaration,
       which tells dhcpd how to recognize that an address is on that subnet.
       A subnet declaration is required for each subnet even if no addresses
       will be dynamically allocated on that subnet.

       Some installations have physical networks on which more than one IP
       subnet operates.  For example, if there is a site-wide requirement that
       8-bit subnet masks be used, but a department with a single physical
       ethernet network expands to the point where it has more than 254 nodes,
       it may be necessary to run two 8-bit subnets on the same ethernet until
       such time as a new physical network can be added.  In this case, the
       subnet declarations for these two networks must be enclosed in a
       shared-network declaration.

       Note that even when the shared-network declaration is absent, an empty
       one is created by the server to contain the subnet (and any scoped
       parameters included in the subnet).  For practical purposes, this means
       that "stateless" DHCP clients, which are not tied to addresses (and
       therefore subnets) will receive the same configuration as stateful
       ones.

       Some sites may have departments which have clients on more than one
       subnet, but it may be desirable to offer those clients a uniform set of
       parameters which are different than what would be offered to clients
       from other departments on the same subnet.  For clients which will be
       declared explicitly with host declarations, these declarations can be
       enclosed in a group declaration along with the parameters which are
       common to that department.  For clients whose addresses will be
       dynamically assigned, class declarations and conditional declarations
       may be used to group parameter assignments based on information the
       client sends.

       When a client is to be booted, its boot parameters are determined by
       consulting that client's host declaration (if any), and then consulting
       any class declarations matching the client, followed by the pool,
       subnet and shared-network declarations for the IP address assigned to
       the client.  Each of these declarations itself appears within a lexical
       scope, and all declarations at less specific lexical scopes are also
       consulted for client option declarations.  Scopes are never considered
       twice, and if parameters are declared in more than one scope, the
       parameter declared in the most specific scope is the one that is used.

       When dhcpd tries to find a host declaration for a client, it first
       looks for a host declaration which has a fixed-address declaration that
       lists an IP address that is valid for the subnet or shared network on
       which the client is booting.  If it doesn't find any such entry, it
       tries to find an entry which has no fixed-address declaration.

EXAMPLES
       A typical dhcpd.conf file will look something like this:

       global parameters...

       subnet 204.254.239.0 netmask 255.255.255.224 {
         subnet-specific parameters...
         range 204.254.239.10 204.254.239.30;
       }

       subnet 204.254.239.32 netmask 255.255.255.224 {
         subnet-specific parameters...
         range 204.254.239.42 204.254.239.62;
       }

       subnet 204.254.239.64 netmask 255.255.255.224 {
         subnet-specific parameters...
         range 204.254.239.74 204.254.239.94;
       }

       group {
         group-specific parameters...
         host zappo.test.isc.org {
           host-specific parameters...
         }
         host beppo.test.isc.org {
           host-specific parameters...
         }
         host harpo.test.isc.org {
           host-specific parameters...
         }
       }

                                      Figure 1


       Notice that at the beginning of the file, there's a place for global
       parameters.  These might be things like the organization's domain name,
       the addresses of the name servers (if they are common to the entire
       organization), and so on.  So, for example:

            option domain-name "isc.org";
            option domain-name-servers ns1.isc.org, ns2.isc.org;

                                      Figure 2


       As you can see in Figure 2, you can specify host addresses in
       parameters using their domain names rather than their numeric IP
       addresses.  If a given hostname resolves to more than one IP address
       (for example, if that host has two ethernet interfaces), then where
       possible, both addresses are supplied to the client.

       The most obvious reason for having subnet-specific parameters as shown
       in Figure 1 is that each subnet, of necessity, has its own router.  So
       for the first subnet, for example, there should be something like:

            option routers 204.254.239.1;

       Note that the address here is specified numerically.  This is not
       required - if you have a different domain name for each interface on
       your router, it's perfectly legitimate to use the domain name for that
       interface instead of the numeric address.  However, in many cases there
       may be only one domain name for all of a router's IP addresses, and it
       would not be appropriate to use that name here.

       In Figure 1 there is also a group statement, which provides common
       parameters for a set of three hosts - zappo, beppo and harpo.  As you
       can see, these hosts are all in the test.isc.org domain, so it might
       make sense for a group-specific parameter to override the domain name
       supplied to these hosts:

            option domain-name "test.isc.org";

       Also, given the domain they're in, these are probably test machines.
       If we wanted to test the DHCP leasing mechanism, we might set the lease
       timeout somewhat shorter than the default:

            max-lease-time 120;
            default-lease-time 120;

       You may have noticed that while some parameters start with the option
       keyword, some do not.  Parameters starting with the option keyword
       correspond to actual DHCP options, while parameters that do not start
       with the option keyword either control the behavior of the DHCP server
       (e.g., how long a lease dhcpd will give out), or specify client
       parameters that are not optional in the DHCP protocol (for example,
       server-name and filename).

       In Figure 1, each host had host-specific parameters.  These could
       include such things as the hostname option, the name of a file to
       upload (the filename parameter) and the address of the server from
       which to upload the file (the next-server parameter).  In general, any
       parameter can appear anywhere that parameters are allowed, and will be
       applied according to the scope in which the parameter appears.

       Imagine that you have a site with a lot of NCD X-Terminals.  These
       terminals come in a variety of models, and you want to specify the boot
       files for each model.  One way to do this would be to have host
       declarations for each server and group them by model:

       group {
         filename "Xncd19r";
         next-server ncd-booter;

         host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; }
         host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; }
         host ncd8 { hardware ethernet 0:c0:c3:22:46:81; }
       }

       group {
         filename "Xncd19c";
         next-server ncd-booter;

         host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; }
         host ncd3 { hardware ethernet 0:c0:c3:00:14:11; }
       }

       group {
         filename "XncdHMX";
         next-server ncd-booter;

         host ncd1 { hardware ethernet 0:c0:c3:11:90:23; }
         host ncd4 { hardware ethernet 0:c0:c3:91:a7:8; }
         host ncd8 { hardware ethernet 0:c0:c3:cc:a:8f; }
       }

ADDRESS POOLS
       The pool and pool6 declarations can be used to specify a pool of
       addresses that will be treated differently than another pool of
       addresses, even on the same network segment or subnet.  For example,
       you may want to provide a large set of addresses that can be assigned
       to DHCP clients that are registered to your DHCP server, while
       providing a smaller set of addresses, possibly with short lease times,
       that are available for unknown clients.  If you have a firewall, you
       may be able to arrange for addresses from one pool to be allowed access
       to the Internet, while addresses in another pool are not, thus
       encouraging users to register their DHCP clients.  To do this, you
       would set up a pair of pool declarations:

       subnet 10.0.0.0 netmask 255.255.255.0 {
         option routers 10.0.0.254;

         # Unknown clients get this pool.
         pool {
           option domain-name-servers bogus.example.com;
           max-lease-time 300;
           range 10.0.0.200 10.0.0.253;
           allow unknown-clients;
         }

         # Known clients get this pool.
         pool {
           option domain-name-servers ns1.example.com, ns2.example.com;
           max-lease-time 28800;
           range 10.0.0.5 10.0.0.199;
           deny unknown-clients;
         }
       }

       It is also possible to set up entirely different subnets for known and
       unknown clients - address pools exist at the level of shared networks,
       so address ranges within pool declarations can be on different subnets.

       As you can see in the preceding example, pools can have permit lists
       that control which clients are allowed access to the pool and which
       aren't.  Each entry in a pool's permit list is introduced with the
       allow or deny keyword.  If a pool has a permit list, then only those
       clients that match specific entries on the permit list will be eligible
       to be assigned addresses from the pool.  If a pool has a deny list,
       then only those clients that do not match any entries on the deny list
       will be eligible.   If both permit and deny lists exist for a pool,
       then only clients that match the permit list and do not match the deny
       list will be allowed access.

       The pool6 declaration is similar to the pool declaration.  Currently it
       is only allowed within a subnet6 declaration, and may not be included
       directly in a shared network declaration.  In addition to the range6
       statement it allows the prefix6 statement to be included.  You may
       include range6 statements for both NA and TA and prefixy6 statements in
       a single pool6 statement.

DYNAMIC ADDRESS ALLOCATION
       Address allocation is actually only done when a client is in the INIT
       state and has sent a DHCPDISCOVER message.  If the client thinks it has
       a valid lease and sends a DHCPREQUEST to initiate or renew that lease,
       the server has only three choices - it can ignore the DHCPREQUEST, send
       a DHCPNAK to tell the client it should stop using the address, or send
       a DHCPACK, telling the client to go ahead and use the address for a
       while.

       If the server finds the address the client is requesting, and that
       address is available to the client, the server will send a DHCPACK.  If
       the address is no longer available, or the client isn't permitted to
       have it, the server will send a DHCPNAK.  If the server knows nothing
       about the address, it will remain silent, unless the address is
       incorrect for the network segment to which the client has been attached
       and the server is authoritative for that network segment, in which case
       the server will send a DHCPNAK even though it doesn't know about the
       address.

       There may be a host declaration matching the client's identification.
       If that host declaration contains a fixed-address declaration that
       lists an IP address that is valid for the network segment to which the
       client is connected, the DHCP server will never do dynamic address
       allocation.  In this case, the client is required to take the address
       specified in the host declaration.  If the client sends a DHCPREQUEST
       for some other address, the server will respond with a DHCPNAK.

       When the DHCP server allocates a new address for a client (remember,
       this only happens if the client has sent a DHCPDISCOVER), it first
       looks to see if the client already has a valid lease on an IP address,
       or if there is an old IP address the client had before that hasn't yet
       been reassigned.  In that case, the server will take that address and
       check it to see if the client is still permitted to use it.  If the
       client is no longer permitted to use it, the lease is freed if the
       server thought it was still in use - the fact that the client has sent
       a DHCPDISCOVER proves to the server that the client is no longer using
       the lease.

       If no existing lease is found, or if the client is forbidden to receive
       the existing lease, then the server will look in the list of address
       pools for the network segment to which the client is attached for a
       lease that is not in use and that the client is permitted to have.  It
       looks through each pool declaration in sequence (all range declarations
       that appear outside of pool declarations are grouped into a single pool
       with no permit list).  If the permit list for the pool allows the
       client to be allocated an address from that pool, the pool is examined
       to see if there is an address available.  If so, then the client is
       tentatively assigned that address.  Otherwise, the next pool is tested.
       If no addresses are found that can be assigned to the client, no
       response is sent to the client.

       If an address is found that the client is permitted to have, and that
       has never been assigned to any client before, the address is
       immediately allocated to the client.  If the address is available for
       allocation but has been previously assigned to a different client, the
       server will keep looking in hopes of finding an address that has never
       before been assigned to a client.

       The DHCP server generates the list of available IP addresses from a
       hash table.  This means that the addresses are not sorted in any
       particular order, and so it is not possible to predict the order in
       which the DHCP server will allocate IP addresses.  Users of previous
       versions of the ISC DHCP server may have become accustomed to the DHCP
       server allocating IP addresses in ascending order, but this is no
       longer possible, and there is no way to configure this behavior with
       version 3 of the ISC DHCP server.

IP ADDRESS CONFLICT PREVENTION
       The DHCP server checks IP addresses to see if they are in use before
       allocating them to clients.  It does this by sending an ICMP Echo
       request message to the IP address being allocated.  If no ICMP Echo
       reply is received within a second, the address is assumed to be free.
       This is only done for leases that have been specified in range
       statements, and only when the lease is thought by the DHCP server to be
       free - i.e., the DHCP server or its failover peer has not listed the
       lease as in use.

       If a response is received to an ICMP Echo request, the DHCP server
       assumes that there is a configuration error - the IP address is in use
       by some host on the network that is not a DHCP client.  It marks the
       address as abandoned, and will not assign it to clients. The lease will
       remain abandoned for a minimum of abandon-lease-time seconds.

       If a DHCP client tries to get an IP address, but none are available,
       but there are abandoned IP addresses, then the DHCP server will attempt
       to reclaim an abandoned IP address.  It marks one IP address as free,
       and then does the same ICMP Echo request check described previously.
       If there is no answer to the ICMP Echo request, the address is assigned
       to the client.

       The DHCP server does not cycle through abandoned IP addresses if the
       first IP address it tries to reclaim is free.  Rather, when the next
       DHCPDISCOVER comes in from the client, it will attempt a new allocation
       using the same method described here, and will typically try a new IP
       address.

DHCP FAILOVER
       This version of the ISC DHCP server supports the DHCP failover protocol
       as documented in draft-ietf-dhc-failover-12.txt.  This is not a final
       protocol document, and we have not done interoperability testing with
       other vendors' implementations of this protocol, so you must not assume
       that this implementation conforms to the standard.  If you wish to use
       the failover protocol, make sure that both failover peers are running
       the same version of the ISC DHCP server.

       The failover protocol allows two DHCP servers (and no more than two) to
       share a common address pool.  Each server will have about half of the
       available IP addresses in the pool at any given time for allocation.
       If one server fails, the other server will continue to renew leases out
       of the pool, and will allocate new addresses out of the roughly half of
       available addresses that it had when communications with the other
       server were lost.

       It is possible during a prolonged failure to tell the remaining server
       that the other server is down, in which case the remaining server will
       (over time) reclaim all the addresses the other server had available
       for allocation, and begin to reuse them.  This is called putting the
       server into the PARTNER-DOWN state.

       You can put the server into the PARTNER-DOWN state either by using the
       omshell (1) command or by stopping the server, editing the last
       failover state declaration in the lease file, and restarting the
       server.  If you use this last method, change the "my state" line to:

       failover peer name state {
       my state partner-down;.
       peer state state at date;
       }

       It is only required to change "my state" as shown above.

       When the other server comes back online, it should automatically detect
       that it has been offline and request a complete update from the server
       that was running in the PARTNER-DOWN state, and then both servers will
       resume processing together.

       It is possible to get into a dangerous situation: if you put one server
       into the PARTNER-DOWN state, and then *that* server goes down, and the
       other server comes back up, the other server will not know that the
       first server was in the PARTNER-DOWN state, and may issue addresses
       previously issued by the other server to different clients, resulting
       in IP address conflicts.  Before putting a server into PARTNER-DOWN
       state, therefore, make sure that the other server will not restart
       automatically.

       The failover protocol defines a primary server role and a secondary
       server role.  There are some differences in how primaries and
       secondaries act, but most of the differences simply have to do with
       providing a way for each peer to behave in the opposite way from the
       other.  So one server must be configured as primary, and the other must
       be configured as secondary, and it doesn't matter too much which one is
       which.

FAILOVER STARTUP
       When a server starts that has not previously communicated with its
       failover peer, it must establish communications with its failover peer
       and synchronize with it before it can serve clients.  This can happen
       either because you have just configured your DHCP servers to perform
       failover for the first time, or because one of your failover servers
       has failed catastrophically and lost its database.

       The initial recovery process is designed to ensure that when one
       failover peer loses its database and then resynchronizes, any leases
       that the failed server gave out before it failed will be honored.  When
       the failed server starts up, it notices that it has no saved failover
       state, and attempts to contact its peer.

       When it has established contact, it asks the peer for a complete copy
       its peer's lease database.  The peer then sends its complete database,
       and sends a message indicating that it is done.  The failed server then
       waits until MCLT has passed, and once MCLT has passed both servers make
       the transition back into normal operation.  This waiting period ensures
       that any leases the failed server may have given out while out of
       contact with its partner will have expired.

       While the failed server is recovering, its partner remains in the
       partner-down state, which means that it is serving all clients.  The
       failed server provides no service at all to DHCP clients until it has
       made the transition into normal operation.

       In the case where both servers detect that they have never before
       communicated with their partner, they both come up in this recovery
       state and follow the procedure we have just described.  In this case,
       no service will be provided to DHCP clients until MCLT has expired.

CONFIGURING FAILOVER
       In order to configure failover, you need to write a peer declaration
       that configures the failover protocol, and you need to write peer
       references in each pool declaration for which you want to do failover.
       You do not have to do failover for all pools on a given network
       segment.   You must not tell one server it's doing failover on a
       particular address pool and tell the other it is not.  You must not
       have any common address pools on which you are not doing failover.  A
       pool declaration that utilizes failover would look like this:

       pool {
            failover peer "foo";
            pool specific parameters
       };

       The  server currently  does very  little  sanity checking,  so if  you
       configure it wrong, it will just  fail in odd ways.  I would recommend
       therefore that you either do  failover or don't do failover, but don't
       do any mixed pools.  Also,  use the same master configuration file for
       both  servers,  and  have  a  separate file  that  contains  the  peer
       declaration and includes the master file.  This will help you to avoid
       configuration  mismatches.  As our  implementation evolves,  this will
       become  less of  a  problem.  A  basic  sample dhcpd.conf  file for  a
       primary server might look like this:

       failover peer "foo" {
         primary;
         address anthrax.rc.example.com;
         port 519;
         peer address trantor.rc.example.com;
         peer port 520;
         max-response-delay 60;
         max-unacked-updates 10;
         mclt 3600;
         split 128;
         load balance max seconds 3;
       }

       include "/etc/dhcpd.master";

       The statements in the peer declaration are as follows:

       The primary and secondary statements

         [ primary | secondary ];

         This determines whether the server is primary or secondary, as
         described earlier under DHCP FAILOVER.

       The address statement

         address address;

         The address statement declares the IP address or DNS name on which
         the server should listen for connections from its failover peer, and
         also the value to use for the DHCP Failover Protocol server
         identifier.  Because this value is used as an identifier, it may not
         be omitted.

       The peer address statement

         peer address address;

         The peer address statement declares the IP address or DNS name to
         which the server should connect to reach its failover peer for
         failover messages.

       The port statement

         port port-number;

         The port statement declares the TCP port on which the server should
         listen for connections from its failover peer.  This statement may be
         omitted, in which case the IANA assigned port number 647 will be used
         by default.

       The peer port statement

         peer port port-number;

         The peer port statement declares the TCP port to which the server
         should connect to reach its failover peer for failover messages.
         This statement may be omitted, in which case the IANA assigned port
         number 647 will be used by default.

       The max-response-delay statement

         max-response-delay seconds;

         The max-response-delay statement tells the DHCP server how many
         seconds may pass without receiving a message from its failover peer
         before it assumes that connection has failed.  This number should be
         small enough that a transient network failure that breaks the
         connection will not result in the servers being out of communication
         for a long time, but large enough that the server isn't constantly
         making and breaking connections.  This parameter must be specified.

       The max-unacked-updates statement

         max-unacked-updates count;

         The max-unacked-updates statement tells the remote DHCP server how
         many BNDUPD messages it can send before it receives a BNDACK from the
         local system.  We don't have enough operational experience to say
         what a good value for this is, but 10 seems to work.  This parameter
         must be specified.

       The mclt statement

         mclt seconds;

         The mclt statement defines the Maximum Client Lead Time.  It must be
         specified on the primary, and may not be specified on the secondary.
         This is the length of time for which a lease may be renewed by either
         failover peer without contacting the other.  The longer you set this,
         the longer it will take for the running server to recover IP
         addresses after moving into PARTNER-DOWN state.  The shorter you set
         it, the more load your servers will experience when they are not
         communicating.  A value of something like 3600 is probably
         reasonable, but again bear in mind that we have no real operational
         experience with this.

       The split statement

         split bits;

         The split statement specifies the split between the primary and
         secondary for the purposes of load balancing.  Whenever a client
         makes a DHCP request, the DHCP server runs a hash on the client
         identification, resulting in value from 0 to 255.  This is used as an
         index into a 256 bit field.  If the bit at that index is set, the
         primary is responsible.  If the bit at that index is not set, the
         secondary is responsible.  The split value determines how many of the
         leading bits are set to one.  So, in practice, higher split values
         will cause the primary to serve more clients than the secondary.
         Lower split values, the converse.  Legal values are between 0 and 256
         inclusive, of which the most reasonable is 128.  Note that a value of
         0 makes the secondary responsible for all clients and a value of 256
         makes the primary responsible for all clients.

       The hba statement

         hba colon-separated-hex-list;

         The hba statement specifies the split between the primary and
         secondary as a bitmap rather than a cutoff, which theoretically
         allows for finer-grained control.  In practice, there is probably no
         need for such fine-grained control, however.  An example hba
         statement:

           hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:
               00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00;

         This is equivalent to a split 128; statement, and identical.  The
         following two examples are also equivalent to a split of 128, but are
         not identical:

           hba aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:
               aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa;

           hba 55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:
               55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55;

         They are equivalent, because half the bits are set to 0, half are set
         to 1 (0xa and 0x5 are 1010 and 0101 binary respectively) and
         consequently this would roughly divide the clients equally between
         the servers.  They are not identical, because the actual peers this
         would load balance to each server are different for each example.

         You must only have split or hba defined, never both.  For most cases,
         the fine-grained control that hba offers isn't necessary, and split
         should be used.

       The load balance max seconds statement

         load balance max seconds seconds;

         This statement allows you to configure a cutoff after which load
         balancing is disabled.  The cutoff is based on the number of seconds
         since the client sent its first DHCPDISCOVER or DHCPREQUEST message,
         and only works with clients that correctly implement the secs field -
         fortunately most clients do.  We recommend setting this to something
         like 3 or 5.  The effect of this is that if one of the failover peers
         gets into a state where it is responding to failover messages but not
         responding to some client requests, the other failover peer will take
         over its client load automatically as the clients retry.

         It is possible to disable load balancing between peers by setting
         this value to 0 on both peers.  Bear in mind that this means both
         peers will respond to all DHCPDISCOVERs or DHCPREQUESTs.

       The auto-partner-down statement

         auto-partner-down seconds;

         This statement instructs the server to initiate a timed delay upon
         entering the communications-interrupted state (any situation of being
         out-of-contact with the remote failover peer).  At the conclusion of
         the timer, the server will automatically enter the partner-down
         state.  This permits the server to allocate leases from the partner's
         free lease pool after an STOS+MCLT timer expires, which can be
         dangerous if the partner is in fact operating at the time (the two
         servers will give conflicting bindings).

         Think very carefully before enabling this feature.  The partner-down
         and communications-interrupted states are intentionally segregated
         because there do exist situations where a failover server can fail to
         communicate with its peer, but still has the ability to receive and
         reply to requests from DHCP clients.  In general, this feature should
         only be used in those deployments where the failover servers are
         directly connected to one another, such as by a dedicated hardwired
         link ("a heartbeat cable").

         A zero value disables the auto-partner-down feature (also the
         default), and any positive value indicates the time in seconds to
         wait before automatically entering partner-down.

       The Failover pool balance statements.

          max-lease-misbalance percentage;
          max-lease-ownership percentage;
          min-balance seconds;
          max-balance seconds;

         This version of the DHCP Server evaluates pool balance on a schedule,
         rather than on demand as leases are allocated.  The latter approach
         proved to be slightly klunky when pool misbalanced reach total
         saturation -- when any server ran out of leases to assign, it also
         lost its ability to notice it had run dry.

         In order to understand pool balance, some elements of its operation
         first need to be defined.  First, there are 'free' and 'backup'
         leases.  Both of these are referred to as 'free state leases'.
         'free' and 'backup' are 'the free states' for the purpose of this
         document.  The difference is that only the primary may allocate from
         'free' leases unless under special circumstances, and only the
         secondary may allocate 'backup' leases.

         When pool balance is performed, the only plausible expectation is to
         provide a 50/50 split of the free state leases between the two
         servers.  This is because no one can predict which server will fail,
         regardless of the relative load placed upon the two servers, so
         giving each server half the leases gives both servers the same amount
         of 'failure endurance'.  Therefore, there is no way to configure any
         different behaviour, outside of some very small windows we will
         describe shortly.

         The first thing calculated on any pool balance run is a value
         referred to as 'lts', or "Leases To Send".  This, simply, is the
         difference in the count of free and backup leases, divided by two.
         For the secondary, it is the difference in the backup and free
         leases, divided by two.  The resulting value is signed: if it is
         positive, the local server is expected to hand out leases to retain a
         50/50 balance.  If it is negative, the remote server would need to
         send leases to balance the pool.  Once the lts value reaches zero,
         the pool is perfectly balanced (give or take one lease in the case of
         an odd number of total free state leases).

         The current approach is still something of a hybrid of the old
         approach, marked by the presence of the max-lease-misbalance
         statement.  This parameter configures what used to be a 10% fixed
         value in previous versions: if lts is less than free+backup * max-
         lease-misbalance percent, then the server will skip balancing a given
         pool (it won't bother moving any leases, even if some leases "should"
         be moved).  The meaning of this value is also somewhat overloaded,
         however, in that it also governs the estimation of when to attempt to
         balance the pool (which may then also be skipped over).  The oldest
         leases in the free and backup states are examined.  The time they
         have resided in their respective queues is used as an estimate to
         indicate how much time it is probable it would take before the leases
         at the top of the list would be consumed (and thus, how long it would
         take to use all leases in that state).  This percentage is directly
         multiplied by this time, and fit into the schedule if it falls within
         the min-balance and max-balance configured values.  The scheduled
         pool check time is only moved in a downwards direction, it is never
         increased.  Lastly, if the lts is more than double this number in the
         negative direction, the local server will 'panic' and transmit a
         Failover protocol POOLREQ message, in the hopes that the remote
         system will be woken up into action.

         Once the lts value exceeds the max-lease-misbalance percentage of
         total free state leases as described above, leases are moved to the
         remote server.  This is done in two passes.

         In the first pass, only leases whose most recent bound client would
         have been served by the remote server - according to the Load Balance
         Algorithm (see above split and hba configuration statements) - are
         given away to the peer.  This first pass will happily continue to
         give away leases, decrementing the lts value by one for each, until
         the lts value has reached the negative of the total number of leases
         multiplied by the max-lease-ownership percentage.  So it is through
         this value that you can permit a small misbalance of the lease pools
         - for the purpose of giving the peer more than a 50/50 share of
         leases in the hopes that their clients might some day return and be
         allocated by the peer (operating normally).  This process is referred
         to as 'MAC Address Affinity', but this is somewhat misnamed: it
         applies equally to DHCP Client Identifier options.  Note also that
         affinity is applied to leases when they enter the state 'free' from
         'expired' or 'released'.  In this case also, leases will not be moved
         from free to backup if the secondary already has more than its share.

         The second pass is only entered into if the first pass fails to
         reduce the lts underneath the total number of free state leases
         multiplied by the max-lease-ownership percentage.  In this pass, the
         oldest leases are given over to the peer without second thought about
         the Load Balance Algorithm, and this continues until the lts falls
         under this value.  In this way, the local server will also happily
         keep a small percentage of the leases that would normally load
         balance to itself.

         So, the max-lease-misbalance value acts as a behavioural gate.
         Smaller values will cause more leases to transition states to balance
         the pools over time, higher values will decrease the amount of change
         (but may lead to pool starvation if there's a run on leases).

         The max-lease-ownership value permits a small (percentage) skew in
         the lease balance of a percentage of the total number of free state
         leases.

         Finally, the min-balance and max-balance make certain that a
         scheduled rebalance event happens within a reasonable timeframe (not
         to be thrown off by, for example, a 7 year old free lease).

         Plausible values for the percentages lie between 0 and 100,
         inclusive, but values over 50 are indistinguishable from one another
         (once lts exceeds 50% of the free state leases, one server must
         therefore have 100% of the leases in its respective free state).  It
         is recommended to select a max-lease-ownership value that is lower
         than the value selected for the max-lease-misbalance value.  max-
         lease-ownership defaults to 10, and max-lease-misbalance defaults to
         15.

         Plausible values for the min-balance and max-balance times also range
         from 0 to (2^32)-1 (or the limit of your local time_t value), but
         default to values 60 and 3600 respectively (to place balance events
         between 1 minute and 1 hour).

CLIENT CLASSING
       Clients can be separated into classes, and treated differently
       depending on what class they are in.  This separation can be done
       either with a conditional statement, or with a match statement within
       the class declaration.  It is possible to specify a limit on the total
       number of clients within a particular class or subclass that may hold
       leases at one time, and it is possible to specify automatic subclassing
       based on the contents of the client packet.

       Classing support for DHCPv6 clients was added in 4.3.0.  It follows the
       same rules as for DHCPv4 except that support for billing classes has
       not been added yet.

       To add clients to classes based on conditional evaluation, you can
       specify a matching expression in the class statement:

       class "ras-clients" {
         match if substring (option dhcp-client-identifier, 1, 3) = "RAS";
       }

       Please note that the values used in match expressions may only come
       from data or options that are part of the client packet. It is not
       possible to use values constructed through one or more executable
       statements.  This stems from the fact that client classification occurs
       before any statements are executed. Attempting to do so will yield
       indeterminate results.

       Note that whether you use matching expressions or add statements (or
       both) to classify clients, you must always write a class declaration
       for any class that you use.  If there will be no match statement and no
       in-scope statements for a class, the declaration should look like this:

       class "ras-clients" {
       }

SUBCLASSES
       In addition to classes, it is possible to declare subclasses.  A
       subclass is a class with the same name as a regular class, but with a
       specific submatch expression which is hashed for quick matching.  This
       is essentially a speed hack - the main difference between five classes
       with match expressions and one class with five subclasses is that it
       will be quicker to find the subclasses.  Subclasses work as follows:

       class "allocation-class-1" {
         match pick-first-value (option dhcp-client-identifier, hardware);
       }

       class "allocation-class-2" {
         match pick-first-value (option dhcp-client-identifier, hardware);
       }

       subclass "allocation-class-1" 1:8:0:2b:4c:39:ad;
       subclass "allocation-class-2" 1:8:0:2b:a9:cc:e3;
       subclass "allocation-class-1" 1:0:0:c4:aa:29:44;

       subnet 10.0.0.0 netmask 255.255.255.0 {
         pool {
           allow members of "allocation-class-1";
           range 10.0.0.11 10.0.0.50;
         }
         pool {
           allow members of "allocation-class-2";
           range 10.0.0.51 10.0.0.100;
         }
       }

       The data following the class name in the subclass declaration is a
       constant value to use in matching the match expression for the class.
       When class matching is done, the server will evaluate the match
       expression and then look the result up in the hash table.  If it finds
       a match, the client is considered a member of both the class and the
       subclass.

       Subclasses can be declared with or without scope.  In the above
       example, the sole purpose of the subclass is to allow some clients
       access to one address pool, while other clients are given access to the
       other pool, so these subclasses are declared without scopes.  If part
       of the purpose of the subclass were to define different parameter
       values for some clients, you might want to declare some subclasses with
       scopes.

       In the above example, if you had a single client that needed some
       configuration parameters, while most didn't, you might write the
       following subclass declaration for that client:

       subclass "allocation-class-2" 1:08:00:2b:a1:11:31 {
         option root-path "samsara:/var/diskless/alphapc";
         filename "/tftpboot/netbsd.alphapc-diskless";
       }

       In this example, we've used subclassing as a way to control address
       allocation on a per-client basis.  However, it's also possible to use
       subclassing in ways that are not specific to clients - for example, to
       use the value of the vendor-class-identifier option to determine what
       values to send in the vendor-encapsulated-options option.  An example
       of this is shown under the VENDOR ENCAPSULATED OPTIONS head in the
       dhcp-options(5) manual page.

PER-CLASS LIMITS ON DYNAMIC ADDRESS ALLOCATION
       You may specify a limit to the number of clients in a class that can be
       assigned leases.  The effect of this will be to make it difficult for a
       new client in a class to get an address.  Once a class with such a
       limit has reached its limit, the only way a new client in that class
       can get a lease is for an existing client to relinquish its lease,
       either by letting it expire, or by sending a DHCPRELEASE packet.
       Classes with lease limits are specified as follows:

       class "limited-1" {
         lease limit 4;
       }

       This will produce a class in which a maximum of four members may hold a
       lease at one time.

SPAWNING CLASSES
       It is possible to declare a spawning class.  A spawning class is a
       class that automatically produces subclasses based on what the client
       sends.  The reason that spawning classes were created was to make it
       possible to create lease-limited classes on the fly.  The envisioned
       application is a cable-modem environment where the ISP wishes to
       provide clients at a particular site with more than one IP address, but
       does not wish to provide such clients with their own subnet, nor give
       them an unlimited number of IP addresses from the network segment to
       which they are connected.

       Many cable modem head-end systems can be configured to add a Relay
       Agent Information option to DHCP packets when relaying them to the DHCP
       server.  These systems typically add a circuit ID or remote ID option
       that uniquely identifies the customer site.  To take advantage of this,
       you can write a class declaration as follows:

       class "customer" {
         spawn with option agent.circuit-id;
         lease limit 4;
       }

       Now whenever a request comes in from a customer site, the circuit ID
       option will be checked against the class's hash table.  If a subclass
       is found that matches the circuit ID, the client will be classified in
       that subclass and treated accordingly.  If no subclass is found
       matching the circuit ID, a new one will be created and logged in the
       dhcpd.leases file, and the client will be classified in this new class.
       Once the client has been classified, it will be treated according to
       the rules of the class, including, in this case, being subject to the
       per-site limit of four leases.

       The use of the subclass spawning mechanism is not restricted to relay
       agent options - this particular example is given only because it is a
       fairly straightforward one.

COMBINING MATCH, MATCH IF AND SPAWN WITH
       In some cases, it may be useful to use one expression to assign a
       client to a particular class, and a second expression to put it into a
       subclass of that class.  This can be done by combining the match if and
       spawn with statements, or the match if and match statements.  For
       example:

       class "jr-cable-modems" {
         match if option dhcp-vendor-identifier = "jrcm";
         spawn with option agent.circuit-id;
         lease limit 4;
       }

       class "dv-dsl-modems" {
         match if option dhcp-vendor-identifier = "dvdsl";
         spawn with option agent.circuit-id;
         lease limit 16;
       }

       This allows you to have two classes that both have the same spawn with
       expression without getting the clients in the two classes confused with
       each other.

DYNAMIC DNS UPDATES
       The DHCP server has the ability to dynamically update the Domain Name
       System.  Within the configuration files, you can define how you want
       the Domain Name System to be updated.  These updates are RFC 2136
       compliant so any DNS server supporting RFC 2136 should be able to
       accept updates from the DHCP server.

       There are two DNS schemes implemented.  The interim option is based on
       draft revisions of the DDNS documents while the standard option is
       based on the RFCs for DHCP-DNS interaction and DHCIDs.  A third option,
       ad-hoc, was deprecated and has now been removed from the code base.
       The DHCP server must be configured to use one of the two currently-
       supported methods, or not to do DNS updates.

       New installations should use the standard option. Older installations
       may want to continue using the interim option for backwards
       compatibility with the DNS database until the database can be updated.
       This can be done with the ddns-update-style configuration parameter.

THE DNS UPDATE SCHEME
       the interim and standard DNS update schemes operate mostly according to
       work from the IETF.  The interim version was based on the drafts in
       progress at the time while the standard is based on the completed RFCs.
       The standard RFCs are:

                            RFC 4701 (updated by RF5494)
                                      RFC 4702
                                      RFC 4703


       And the corresponding drafts were:

                          draft-ietf-dnsext-dhcid-rr-??.txt
                          draft-ietf-dhc-fqdn-option-??.txt
                        draft-ietf-dhc-ddns-resolution-??.txt


       The basic framework for the two schemes is similar with the main
       material difference being that a DHCID RR is used in the standard
       version while the interim versions uses a TXT RR.  The format of the
       TXT record bears a resemblance to the DHCID RR but it is not equivalent
       (MD5 vs SHA2, field length differences etc).

       In these two schemes the DHCP server does not necessarily always update
       both the A and the PTR records.  The FQDN option includes a flag which,
       when sent by the client, indicates that the client wishes to update its
       own A record.  In that case, the server can be configured either to
       honor the client's intentions or ignore them.  This is done with the
       statement allow client-updates; or the statement ignore client-
       updates;.  By default, client updates are allowed.

       If the server is configured to allow client updates, then if the client
       sends a fully-qualified domain name in the FQDN option, the server will
       use that name the client sent in the FQDN option to update the PTR
       record.  For example, let us say that the client is a visitor from the
       "radish.org" domain, whose hostname is "jschmoe".  The server is for
       the "example.org" domain.  The DHCP client indicates in the FQDN option
       that its FQDN is "jschmoe.radish.org.".  It also indicates that it
       wants to update its own A record.  The DHCP server therefore does not
       attempt to set up an A record for the client, but does set up a PTR
       record for the IP address that it assigns the client, pointing at
       jschmoe.radish.org.  Once the DHCP client has an IP address, it can
       update its own A record, assuming that the "radish.org" DNS server will
       allow it to do so.

       If the server is configured not to allow client updates, or if the
       client doesn't want to do its own update, the server will simply choose
       a name for the client. By default, the server will choose from the
       following three values:

            1. fqdn option (if present)
            2. hostname option (if present)
            3. Configured hostname option (if defined).

       If these defaults for choosing the host name are not appropriate you
       can write your own statement to set the ddns-hostname variable as you
       wish.  If none of the above are found the server will use the host
       declaration name (if one) and use-host-decl-names is on.

       It will use its own domain name for the client.  It will then update
       both the A and PTR record, using the name that it chose for the client.
       If the client sends a fully-qualified domain name in the fqdn option,
       the server uses only the leftmost part of the domain name - in the
       example above, "jschmoe" instead of "jschmoe.radish.org".

       Further, if the ignore client-updates; directive is used, then the
       server will in addition send a response in the DHCP packet, using the
       FQDN Option, that implies to the client that it should perform its own
       updates if it chooses to do so.  With deny client-updates;, a response
       is sent which indicates the client may not perform updates.

       Both the standard and interim options also include a method to allow
       more than one DHCP server to update the DNS database without
       accidentally deleting A records that shouldn't be deleted nor failing
       to add A records that should be added.  For the standard option the
       method works as follows:

       When the DHCP server issues a client a new lease, it creates a text
       string that is an SHA hash over the DHCP client's identification (see
       RFCs 4701 & 4702 for details).  The update attempts to add an A record
       with the name the server chose and a DHCID record containing the hashed
       identifier string (hashid).  If this update succeeds, the server is
       done.

       If the update fails because the A record already exists, then the DHCP
       server attempts to add the A record with the prerequisite that there
       must be a DHCID record in the same name as the new A record, and that
       DHCID record's contents must be equal to hashid.  If this update
       succeeds, then the client has its A record and PTR record.  If it
       fails, then the name the client has been assigned (or requested) is in
       use, and can't be used by the client.  At this point the DHCP server
       gives up trying to do a DNS update for the client until the client
       chooses a new name.

       The server also does not update very aggressively.  Because each DNS
       update involves a round trip to the DNS server, there is a cost
       associated with doing updates even if they do not actually modify the
       DNS database.  So the DHCP server tracks whether or not it has updated
       the record in the past (this information is stored on the lease) and
       does not attempt to update records that it thinks it has already
       updated.

       This can lead to cases where the DHCP server adds a record, and then
       the record is deleted through some other mechanism, but the server
       never again updates the DNS because it thinks the data is already
       there.  In this case the data can be removed from the lease through
       operator intervention, and once this has been done, the DNS will be
       updated the next time the client renews.

       The interim DNS update scheme was written before the RFCs were
       finalized and does not quite follow them.  The RFCs call for a new
       DHCID RRtype while the interim DNS update scheme uses a TXT record.  In
       addition the ddns-resolution draft called for the DHCP server to put a
       DHCID RR on the PTR record, but the interim update method does not do
       this.  In the final RFC this requirement was relaxed such that a server
       may add a DHCID RR to the PTR record.

DDNS IN DUAL STACK ENVIRONMENTS
       As described in RFC 4703, section 5.2, in order to perform DDNS in dual
       stack environments, both IPv4 and IPv6 servers would need to be
       configured to use the standard update style and participating IPv4
       clients MUST convey DUIDs as described in RFC 4361, section 6.1., in
       their dhcp-client-identifiers.

       In a nutshell, this mechanism is intended to use globally unique DUIDs
       to idenfity both IPv4 and IPv6 clients, and where a device has both
       IPv4 and IPv6 leases it is identified by the same DUID.  This allows a
       dual stack client to use the same FQDN for both mappings, while being
       protected from updates for other clients by the rules of conflict
       detection.

       However, not all IPv4 clients implement this behavior which makes
       supporting them dual stack environments problematic.  In order to
       address this issue ISC DHCP (as of 4.4.0) supports a new mode of DDNS
       conflict resolution referred to as Dual Stack Mixed Mode (DSMM).

       The concept behind DSMM is relatively simple.  All dhcp servers of one
       protocol (IPv4 or v6) use one ddns-update-style (interim or standard)
       while all servers of the "other" protocol will use the "other" ddns-
       udpate-style.  In this way, all servers of a given protocol are using
       the same record type (TXT or DHCID) for their DHCID RR entries.  This
       allows conflict detection to be enforced within each protocol without
       interferring with the other's entries.

       DSMM modifications now ensure that IPv4 DSMM servers only ever modify A
       records, their associated PTR records and DHCID records, while DSMM
       IPv6 severs only modify AAAA records, their associated PTR records, and
       DHCID records.

       Note that DSMM is not a perfect solution, it is a compromise that can
       work well provided all participating DNS updaters play by DSMM rules.
       As with anything else in life, it only works as well as those who
       particpate behave.

       While conflict detection is enabled by default, DSMM is not.  To enable
       DSMM, both update-conflict-detection and ddns-dual-stack-mixed-mode
       must be true.

PROTECTING DNS ENTRIES FOR STATIC CLIENTS
       Built into conflict resolution is the protection of manually made
       entries for static clients.  Per the rules of conflict resolution,  a
       DNS updater may not alter forward DNS entries unless there is a DHCID
       RR which matches for whom the update is being made.  Therefore, any
       forward DNS entries without a corresponding DHCID RR cannot be altered
       by such an updater.

       In some environments, it may be desirable to use only this aspect of
       conflict resolution and allow DNS updaters to overwrite entries for
       dynamic clients regardless of what client owns them.  In other words,
       the presence or lack of a DHCID RR is used to determine whether entries
       may or may not be overwritten.  Whether or not the client matches the
       data value of the DHCID RR is irrelevant.   This behavior, off by
       default, can be configured through the parameter, ddns-guard-id-must-
       match.  As with DSMM, this behavior is can only be enabled if conflict
       resolution is enabled.   This behavior should be considered carefully
       before electing to use it.

       There is an additional parameter that can be used with DSMM ddns-other-
       guard-is-dynamic.  When enabled along with DSMM, a server will regard
       the presence of a DHCID RR of the other style type as indicating that
       the forward DNS entries for that FQDN should be dynamic and may be
       overwritten.  For example, such a server using interim style could
       overwrite the DNS entries for an FQDN if there is only a DHDID type
       DHDID RR for the FQDN.  Essentially, if there are dynamic entries for
       one protocol, that is enough to overcome the static protection of
       entries for the other protocol.  This behavior warrants careful
       consideration before electing to use it.

DYNAMIC DNS UPDATE SECURITY
       When you set your DNS server up to allow updates from the DHCP server,
       you may be exposing it to unauthorized updates.  To avoid this, you
       should use TSIG signatures - a method of cryptographically signing
       updates using a shared secret key.  As long as you protect the secrecy
       of this key, your updates should also be secure.  Note, however, that
       the DHCP protocol itself provides no security, and that clients can
       therefore provide information to the DHCP server which the DHCP server
       will then use in its updates, with the constraints described
       previously.

       The DNS server must be configured to allow updates for any zone that
       the DHCP server will be updating.  For example, let us say that clients
       in the sneedville.edu domain will be assigned addresses on the
       10.10.17.0/24 subnet.  In that case, you will need a key declaration
       for the TSIG key you will be using, and also two zone declarations -
       one for the zone containing A records that will be updates and one for
       the zone containing PTR records - for ISC BIND, something like this:

       key DHCP_UPDATER {
         algorithm HMAC-MD5.SIG-ALG.REG.INT;
         secret pRP5FapFoJ95JEL06sv4PQ==;
       };

       zone "example.org" {
            type master;
            file "example.org.db";
            allow-update { key DHCP_UPDATER; };
       };

       zone "17.10.10.in-addr.arpa" {
            type master;
            file "10.10.17.db";
            allow-update { key DHCP_UPDATER; };
       };

       You will also have to configure your DHCP server to do updates to these
       zones.  To do so, you need to add something like this to your
       dhcpd.conf file:

       key DHCP_UPDATER {
         algorithm HMAC-MD5.SIG-ALG.REG.INT;
         secret pRP5FapFoJ95JEL06sv4PQ==;
       };

       zone EXAMPLE.ORG. {
         primary 127.0.0.1;
         key DHCP_UPDATER;
       }

       zone 17.127.10.in-addr.arpa. {
         primary 127.0.0.1;
         key DHCP_UPDATER;
       }

       The primary statement specifies the IP address of the name server whose
       zone information is to be updated.  In addition to the primary
       statement there are also the primary6 , secondary and secondary6
       statements.  The primary6 statement specifies an IPv6 address for the
       name server.  The secondaries provide for additional addresses for name
       servers to be used if the primary does not respond.  The number of name
       servers the DDNS code will attempt to use before giving up is limited
       and is currently set to three.

       Note that the zone declarations have to correspond to authority records
       in your name server - in the above example, there must be an SOA record
       for "example.org." and for "17.10.10.in-addr.arpa.".  For example, if
       there were a subdomain "foo.example.org" with no separate SOA, you
       could not write a zone declaration for "foo.example.org."  Also keep in
       mind that zone names in your DHCP configuration should end in a ".";
       this is the preferred syntax.  If you do not end your zone name in a
       ".", the DHCP server will figure it out.  Also note that in the DHCP
       configuration, zone names are not encapsulated in quotes where there
       are in the DNS configuration.

       You should choose your own secret key, of course.  The ISC BIND 9
       distribution comes with a program for generating secret keys called
       dnssec-keygen.  If you are using BIND 9's dnssec-keygen, the above key
       would be created as follows:

            dnssec-keygen -a HMAC-MD5 -b 128 -n USER DHCP_UPDATER

       The key name, algorithm, and secret must match that being used by the
       DNS server. The DHCP server currently supports the following
       algorithms:

               HMAC-MD5
               HMAC-SHA1
               HMAC-SHA224
               HMAC-SHA256
               HMAC-SHA384
               HMAC-SHA512

       You may wish to enable logging of DNS updates on your DNS server.  To
       do so, you might write a logging statement like the following:

       logging {
            channel update_debug {
                 file "/var/log/update-debug.log";
                 severity  debug 3;
                 print-category yes;
                 print-severity yes;
                 print-time     yes;
            };
            channel security_info    {
                 file "/var/log/named-auth.info";
                 severity  info;
                 print-category yes;
                 print-severity yes;
                 print-time     yes;
            };

            category update { update_debug; };
            category security { security_info; };
       };

       You must create the /var/log/named-auth.info and /var/log/update-
       debug.log files before starting the name server.  For more information
       on configuring ISC BIND, consult the documentation that accompanies it.

REFERENCE: EVENTS
       There are three kinds of events that can happen regarding a lease, and
       it is possible to declare statements that occur when any of these
       events happen.  These events are the commit event, when the server has
       made a commitment of a certain lease to a client, the release event,
       when the client has released the server from its commitment, and the
       expiry event, when the commitment expires.

       To declare a set of statements to execute when an event happens, you
       must use the on statement, followed by the name of the event, followed
       by a series of statements to execute when the event happens, enclosed
       in braces.

REFERENCE: DECLARATIONS
       The include statement

        include "filename";

       The include statement is used to read in a named file, and process the
       contents of that file as though it were entered in place of the include
       statement.

       The shared-network statement

        shared-network name {
          [ parameters ]
          [ declarations ]
        }

       The shared-network statement is used to inform the DHCP server that
       some IP subnets actually share the same physical network.  Any subnets
       in a shared network should be declared within a shared-network
       statement.  Parameters specified in the shared-network statement will
       be used when booting clients on those subnets unless parameters
       provided at the subnet or host level override them.  If any subnet in a
       shared network has addresses available for dynamic allocation, those
       addresses are collected into a common pool for that shared network and
       assigned to clients as needed.  There is no way to distinguish on which
       subnet of a shared network a client should boot.

       Name should be the name of the shared network.  This name is used when
       printing debugging messages, so it should be descriptive for the shared
       network.  The name may have the syntax of a valid domain name (although
       it will never be used as such), or it may be any arbitrary name,
       enclosed in quotes.

       The subnet statement

        subnet subnet-number netmask netmask {
          [ parameters ]
          [ declarations ]
        }

       The subnet statement is used to provide dhcpd with enough information
       to tell whether or not an IP address is on that subnet.  It may also be
       used to provide subnet-specific parameters and to specify what
       addresses may be dynamically allocated to clients booting on that
       subnet.  Such addresses are specified using the range declaration.

       The subnet-number should be an IP address or domain name which resolves
       to the subnet number of the subnet being described.  The netmask should
       be an IP address or domain name which resolves to the subnet mask of
       the subnet being described.  The subnet number, together with the
       netmask, are sufficient to determine whether any given IP address is on
       the specified subnet.

       Although a netmask must be given with every subnet declaration, it is
       recommended that if there is any variance in subnet masks at a site, a
       subnet-mask option statement be used in each subnet declaration to set
       the desired subnet mask, since any subnet-mask option statement will
       override the subnet mask declared in the subnet statement.

       The subnet6 statement

        subnet6 subnet6-number {
          [ parameters ]
          [ declarations ]
        }

       The subnet6 statement is used to provide dhcpd with enough information
       to tell whether or not an IPv6 address is on that subnet6.  It may also
       be used to provide subnet-specific parameters and to specify what
       addresses may be dynamically allocated to clients booting on that
       subnet.

       The subnet6-number should be an IPv6 network identifier, specified as
       ip6-address/bits.

       The range statement

       range [ dynamic-bootp ] low-address [ high-address];

       For any subnet on which addresses will be assigned dynamically, there
       must be at least one range statement.  The range statement gives the
       lowest and highest IP addresses in a range.  All IP addresses in the
       range should be in the subnet in which the range statement is declared.
       The dynamic-bootp flag may be specified if addresses in the specified
       range may be dynamically assigned to BOOTP clients as well as DHCP
       clients.  When specifying a single address, high-address can be
       omitted.

       The range6 statement

       range6 low-address high-address;
       range6 subnet6-number;
       range6 subnet6-number temporary;
       range6 address temporary;

       For any IPv6 subnet6 on which addresses will be assigned dynamically,
       there must be at least one range6 statement. The range6 statement can
       either be the lowest and highest IPv6 addresses in a range6, or use
       CIDR notation, specified as ip6-address/bits. All IP addresses in the
       range6 should be in the subnet6 in which the range6 statement is
       declared.

       The temporary variant makes the prefix (by default on 64 bits)
       available for temporary (RFC 4941) addresses. A new address per prefix
       in the shared network is computed at each request with an IA_TA option.
       Release and Confirm ignores temporary addresses.

       Any IPv6 addresses given to hosts with fixed-address6 are excluded from
       the range6, as are IPv6 addresses on the server itself.

       The prefix6 statement

       prefix6 low-address high-address / bits;

       The prefix6 is the range6 equivalent for Prefix Delegation (RFC 3633).
       Prefixes of bits length are assigned between low-address and high-
       address.

       Any IPv6 prefixes given to static entries (hosts) with fixed-prefix6
       are excluded from the prefix6.

       This statement is currently global but it should have a shared-network
       scope.

       The host statement

        host hostname {
          [ parameters ]
          [ declarations ]
        }

       The host declaration provides a way for the DHCP server to identify a
       DHCP or BOOTP client.  This allows the server to provide configuration
       information including fixed addresses or, in DHCPv6, fixed prefixes for
       a specific client.

       If it is desirable to be able to boot a DHCP or BOOTP client on more
       than one subnet with fixed v4 addresses, more than one address may be
       specified in the fixed-address declaration, or more than one host
       statement may be specified matching the same client.

       The fixed-address6 declaration is used for v6 addresses.  At this time
       it only works with a single address.  For multiple addresses specify
       multiple host statements.

       If client-specific boot parameters must change based on the network to
       which the client is attached, then multiple host declarations should be
       used.  The host declarations will only match a client if one of their
       fixed-address statements is viable on the subnet (or shared network)
       where the client is attached.  Conversely, for a host declaration to
       match a client being allocated a dynamic address, it must not have any
       fixed-address statements.  You may therefore need a mixture of host
       declarations for any given client...some having fixed-address
       statements, others without.

       hostname should be a name identifying the host.  If a hostname option
       is not specified for the host, hostname is used.

       Host declarations are matched to actual DHCP or BOOTP clients by
       matching the dhcp-client-identifier option specified in the host
       declaration to the one supplied by the client, or, if the host
       declaration or the client does not provide a dhcp-client-identifier
       option, by matching the hardware parameter in the host declaration to
       the network hardware address supplied by the client.  BOOTP clients do
       not normally provide a dhcp-client-identifier, so the hardware address
       must be used for all clients that may boot using the BOOTP protocol.

       DHCPv6 servers can use the host-identifier option parameter in the host
       declaration, and specify any option with a fixed value to identify
       hosts.

       Please be aware that only the dhcp-client-identifier option and the
       hardware address can be used to match a host declaration, or the host-
       identifier option parameter for DHCPv6 servers.  For example, it is not
       possible to match a host declaration to a host-name option.  This is
       because the host-name option cannot be guaranteed to be unique for any
       given client, whereas both the hardware address and dhcp-client-
       identifier option are at least theoretically guaranteed to be unique to
       a given client.

       The group statement

        group {
          [ parameters ]
          [ declarations ]
        }

       The group statement is used simply to apply one or more parameters to a
       group of declarations.  It can be used to group hosts, shared networks,
       subnets, or even other groups.

REFERENCE: ALLOW AND DENY
       The allow and deny statements can be used to control the response of
       the DHCP server to various sorts of requests.  The allow and deny
       keywords actually have different meanings depending on the context.  In
       a pool context, these keywords can be used to set up access lists for
       address allocation pools.  In other contexts, the keywords simply
       control general server behavior with respect to clients based on scope.
       In a non-pool context, the ignore keyword can be used in place of the
       deny keyword to prevent logging of denied requests.

ALLOW DENY AND IGNORE IN SCOPE
       The following usages of allow and deny will work in any scope, although
       it is not recommended that they be used in pool declarations.

       The unknown-clients keyword

        allow unknown-clients;
        deny unknown-clients;
        ignore unknown-clients;

       The unknown-clients flag is used to tell dhcpd whether or not to
       dynamically assign addresses to unknown clients.  Dynamic address
       assignment to unknown clients is allowed by default.  An unknown client
       is simply a client that has no host declaration.

       The use of this option is now deprecated.  If you are trying to
       restrict access on your network to known clients, you should use deny
       unknown-clients; inside of your address pool, as described under the
       heading ALLOW AND DENY WITHIN POOL DECLARATIONS.

       The bootp keyword

        allow bootp;
        deny bootp;
        ignore bootp;

       The bootp flag is used to tell dhcpd whether or not to respond to bootp
       queries.  Bootp queries are allowed by default.

       The booting keyword

        allow booting;
        deny booting;
        ignore booting;

       The booting flag is used to tell dhcpd whether or not to respond to
       queries from a particular client.  This keyword only has meaning when
       it appears in a host declaration.  By default, booting is allowed, but
       if it is disabled for a particular client, then that client will not be
       able to get an address from the DHCP server.

       The duplicates keyword

        allow duplicates;
        deny duplicates;

       Host declarations can match client messages based on the DHCP Client
       Identifier option or based on the client's network hardware type and
       MAC address.  If the MAC address is used, the host declaration will
       match any client with that MAC address - even clients with different
       client identifiers.  This doesn't normally happen, but is possible when
       one computer has more than one operating system installed on it - for
       example, Microsoft Windows and NetBSD or Linux.

       The duplicates flag tells the DHCP server that if a request is received
       from a client that matches the MAC address of a host declaration, any
       other leases matching that MAC address should be discarded by the
       server, even if the UID is not the same.  This is a violation of the
       DHCP protocol, but can prevent clients whose client identifiers change
       regularly from holding many leases at the same time.  By default,
       duplicates are allowed.

       The declines keyword

        allow declines;
        deny declines;
        ignore declines;

       The DHCPDECLINE message is used by DHCP clients to indicate that the
       lease the server has offered is not valid.  When the server receives a
       DHCPDECLINE for a particular address, it normally abandons that
       address, assuming that some unauthorized system is using it.
       Unfortunately, a malicious or buggy client can, using DHCPDECLINE
       messages, completely exhaust the DHCP server's allocation pool.  The
       server will eventually reclaim these leases, but not while the client
       is running through the pool. This may cause serious thrashing in the
       DNS, and it will also cause the DHCP server to forget old DHCP client
       address allocations.

       The declines flag tells the DHCP server whether or not to honor
       DHCPDECLINE messages.  If it is set to deny or ignore in a particular
       scope, the DHCP server will not respond to DHCPDECLINE messages.

       The declines flag is only supported by DHCPv4 servers.  Given the large
       IPv6 address space and the internal limits imposed by the server's
       address generation mechanism we don't think it is necessary for DHCPv6
       servers at this time.

       Currently, abandoned IPv6 addresses are reclaimed in one of two ways:
           a) Client renews a specific address:
           If a client using a given DUID submits a DHCP REQUEST containing
           the last address abandoned by that DUID, the address will be
           reassigned to that client.

           b) Upon the second restart following an address abandonment.  When
           an address is abandoned it is both recorded as such in the lease
           file and retained as abandoned in server memory until the server
           is restarted. Upon restart, the server will process the lease file
           and all addresses whose last known state is abandoned will be
           retained as such in memory but not rewritten to the lease file.
           This means that a subsequent restart of the server will not see the
           abandoned addresses in the lease file and therefore have no record
           of them as abandoned in memory and as such perceive them as free
           for assignment.

       The total number addresses in a pool, available for a given DUID value,
       is internally limited by the server's address generation mechanism.  If
       through mistaken configuration, multiple clients are using the same
       DUID they will competing for the same addresses causing the server to
       reach this internal limit rather quickly.  The internal limit isolates
       this type of activity such that address range is not exhausted for
       other DUID values.  The appearance of the following error log, can be
       an indication of this condition:

           "Best match for DUID <XX> is an abandoned address, This may be a
            result of multiple clients attempting to use this DUID"

           where <XX> is an actual DUID value depicted as colon separated
           string of bytes in hexadecimal values.

       The client-updates keyword

        allow client-updates;
        deny client-updates;

       The client-updates flag tells the DHCP server whether or not to honor
       the client's intention to do its own update of its A record.  See the
       documentation under the heading THE DNS UPDATE SCHEME for details.

       The leasequery keyword

        allow leasequery;
        deny leasequery;

       The leasequery flag tells the DHCP server whether or not to answer
       DHCPLEASEQUERY packets. The answer to a DHCPLEASEQUERY packet includes
       information about a specific lease, such as when it was issued and when
       it will expire. By default, the server will not respond to these
       packets.

ALLOW AND DENY WITHIN POOL DECLARATIONS
       The uses of the allow and deny keywords shown in the previous section
       work pretty much the same way whether the client is sending a
       DHCPDISCOVER or a DHCPREQUEST message - an address will be allocated to
       the client (either the old address it's requesting, or a new address)
       and then that address will be tested to see if it's okay to let the
       client have it.  If the client requested it, and it's not okay, the
       server will send a DHCPNAK message.  Otherwise, the server will simply
       not respond to the client.  If it is okay to give the address to the
       client, the server will send a DHCPACK message.

       The primary motivation behind pool declarations is to have address
       allocation pools whose allocation policies are different.  A client may
       be denied access to one pool, but allowed access to another pool on the
       same network segment.  In order for this to work, access control has to
       be done during address allocation, not after address allocation is
       done.

       When a DHCPREQUEST message is processed, address allocation simply
       consists of looking up the address the client is requesting and seeing
       if it's still available for the client.  If it is, then the DHCP server
       checks both the address pool permit lists and the relevant in-scope
       allow and deny statements to see if it's okay to give the lease to the
       client.  In the case of a DHCPDISCOVER message, the allocation process
       is done as described previously in the ADDRESS ALLOCATION section.

       When declaring permit lists for address allocation pools, the following
       syntaxes are recognized following the allow or deny keywords:

        known-clients;

       If specified, this statement either allows or prevents allocation from
       this pool to any client that has a host declaration (i.e., is known).
       A client is known if it has a host declaration in any scope, not just
       the current scope.

        unknown-clients;

       If specified, this statement either allows or prevents allocation from
       this pool to any client that has no host declaration (i.e., is not
       known).

        members of "class";

       If specified, this statement either allows or prevents allocation from
       this pool to any client that is a member of the named class.

        dynamic bootp clients;

       If specified, this statement either allows or prevents allocation from
       this pool to any bootp client.

        authenticated clients;

       If specified, this statement either allows or prevents allocation from
       this pool to any client that has been authenticated using the DHCP
       authentication protocol.  This is not yet supported.

        unauthenticated clients;

       If specified, this statement either allows or prevents allocation from
       this pool to any client that has not been authenticated using the DHCP
       authentication protocol.  This is not yet supported.

        all clients;

       If specified, this statement either allows or prevents allocation from
       this pool to all clients.  This can be used when you want to write a
       pool declaration for some reason, but hold it in reserve, or when you
       want to renumber your network quickly, and thus want the server to
       force all clients that have been allocated addresses from this pool to
       obtain new addresses immediately when they next renew.

        after time;

       If specified, this statement either allows or prevents allocation from
       this pool after a given date. This can be used when you want to move
       clients from one pool to another. The server adjusts the regular lease
       time so that the latest expiry time is at the given time+min-lease-
       time.  A short min-lease-time enforces a step change, whereas a longer
       min-lease-time allows for a gradual change.  time is either second
       since epoch, or a UTC time string e.g.  4 2007/08/24 09:14:32 or a
       string with time zone offset in seconds e.g. 4 2007/08/24 11:14:32
       -7200

REFERENCE: PARAMETERS
       The abandon-lease-time statement

         abandon-lease-time time;

         Time should be the maximum amount of time (in seconds) that an
         abandoned IPv4 lease remains unavailable for assignment to a client.
         Abandoned leases will only be offered to clients if there are no free
         leases.  If not defined, the default abandon lease time is 86400
         seconds (24 hours).  Note the abandoned lease time for a given lease
         is preserved across server restarts.  The parameter may only be set
         at the global scope and is evaluated only once during server startup.

         Values less than sixty seconds are not recommended as this is below
         the ping check threshold and can cause leases once abandoned but
         since returned to the free state to not be pinged before being
         offered.  If the requested time is larger than 0x7FFFFFFF - 1 or the
         sum of the current time plus the abandoned time isgreater than
         0x7FFFFFFF it is treated as infinite.

       The adaptive-lease-time-threshold statement

         adaptive-lease-time-threshold percentage;

         When the number of allocated leases within a pool rises above the
         percentage given in this statement, the DHCP server decreases the
         lease length for new clients within this pool to min-lease-time
         seconds. Clients renewing an already valid (long) leases get at least
         the remaining time from the current lease. Since the leases expire
         faster, the server may either recover more quickly or avoid pool
         exhaustion entirely.  Once the number of allocated leases drop below
         the threshold, the server reverts back to normal lease times.  Valid
         percentages are between 1 and 99.

       The always-broadcast statement

         always-broadcast flag;

         The DHCP and BOOTP protocols both require DHCP and BOOTP clients to
         set the broadcast bit in the flags field of the BOOTP message header.
         Unfortunately, some DHCP and BOOTP clients do not do this, and
         therefore may not receive responses from the DHCP server.  The DHCP
         server can be made to always broadcast its responses to clients by
         setting this flag to 'on' for the relevant scope; relevant scopes
         would be inside a conditional statement, as a parameter for a class,
         or as a parameter for a host declaration.  To avoid creating excess
         broadcast traffic on your network, we recommend that you restrict the
         use of this option to as few clients as possible.  For example, the
         Microsoft DHCP client is known not to have this problem, as are the
         OpenTransport and ISC DHCP clients.

       The always-reply-rfc1048 statement

         always-reply-rfc1048 flag;

         Some BOOTP clients expect RFC1048-style responses, but do not follow
         RFC1048 when sending their requests.  You can tell that a client is
         having this problem if it is not getting the options you have
         configured for it and if you see in the server log the message "(non-
         rfc1048)" printed with each BOOTREQUEST that is logged.

         If you want to send rfc1048 options to such a client, you can set the
         always-reply-rfc1048 option in that client's host declaration, and
         the DHCP server will respond with an RFC-1048-style vendor options
         field.  This flag can be set in any scope, and will affect all
         clients covered by that scope.

       The authoritative statement

         authoritative;

         not authoritative;

         The DHCP server will normally assume that the configuration
         information about a given network segment is not known to be correct
         and is not authoritative.  This is so that if a naive user installs a
         DHCP server not fully understanding how to configure it, it does not
         send spurious DHCPNAK messages to clients that have obtained
         addresses from a legitimate DHCP server on the network.

         Network administrators setting up authoritative DHCP servers for
         their networks should always write authoritative; at the top of their
         configuration file to indicate that the DHCP server should send
         DHCPNAK messages to misconfigured clients.  If this is not done,
         clients will be unable to get a correct IP address after changing
         subnets until their old lease has expired, which could take quite a
         long time.

         Usually, writing authoritative; at the top level of the file should
         be sufficient.  However, if a DHCP server is to be set up so that it
         is aware of some networks for which it is authoritative and some
         networks for which it is not, it may be more appropriate to declare
         authority on a per-network-segment basis.

         Note that the most specific scope for which the concept of authority
         makes any sense is the physical network segment - either a shared-
         network statement or a subnet statement that is not contained within
         a shared-network statement.  It is not meaningful to specify that the
         server is authoritative for some subnets within a shared network, but
         not authoritative for others, nor is it meaningful to specify that
         the server is authoritative for some host declarations and not
         others.

         In order for DHCPINFORMs to be responded to by the server, they must
         match to subnets over which the server has authority; otherwise they
         will be ignored and logged.  To minimize the impact on logging
         volume, only the first and every subsequent 100th occurrence of an
         ignored DHCPINFORM is logged.

       The boot-unknown-clients statement

         boot-unknown-clients flag;

         If the boot-unknown-clients statement is present and has a value of
         false or off, then clients for which there is no host declaration
         will not be allowed to obtain IP addresses.  If this statement is not
         present or has a value of true or on, then clients without host
         declarations will be allowed to obtain IP addresses, as long as those
         addresses are not restricted by allow and deny statements within
         their pool declarations.

       The check-secs-byte-order statement

         check-secs-byte-order flag;

         When check-secs-byte-order is enabled, the server will check for
         DHCPv4 clients that do the byte ordering on the secs field
         incorrectly. This field should be in network byte order but some
         clients get it wrong. When this parameter is enabled the server will
         examine the secs field and if it looks wrong (high byte non zero and
         low byte zero) swap the bytes.  The default is disabled. This
         parameter is only useful when doing load balancing within failover.
         (Formerly, this behavior had to be enabled during compilation
         configuration via --enable-secs-byteorder).

         The db-time-format statement

           db-time-format [ default | local ] ;

           The DHCP server software outputs several timestamps when writing
           leases to persistent storage.  This configuration parameter selects
           one of two output formats.  The default format prints the day,
           date, and time in UTC, while the local format prints the system
           seconds-since-epoch, and helpfully provides the day and time in the
           system timezone in a comment.  The time formats are described in
           detail in the dhcpd.leases(5) manpage.

         The ddns-hostname statement

           ddns-hostname name;

           The name parameter should be the hostname that will be used in
           setting up the client's A and PTR records.  If no ddns-hostname is
           specified in scope, then the server will derive the hostname
           automatically, using an algorithm that varies for each of the
           different update methods.

         The ddns-domainname statement

           ddns-domainname name;

           The name parameter should be the domain name that will be appended
           to the client's hostname to form a fully-qualified domain-name
           (FQDN).

         The ddns-dual-stack-mixed-mode statement

           ddns-dual-stack-mixed-mode flag;

           The ddns-dual-stack-mixed-mode parameter controls whether or not
           the server applies Dual Stack Mixed Mode rules during DDNS conflict
           resolution.  This parameter is off by default, has no effect unless
           update-conflict-detection is enabled, and may only be specified at
           the global scope.

         The ddns-guard-id-must-match statement

           ddns-guard-id-must-match flag;

           The ddns-guard-id-must-match parameter controls whether or not a
           the client id within a DHCID RR must match that of the DNS update's
           client to permit DNS entries associated with that DHCID RR to be
           ovewritten.  Proper conflict resolution requires ID matching and
           should only be disabled after careful consideration.  When
           disabled, it is allows any DNS updater to replace DNS entries that
           have an associated DHCID RR, regardless of client identity. This
           parameter is on by default, has no effect unless update-conflict-
           detection is enabled, and may only be specified at the global
           scope.

         The dns-local-address4 and dns-local-address6 statements

           ddns-local-address4 address;

           ddns-local-address6 address;

           The address parameter should be the local IPv4 or IPv6 address the
           server should use as the from address when sending DDNS update
           requests.

         The ddns-other-guard-is-dynamic statement

           ddns-other-guard-is-dynamic flag;

           The ddns-other-guard-is-dynamic parameter controls whether or not a
           a server running DSMM will consider the presence of the other
           update style DHCID RR as an indcation that a DNS entries may be
           overwritten. It should only be enabled after careful study as it
           allows DNS entries that would otherwise be protected as static, to
           be overwritten in certain cases. This paramater is off by default,
           has no effect unless ddns-dual-stack-mixed-mode is enabled, and may
           only be specified at the global scope.

         The ddns-rev-domainname statement

           ddns-rev-domainname name;

           The name parameter should be the domain name that will be appended
           to the client's reversed IP address to produce a name for use in
           the client's PTR record.  By default, this is "in-addr.arpa.", but
           the default can be overridden here.

           The reversed IP address to which this domain name is appended is
           always the IP address of the client, in dotted quad notation,
           reversed - for example, if the IP address assigned to the client is
           10.17.92.74, then the reversed IP address is 74.92.17.10.  So a
           client with that IP address would, by default, be given a PTR
           record of 10.17.92.74.in-addr.arpa.

         The ddns-update-style parameter

           ddns-update-style style;

           The style parameter must be one of standard, interim or none.  The
           ddns-update-style statement is only meaningful in the outer scope -
           it is evaluated once after reading the dhcpd.conf file, rather than
           each time a client is assigned an IP address, so there is no way to
           use different DNS update styles for different clients. The default
           is none.

         The ddns-updates statement

            ddns-updates flag;

           The ddns-updates parameter controls whether or not the server will
           attempt to do a DNS update when a lease is confirmed.  Set this to
           off if the server should not attempt to do updates within a certain
           scope.  The ddns-updates parameter is on by default.  To disable
           DNS updates in all scopes, it is preferable to use the ddns-update-
           style statement, setting the style to none.

         The default-lease-time statement

           default-lease-time time;

           Time should be the length in seconds that will be assigned to a
           lease if the client requesting the lease does not ask for a
           specific expiration time.  This is used for both DHCPv4 and DHCPv6
           leases (it is also known as the "valid lifetime" in DHCPv6).  The
           default is 43200 seconds.

         The delayed-ack and max-ack-delay statements

           delayed-ack count;

           max-ack-delay microseconds;

           Count should be an integer value from zero to 2^16-1 and defaults
           to 0, which means that the feature is disabled.  Otherwise, 28 may
           be a sensible starting point for many configurations (SO_SNDBUF
           size / 576 bytes.)  The count represents how many DHCPv4 replies
           maximum will be queued pending transmission until after a database
           commit event.  If this number is reached, a database commit event
           (commonly resulting in fsync() and representing a performance
           penalty) will be made, and the reply packets will be transmitted in
           a batch afterwards.  This preserves the RFC2131 direction that
           "stable storage" be updated prior to replying to clients.  Should
           the DHCPv4 sockets "go dry" (select() returns immediately with no
           read sockets), the commit is made and any queued packets are
           transmitted.

           Similarly, microseconds indicates how many microseconds are
           permitted to pass inbetween queuing a packet pending an fsync, and
           performing the fsync.  Valid values range from 0 to 2^32-1, and
           defaults to 250,000 (1/4 of a second).

           The delayed-ack feature is compiled in by default, but can be
           disabled at compile time with './configure --disable-delayed-ack'.
           Please note that the delayed-ack feature is not currently
           compatible with support for DHPCv4-over-DHCPv6 so when a 4to6 port
           ommand line argument enables this in the server the delayed-ack
           value is reset to 0.

         The dhcp-cache-threshold statement

           dhcp-cache-threshold percentage;

           The dhcp-cache-threshold statement takes one integer parameter with
           allowed values between 0 and 100. The default value is 25 (25% of
           the lease time). This parameter expresses the percentage of the
           total lease time, measured from the beginning, during which a
           client's attempt to renew its lease will result in getting the
           already assigned lease, rather than an extended lease.  This
           feature is supported for both IPv4 and IPv6 and down to the pool
           level and for IPv6 all three pool types: NA, TA and PD.

           Clients that attempt renewal frequently can cause the server to
           update and write the database frequently resulting in a performance
           impact on the server.  The dhcp-cache-threshold statement instructs
           the DHCP server to avoid updating leases too frequently thus
           avoiding this behavior.  Instead the server replies with the same
           lease (i.e. reuses it) with no modifications except for CLTT
           (Client Last Transmission Time) and for IPv4:

               the lease time sent to the client is shortened by the age of
               the lease

           while for IPv6:

               the preferred and valid lifetimes sent to the client are
               shortened by the age of the lease.

           None of these changes require writing the lease to disk.


           When an existing lease is matched to a renewing client, it will be
           reused if all of the following conditions are true:
               1. The dhcp-cache-threshold is larger than zero
               2. The current lease is active
               3. The percentage of the lease time that has elapsed is less than
               dhcp-cache-threshold
               4. The client information provided in the renewal does not alter
               any of the following:
                  a. DNS information and DNS updates are enabled
                  b. Billing class to which the lease is associated (IPv4 only)
                  c. The host declaration associated with the lease (IPv4 only)
                  d. The client id - this may happen if a client boots without
                     a client id and then starts using one in subsequent
                     requests. (IPv4 only)

           While lease data is not written to disk when a lease is reused, the
           server will still execute any on-commit statements.

           Note that the lease can be reused if the options the client or
           relay agent sends are changed.  These changes will not be recorded
           in the in-memory or on-disk databases until the client renews after
           the threshold time is reached.

         The do-forward-updates statement

           do-forward-updates flag;

           The do-forward-updates statement instructs the DHCP server as to
           whether it should attempt to update a DHCP client's A record when
           the client acquires or renews a lease.  This statement has no
           effect unless DNS updates are enabled.  Forward updates are enabled
           by default.  If this statement is used to disable forward updates,
           the DHCP server will never attempt to update the client's A record,
           and will only ever attempt to update the client's PTR record if the
           client supplies an FQDN that should be placed in the PTR record
           using the fqdn option.  If forward updates are enabled, the DHCP
           server will still honor the setting of the client-updates flag.

         The dont-use-fsync statement

           dont-use-fsync flag;

           The dont-use-fsync statement instructs the DHCP server if it should
           call fsync() when writing leases to the lease file.  By default and
           if the flag is set to false the server will call fsync().
           Suppressing the call to fsync() may increase the performance of the
           server but it also adds a risk that a lease will not be properly
           written to the disk after it has been issued to a client and before
           the server stops.  This can lead to duplicate leases being issued
           to different clients.  Using this option is not recommended.

         The dynamic-bootp-lease-cutoff statement

           dynamic-bootp-lease-cutoff date;

           The dynamic-bootp-lease-cutoff statement sets the ending time for
           all leases assigned dynamically to BOOTP clients.  Because BOOTP
           clients do not have any way of renewing leases, and don't know that
           their leases could expire, by default dhcpd assigns infinite leases
           to all BOOTP clients.  However, it may make sense in some
           situations to set a cutoff date for all BOOTP leases - for example,
           the end of a school term, or the time at night when a facility is
           closed and all machines are required to be powered off.

           Date should be the date on which all assigned BOOTP leases will
           end.  The date is specified in the form:

                                  W YYYY/MM/DD HH:MM:SS

           W is the day of the week expressed as a number from zero (Sunday)
           to six (Saturday).  YYYY is the year, including the century.  MM is
           the month expressed as a number from 1 to 12.  DD is the day of the
           month, counting from 1.  HH is the hour, from zero to 23.  MM is
           the minute and SS is the second.  The time is always in Coordinated
           Universal Time (UTC), not local time.

         The dynamic-bootp-lease-length statement

           dynamic-bootp-lease-length length;

           The dynamic-bootp-lease-length statement is used to set the length
           of leases dynamically assigned to BOOTP clients.  At some sites, it
           may be possible to assume that a lease is no longer in use if its
           holder has not used BOOTP or DHCP to get its address within a
           certain time period.  The period is specified in length as a number
           of seconds.  If a client reboots using BOOTP during the timeout
           period, the lease duration is reset to length, so a BOOTP client
           that boots frequently enough will never lose its lease.  Needless
           to say, this parameter should be adjusted with extreme caution.

         The echo-client-id statement

           echo-client-id flag;

           The echo-client-id statement is used to enable or disable RFC 6842
           compliant behavior.  If the echo-client-id statement is present and
           has a value of true or on, and a DHCP DISCOVER or REQUEST is
           received which contains the client identifier option (Option code
           61), the server will copy the option into its response (DHCP ACK or
           NAK) per RFC 6842.  In other words if the client sends the option
           it will receive it back. By default, this flag is off and client
           identifiers will not echoed back to the client.

         The filename statement

           filename "filename";

           The filename statement can be used to specify the name of the
           initial boot file which is to be loaded by a client.  The filename
           should be a filename recognizable to whatever file transfer
           protocol the client can be expected to use to load the file.

         The fixed-address declaration

           fixed-address address [, address ... ];

           The fixed-address declaration is used to assign one or more fixed
           IP addresses to a client.  It should only appear in a host
           declaration.  If more than one address is supplied, then when the
           client boots, it will be assigned the address that corresponds to
           the network on which it is booting.  If none of the addresses in
           the fixed-address statement are valid for the network to which the
           client is connected, that client will not match the host
           declaration containing that fixed-address declaration.  Each
           address in the fixed-address declaration should be either an IP
           address or a domain name that resolves to one or more IP addresses.

         The fixed-address6 declaration

           fixed-address6 ip6-address ;

           The fixed-address6 declaration is used to assign a fixed IPv6
           addresses to a client.  It should only appear in a host
           declaration.

         The fixed-prefix6 declaration

           fixed-prefix6 low-address / bits;

           The fixed-prefix6 declaration is used to assign a fixed IPv6 prefix
           to a client.  It should only appear in a host declaration, but
           multiple fixed-prefix6 statements may appear in a single host
           declaration.

           The low-address specifies the start of the prefix and the bits
           specifies the size of the prefix in bits.

           If there are multiple prefixes for a given host entry the server
           will choose one that matches the requested prefix size or, if none
           match, the first one.

           If there are multiple host declarations the server will try to
           choose a declaration where the fixed-address6 matches the client's
           subnet.  If none match it will choose one that doesn't have a
           fixed-address6 statement.

           Note Well: Unlike the fixed address the fixed prefix does not need
           to match a subnet in order to be served.  This allows you to
           provide a prefix to a client that is outside of the subnet on which
           the client makes the request to the the server.

         The get-lease-hostnames statement

           get-lease-hostnames flag;

           The get-lease-hostnames statement is used to tell dhcpd whether or
           not to look up the domain name corresponding to the IP address of
           each address in the lease pool and use that address for the DHCP
           hostname option.  If flag is true, then this lookup is done for all
           addresses in the current scope.  By default, or if flag is false,
           no lookups are done.

         The hardware statement

           hardware hardware-type hardware-address;

           In order for a BOOTP client to be recognized, its network hardware
           address must be declared using a hardware clause in the host
           statement.  hardware-type must be the name of a physical hardware
           interface type.  Currently, only the ethernet and token-ring types
           are recognized, although support for a fddi hardware type (and
           others) would also be desirable.  The hardware-address should be a
           set of hexadecimal octets (numbers from 0 through ff) separated by
           colons.  The hardware statement may also be used for DHCP clients.

         The host-identifier option statement

           host-identifier option option-name option-data;

           or

           host-identifier v6relopt number option-name option-data;

           This identifies a DHCPv6 client in a host statement.  option-name
           is any option, and option-data is the value for the option that the
           client will send. The option-data must be a constant value.  In the
           v6relopts case the additional number is the relay to examine for
           the specified option name and value.  The values are the same as
           for the v6relay option.  0 is a no-op, 1 is the relay closest to
           the client, 2 the next one in and so on.  Values that are larger
           than the maximum number of relays (currently 32) indicate the relay
           closest to the server independent of number.

         The ignore-client-uids statement

           ignore-client-uids flag;

           If the ignore-client-uids statement is present and has a value of
           true or on, the UID for clients will not be recorded.  If this
           statement is not present or has a value of false or off, then
           client UIDs will be recorded.

         The infinite-is-reserved statement

           infinite-is-reserved flag;

           ISC DHCP now supports 'reserved' leases.  See the section on
           RESERVED LEASES below.  If this flag is on, the server will
           automatically reserve leases allocated to clients which requested
           an infinite (0xffffffff) lease-time.

           The default is off.

         The lease-file-name statement

           lease-file-name name;

           Name Where name is the name of the DHCP server's lease file. By
           default, this is DBDIR/dhcpd.leases.  This statement must appear in
           the outer scope of the configuration file - if it appears in some
           other scope, it will have no effect.  The value must be the
           absolute path of the file to use.  The order of precedence the
           server uses for the lease file name is:

               1. lease-file-name configuration file statement.
               2. -lf command line flag.
               3. PATH_DHCPD_DB environment variable.

         The dhcpv6-lease-file-name statement

           dhcpv6-lease-file-name name;

           Where name is the name of the DHCP server's lease file when the
           server is running DHCPv6. By default, this is DBDIR/dhcpd6.leases.
           This statement must appear in the outer scope of the configuration
           file - if it appears in some other scope, it will have no effect.
           The value must be the absolute path of the file to use.  The order
           of precedence the server uses for the lease file name is:

               1. dhcpv6-lease-file-name configuration file statement.
               2. -lf command line flag.
               3. PATH_DHCPD6_DB environment variable.

         The lease-id-format parameter

           lease-id-format format;

           The format parameter must be either octal or hex.  This parameter
           governs the format used to write certain values to lease files.
           With the default format, octal, values are written as quoted
           strings in which non-printable characters are represented as octal
           escapes - a backslash character followed by three octal digits.
           When the hex format is specified, values are written as an unquoted
           series of pairs of hexadecimal digits, separated by colons.

           Currently, the values written out based on lease-id-format are the
           server-duid, the uid (DHCPv4 leases), and the IAID_DUID (DHCPv6
           leases).  Note the server automatically reads the values in either
           format.

         The limit-addrs-per-ia statement

           limit-addrs-per-ia number;

           By default, the DHCPv6 server will limit clients to one IAADDR per
           IA option, meaning one address.  If you wish to permit clients to
           hang onto multiple addresses at a time, configure a larger number
           here.

           Note that there is no present method to configure the server to
           forcibly configure the client with one IP address per each subnet
           on a shared network.  This is left to future work.

         The local-port statement

           local-port port;

           This statement causes the DHCP server to listen for DHCP requests
           on the UDP port specified in port, rather than on port 67.

         The local-address statement

           local-address address;

           This statement causes the DHCP server to listen for DHCP requests
           sent to the specified address, rather than requests sent to all
           addresses.  Since serving directly attached DHCP clients implies
           that the server must respond to requests sent to the all-ones IP
           address, this option cannot be used if clients are on directly
           attached networks; it is only realistically useful for a server
           whose only clients are reached via unicasts, such as via DHCP relay
           agents.

           Note:  This statement is only effective if the server was compiled
           using the USE_SOCKETS #define statement, which is default on a
           small number of operating systems, and must be explicitly chosen at
           compile-time for all others.  You can be sure if your server is
           compiled with USE_SOCKETS if you see lines of this format at
           startup:

            Listening on Socket/eth0

           Note also that since this bind()s all DHCP sockets to the specified
           address, that only one address may be supported in a daemon at a
           given time.

         The local-address6 and bind-local-address6 statements

           local-address6 address;

           bind-local-address6 flag;

           The local-address6 statement causes the DHCP server to send IPv6
           packets as originating from the specified IPv6 address, rather than
           leaving the kernel to fill in the source address field.

           When bind-local-address6 is present and has a value of true or on,
           service sockets are bound to address too.

           By default address is the undefined address and the bind-local-
           address6 is disabled, both may only be set at the global scope.

         The log-facility statement

           log-facility facility;

           This statement causes the DHCP server to do all of its logging on
           the specified log facility once the dhcpd.conf file has been read.
           By default the DHCP server logs to the daemon facility.  Possible
           log facilities include auth, authpriv, cron, daemon, ftp, kern,
           lpr, mail, mark, news, ntp, security, syslog, user, uucp, and
           local0 through local7.  Not all of these facilities are available
           on all systems, and there may be other facilities available on
           other systems.

           In addition to setting this value, you may need to modify your
           syslog.conf file to configure logging of the DHCP server.  For
           example, you might add a line like this:

                local7.debug /var/log/dhcpd.log

           The syntax of the syslog.conf file may be different on some
           operating systems - consult the syslog.conf manual page to be sure.
           To get syslog to start logging to the new file, you must first
           create the file with correct ownership and permissions (usually,
           the same owner and permissions of your /var/log/messages or
           /usr/adm/messages file should be fine) and send a SIGHUP to
           syslogd.  Some systems support log rollover using a shell script or
           program called newsyslog or logrotate, and you may be able to
           configure this as well so that your log file doesn't grow
           uncontrollably.

           Because the log-facility setting is controlled by the dhcpd.conf
           file, log messages printed while parsing the dhcpd.conf file or
           before parsing it are logged to the default log facility.  To
           prevent this, see the README file included with this distribution,
           which describes BUG: where is that mentioned in README?  how to
           change the default log facility.  When this parameter is used, the
           DHCP server prints its startup message a second time after parsing
           the configuration file, so that the log will be as complete as
           possible.

         The log-threshold-high and log-threshold-low statements

           log-threshold-high percentage;

           log-threshold-low percentage;

           The log-threshold-low and log-threshold-high statements are used to
           control when a message is output about pool usage.  The value for
           both of them is the percentage of the pool in use.  If the high
           threshold is 0 or has not been specified, no messages will be
           produced.  If a high threshold is given, a message is output once
           the pool usage passes that level.  After that, no more messages
           will be output until the pool usage falls below the low threshold.
           If the low threshold is not given, it default to a value of zero.

           A special case occurs when the low threshold is set to be higer
           than the high threshold.  In this case, a message will be generated
           each time a lease is acknowledged when the pool usage is above the
           high threshold.

           Note that threshold logging will be automatically disabled for
           shared subnets whose total number of addresses is larger than
           (2^64)-1.  The server will emit a log statement at startup when
           threshold logging is disabled as shown below:

               "Threshold logging disabled for shared subnet of ranges:
           <addresses>"

           This is likely to have no practical runtime effect as CPUs are
           unlikely to support a server actually reaching such a large number
           of leases.

         The max-lease-time statement

           max-lease-time time;

           Time should be the maximum length in seconds that will be assigned
           to a lease.  If not defined, the default maximum lease time is
           86400.  The only exception to this is that Dynamic BOOTP lease
           lengths, which are not specified by the client, are not limited by
           this maximum.

         The min-lease-time statement

           min-lease-time time;

           Time should be the minimum length in seconds that will be assigned
           to a lease.  The default is the minimum of 300 seconds or max-
           lease-time.

         The min-secs statement

           min-secs seconds;

           Seconds should be the minimum number of seconds since a client
           began trying to acquire a new lease before the DHCP server will
           respond to its request.  The number of seconds is based on what the
           client reports, and the maximum value that the client can report is
           255 seconds.  Generally, setting this to one will result in the
           DHCP server not responding to the client's first request, but
           always responding to its second request.

           This can be used to set up a secondary DHCP server which never
           offers an address to a client until the primary server has been
           given a chance to do so.  If the primary server is down, the client
           will bind to the secondary server, but otherwise clients should
           always bind to the primary.  Note that this does not, by itself,
           permit a primary server and a secondary server to share a pool of
           dynamically-allocatable addresses.

         The next-server statement

           next-server server-name;

           The next-server statement is used to specify the host address of
           the server from which the initial boot file (specified in the
           filename statement) is to be loaded.  Server-name should be a
           numeric IP address or a domain name.

         The omapi-port statement

           omapi-port port;

           The omapi-port statement causes the DHCP server to listen for OMAPI
           connections on the specified port.  This statement is required to
           enable the OMAPI protocol, which is used to examine and modify the
           state of the DHCP server as it is running.

         The one-lease-per-client statement

           one-lease-per-client flag;

           If this flag is enabled, whenever a client sends a DHCPREQUEST for
           a particular lease, the server will automatically free any other
           leases the client holds.  This presumes that when the client sends
           a DHCPREQUEST, it has forgotten any lease not mentioned in the
           DHCPREQUEST - i.e., the client has only a single network interface
           and it does not remember leases it's holding on networks to which
           it is not currently attached.  Neither of these assumptions are
           guaranteed or provable, so we urge caution in the use of this
           statement.

         The persist-eui-64-leases statement

           persist-eui-64-leases flag;

           When this flag is enabled, the server will write EUI-64 based
           leases to the leases file. Since such leases can only, ever be
           valid for a single DUID value it can be argued that writing them to
           the leases file isn't essential and not doing so may have
           perfomance advantages.  See use-eui-64 statement for more details
           on EUI-64 based address allocation.  The flag is enabled by default
           and may only be set at the global scope.

         The pid-file-name statement

           pid-file-name name;

           Name should be the name of the DHCP server's process ID file.  This
           is the file in which the DHCP server's process ID is stored when
           the server starts.  By default, this is RUNDIR/dhcpd.pid.  Like the
           lease-file-name statement, this statement must appear in the outer
           scope of the configuration file. The order of precedence used by
           the server is:

               1. pid-file-name configuration file statement.
               2. -lf command line flag.
               3. PATH_DHCPD_PID environment variable.

           The dhcpv6-pid-file-name statement

             dhcpv6-pid-file-name name;

             Name is the name of the pid file to use if and only if the server
             is running in DHCPv6 mode.  By default, this is DBDIR/dhcpd6.pid.
             This statement, like pid-file-name, must appear in the outer
             scope of the configuration file.  The order of precedence used by
             the server is:

                 1. dhcpv6-pid-file-name configuration file statement.
                 2. -lf command line flag.
                 3. PATH_DHCPD6_PID environment variable.

           The ping-check statement

             ping-check flag;

             When the DHCP server is considering dynamically allocating an IP
             address to a client, it first sends an ICMP Echo request (a ping)
             to the address being assigned.  It waits for a second, and if no
             ICMP Echo response has been heard, it assigns the address.  If a
             response is heard, the lease is abandoned, and the server does
             not respond to the client.  The lease will remain abandoned for a
             minimum of abandon-lease-time seconds.

             If a there are no free addressses but there are abandoned IP
             addresses, the DHCP server will attempt to reclaim an abandoned
             IP address regardless of the value of abandon-lease-time.

             This ping check introduces a default one-second delay in
             responding to DHCPDISCOVER messages, which can be a problem for
             some clients.  The default delay of one second may be configured
             using the ping-timeout parameter.  The ping-check configuration
             parameter can be used to control checking - if its value is
             false, no ping check is done.

           The ping-cltt-secs statement

             ping-cltt-secs seconds;

             The server will conduct a ping check if all the following
             conditions are true:

             1. Ping checking is enabled.

             2. The server is responding to a DISCOVER.

             3. The lease to be offered is neither static nor active (i.e.
             still a valid lease).

             4. And any of the following are true:
                 a. This will be the first offer of this lease (CLTT is 0).
                 b. The lease is being offered to a client other than its
             previous owner
                 c. The lease is being offered to its previous owner and more
             than
                 ping-cltt-secs have elapsed since CLTT of the original lease.
                 d. The lease was abandoned and the server is attempting to
             reclaim it.


             The ping-cltt-secs statement allows the user to specify the
             amount of time that must elaspe after CLTT before a ping check
             will be conducted.  The default value is sixty seconds.

           The ping-timeout statement

             ping-timeout seconds;

             If the DHCP server determined it should send an ICMP echo request
             (a ping) because the ping-check statement is true, ping-timeout
             allows you to configure how many seconds the DHCP server should
             wait for an ICMP Echo response to be heard, if no ICMP Echo
             response has been received before the timeout expires, it assigns
             the address.  If a response is heard, the lease is abandoned, and
             the server does not respond to the client.  If no value is set,
             ping-timeout defaults to 1 second. (See also ping-timeout-ms
             below)

           The ping-timeout-ms statement

             ping-timeout-ms milliseconds;

             Allows you to specify the ping timeout in milliseconds rather
             than seconds.  If this value is greater than zero, the server
             will use it in place of ping-timeout.  The default value is zero.

           The preferred-lifetime statement

             preferred-lifetime seconds;

             IPv6 addresses have 'valid' and 'preferred' lifetimes.  The valid
             lifetime determines at what point at lease might be said to have
             expired, and is no longer useable.  A preferred lifetime is an
             advisory condition to help applications move off of the address
             and onto currently valid addresses (should there still be any
             open TCP sockets or similar).

             The preferred lifetime defaults to 5/8 the default lease time.

           The prefix-length-mode statement

             prefix-length-mode mode;

             According to RFC 3633, DHCPv6 clients may specify preferences
             when soliciting prefixes by including an IA_PD Prefix option
             within the IA_PD option. Among the preferences that may be
             conveyed is the "prefix-length". When non-zero it indicates a
             client's desired length for offered prefixes.  The RFC states
             that servers "MAY choose to use the information...to select
             prefix(es)" but does not specify any particular rules for doing
             so. The prefix-length-mode statement can be used to set the
             prefix selection rules employed by the server, when clients send
             a non-zero prefix-length value. The mode parameter must be one of
             ignore, prefer, exact, minimum, or maximum where:

             1. ignore - The requested length is ignored. The server will
             offer the first available prefix.

             2. prefer - The server will offer the first available prefix with
             the same length as the requested length.  If none are found then
             it will offer the first available prefix of any length.  This is
             the default behavior.

             3. exact - The server will offer the first available prefix with
             the same length as the requested length.  If none are found, it
             will return a status indicating no prefixes available.

             4. minimum - The server will offer the first available prefix
             with the same length as the requested length.  If none are found,
             it will return the first available prefix whose length is greater
             than (e.g. longer than), the requested value.  If none of those
             are found, it will return a status indicating no prefixes
             available.  For example, if client requests a length of /60, and
             the server has available prefixes of lengths /56 and /64, it will
             offer prefix of length /64.

             5. maximum - The server will offer the first available prefix
             with the same length as the requested length.  If none are found,
             it will return the first available prefix whose length is less
             than (e.g. shorter than), the requested value.  If none of those
             are found, it will return a status indicating no prefixes
             available.  For example, if client requests a length of /60, and
             the server has available prefixes of lengths /56 and /64, it will
             offer a prefix of length /56.

             In general "first available" is determined by the order in which
             pools are defined in the server's configuration.  For example, if
             a subnet is defined with three prefix pools A,B, and C:

             subnet 3000::/64 {
                  # pool A
                  pool6 {
                       :
                  }
                  # pool B
                  pool6 {
                       :
                  }
                  # pool C
                  pool6 {
                       :
                  }
             }

             then the pools will be checked in the order A, B, C. For modes
             prefer, minimum, and maximum this may mean checking the pools in
             that order twice.  A first pass through is made looking for an
             available prefix of exactly the preferred length.  If none are
             found, then a second pass is performed starting with pool A but
             with appropriately adjusted length criteria.

           The release-on-roam statement

             release-on-roam flag;

             When enabled and the dhcpd server detects that a DHCPv6 client
             (IAID+DUID) has roamed to a new network, it will release the pre-
             existing leases on the old network and emit a log statement
             similiar to the following:

                   "Client: <id> roamed to new network, releasing lease:
             <address>"

             The server will carry out all of the same steps that would
             normally occur when a client explicitly releases a lease.  When
             release-on-roam is disabled (the default) the server makes such
             leases unavailable until they expire or the server is restarted.
             Clients that need leases in multiple networks must supply a
             unique IAID in each IA.  This parameter may only be specified at
             the global level.

           The remote-port statement

             remote-port port;

             This statement causes the DHCP server to transmit DHCP responses
             to DHCP clients upon the UDP port specified in port, rather than
             on port 68.  In the event that the UDP response is transmitted to
             a DHCP Relay, the server generally uses the local-port
             configuration value.  Should the DHCP Relay happen to be
             addressed as 127.0.0.1, however, the DHCP Server transmits its
             response to the remote-port configuration value.  This is
             generally only useful for testing purposes, and this
             configuration value should generally not be used.

           The server-identifier statement

             server-identifier hostname;

             The server-identifier statement can be used to define the value
             that is sent in the DHCP Server Identifier option for a given
             scope.  The value specified must be an IP address for the DHCP
             server, and must be reachable by all clients served by a
             particular scope.

             The use of the server-identifier statement is not recommended -
             the only reason to use it is to force a value other than the
             default value to be sent on occasions where the default value
             would be incorrect.  The default value is the first IP address
             associated with the physical network interface on which the
             request arrived.

             The usual case where the server-identifier statement needs to be
             sent is when a physical interface has more than one IP address,
             and the one being sent by default isn't appropriate for some or
             all clients served by that interface.  Another common case is
             when an alias is defined for the purpose of having a consistent
             IP address for the DHCP server, and it is desired that the
             clients use this IP address when contacting the server.

             Supplying a value for the dhcp-server-identifier option is
             equivalent to using the server-identifier statement.

           The server-id-check statement

             server-id-check flag;

             The server-id-check statement is used to control whether or not a
             server, participating in failover, verifies that the value of the
             dhcp-server-identifier option in received DHCP REQUESTs match the
             server's id before processing the request. Server id checking is
             disabled by default.  Setting this flag enables id checking and
             thereafter the server will only process requests that match.
             Note the flag setting should be consistent between failover
             partners.

             Unless overridden by use of the server-identifier statement, the
             value the server uses as its id will be the first IP address
             associated with the physical network interface on which the
             request arrived.

             In order to reduce runtime overhead the server only checks for a
             server id option in the global and subnet scopes.  Complicated
             configurations may result in different server ids for this check
             and when the server id for a reply packet is determined, which
             would prohibit the server from responding.

             The primary use for this option is when a client broadcasts a
             request but requires that the response come from a specific
             failover peer.  An example of this would be when a client reboots
             while its lease is still active - in this case both servers will
             normally respond.  Most of the time the client won't check the
             server id and can use either of the responses.  However if the
             client does check the server id it may reject the response if it
             came from the wrong peer.  If the timing is such that the "wrong"
             peer responds first most of the time the client may not get an
             address for some time.

             Care should be taken before enabling this option.

           The server-duid statement

             server-duid LLT [ hardware-type timestamp hardware-address ] ;

             server-duid EN enterprise-number enterprise-identifier ;

             server-duid LL [ hardware-type hardware-address ] ;

             The server-duid statement configures the server DUID. You may
             pick either LLT (link local address plus time), EN (enterprise),
             or LL (link local).

             If you choose LLT or LL, you may specify the exact contents of
             the DUID.  Otherwise the server will generate a DUID of the
             specified type.

             If you choose EN, you must include the enterprise number and the
             enterprise-identifier.

             If there is a server-duid statement in the lease file it will
             take precedence over the server-duid statement from the config
             file and a dhcp6.server-id option in the config file will
             override both.

             The default server-duid type is LLT.

           The server-name statement

             server-name name ;

             The server-name statement can be used to inform the client of the
             name of the server from which it is booting.  Name should be the
             name that will be provided to the client.

           The dhcpv6-set-tee-times statement

             dhcpv6-set-tee-times flag;

             The dhcpv6-set-tee-times statement enables setting T1 and T2 to
             the values recommended in RFC 3315 (Section 22.4).  When setting
             T1 and T2, the server will use dhcp-renewal-time and dhcp-
             rebinding-time, respectively.  A value of zero tells the client
             it may choose its own value.

             When those options are not defined then values will be set to
             zero unless the global dhcpv6-set-tee-times is enabled.  When
             this option is enabled the times are calculated as recommended by
             RFC 3315, Section 22.4:

                   T1 will be set to 0.5 times the shortest preferred lifetime
                   in the reply.  If the "shortest" preferred lifetime is
                   0xFFFFFFFF,  T1 will set to 0xFFFFFFFF.

                   T2 will be set to 0.8 times the shortest preferred lifetime
                   in the reply.  If the "shortest" preferred lifetime is
                   0xFFFFFFFF,  T2 will set to 0xFFFFFFFF.

             Keep in mind that given sufficiently small lease lifetimes, the
             above calculations will result in the two values being equal. For
             example, a 9 second lease lifetime would yield T1 = T2 = 4
             seconds, which would cause clients to issue rebinds only.  In
             such a case it would likely be better to explicitly define the
             values.

             Note that dhcpv6-set-tee-times is intended to be transitional and
             will likely be removed in a future release. Once removed the
             behavior will be to use the configured values when present or
             calculate them per the RFC. If you want zeros, define them as
             zeros.

           The site-option-space statement

             site-option-space name ;

             The site-option-space statement can be used to determine from
             what option space site-local options will be taken.  This can be
             used in much the same way as the vendor-option-space statement.
             Site-local options in DHCP are those options whose numeric codes
             are greater than 224.  These options are intended for site-
             specific uses, but are frequently used by vendors of embedded
             hardware that contains DHCP clients.  Because site-specific
             options are allocated on an ad hoc basis, it is quite possible
             that one vendor's DHCP client might use the same option code that
             another vendor's client uses, for different purposes.  The site-
             option-space option can be used to assign a different set of
             site-specific options for each such vendor, using conditional
             evaluation (see dhcp-eval (5) for details).

           The stash-agent-options statement

             stash-agent-options flag;

             If the stash-agent-options parameter is true for a given client,
             the server will record the relay agent information options sent
             during the client's initial DHCPREQUEST message when the client
             was in the SELECTING state and behave as if those options are
             included in all subsequent DHCPREQUEST messages sent in the
             RENEWING state.  This works around a problem with relay agent
             information options, which is that they usually not appear in
             DHCPREQUEST messages sent by the client in the RENEWING state,
             because such messages are unicast directly to the server and not
             sent through a relay agent.

           The update-conflict-detection statement

             update-conflict-detection flag;

             If the update-conflict-detection parameter is true, the server
             will perform standard DHCID multiple-client, one-name conflict
             detection.  If the parameter has been set false, the server will
             skip this check and instead simply tear down any previous
             bindings to install the new binding without question.  The
             default is true and this parameter may only be specified at the
             global scope.

           The update-optimization statement

             update-optimization flag;

             If the update-optimization parameter is false for a given client,
             the server will attempt a DNS update for that client each time
             the client renews its lease, rather than only attempting an
             update when it appears to be necessary.  This will allow the DNS
             to heal from database inconsistencies more easily, but the cost
             is that the DHCP server must do many more DNS updates.  We
             recommend leaving this option enabled, which is the default. If
             this parameter is not specified, or is true, the DHCP server will
             only update when the client information changes, the client gets
             a different lease, or the client's lease expires.

           The update-static-leases statement

             update-static-leases flag;

             The update-static-leases flag, if enabled, causes the DHCP server
             to do DNS updates for clients even if those clients are being
             assigned their IP address using a fixed-address or fixed-address6
             statement - that is, the client is being given a static
             assignment.  It is not recommended because the DHCP server has no
             way to tell that the update has been done, and therefore will not
             delete the record when it is not in use.  Also, the server must
             attempt the update each time the client renews its lease, which
             could have a significant performance impact in environments that
             place heavy demands on the DHCP server.  This feature is
             supported for both DHCPv4 and DHCPv6, and update modes standard
             or interim. It is disabled by default.

           The use-eui-64 statement

             use-eui-64 flag;

             (Support for this must be enabled at compile time, see EUI_64 in
              includes/site.h)

             The use-eui-64 flag, if enabled, instructs the server to
             construct an address using the client's EUI-64 DUID (Type 3, HW
             Type EUI-64), rather than creating an address using the dynamic
             algorithm.  This means that a given DUID will always generate the
             same address for a given pool and further that the address is
             guaranteed to be unique to that DUID.  The IPv6 address will be
             calculated from the EUI-64 link layer address, conforming to RFC
             2373, unless there is a host declaration for the client-id.

             The range6 statement for EUI-64 must define full /64 bit ranges.
             Invalid ranges will be flagged during configuration parsing as
             errors.  See the following example:

                 subnet6 fc00:e4::/64 {
                     use-eui-64 true;
                     range6 fc00:e4::/64;
                 }

             The statement may be specified down to the pool level, allowing a
             mixture of dynamic and EUI-64 based pools.

             During lease file parsing, any leases which map to an EUI-64
             pool, that have a non-EUI-64 DUID or for which the lease address
             is not the EUI-64 address for that DUID in that pool, will be
             discarded.

             If a host declaration exists for the DUID, the server grants the
             address (fixed-prefix6, fixed-address6) according to the host
             declaration, regardless of the DUID type of the client (even for
             EUI-64 DUIDs).

             If a client request's an EUI-64 lease for a given network, and
             the resultant address conflicts with a fixed address reservation,
             the server will send the client a "no addresses available"
             response.

             Any client with a non-conforming DUID (not type 3 or not hw type
             EUI-64) that is not linked to a host declaration, which requests
             an address from an EUI-64 enabled pool will be ignored and the
             event will be logged.

             Pools that are configured for EUI-64 will be skipped for dynamic
             allocation.  If there are no pools in the shared network from
             which to allocate, the client will get back a no addresses
             available status.

             On an EUI-64 enabled pool, any client with a DUID 3, HW Type
             EUI-64, requesting a solicit/renew and including IA_NA that do
             not match the EUI-64 policy, they will be treated as though they
             are "outside" the subnet for a given client message:

                 Solicit - Server will advertise with EUI-64 ia suboption, but
             with rapid
                 commit off
                 Request - Server will send "an address not on link status",
             and no ia
                 suboption Renew/Rebind - Server will send the requested
             address ia
                 suboption with lifetimes of 0, plus an EUI-64 ia

             Whether or not  EUI-64 based leases are written out to the lease
             database may be controlled by persist-eui-64-leases statement.

           The use-host-decl-names statement

             use-host-decl-names flag;

             If the use-host-decl-names parameter is true in a given scope,
             then for every host declaration within that scope, the name
             provided for the host declaration will be supplied to the client
             as its hostname.  So, for example,

                 group {
                   use-host-decl-names on;

                   host joe {
                     hardware ethernet 08:00:2b:4c:29:32;
                     fixed-address joe.example.com;
                   }
                 }

             is equivalent to

                   host joe {
                     hardware ethernet 08:00:2b:4c:29:32;
                     fixed-address joe.example.com;
                     option host-name "joe";
                   }

             Additionally, enabling use-host-decl-names instructs the server
             to use the host declaration name in the the forward DNS name, if
             no other values are available.  This value selection process is
             discussed in more detail under DNS updates.

             An option host-name statement within a host declaration will
             override the use of the name in the host declaration.

             It should be noted here that most DHCP clients completely ignore
             the host-name option sent by the DHCP server, and there is no way
             to configure them not to do this.  So you generally have a choice
             of either not having any hostname to client IP address mapping
             that the client will recognize, or doing DNS updates.  It is
             beyond the scope of this document to describe how to make this
             determination.

           The use-lease-addr-for-default-route statement

             use-lease-addr-for-default-route flag;

             If the use-lease-addr-for-default-route parameter is true in a
             given scope, then instead of sending the value specified in the
             routers option (or sending no value at all), the IP address of
             the lease being assigned is sent to the client.  This supposedly
             causes Win95 machines to ARP for all IP addresses, which can be
             helpful if your router is configured for proxy ARP.  The use of
             this feature is not recommended, because it won't work for many
             DHCP clients.

           The vendor-option-space statement

             vendor-option-space string;

             The vendor-option-space parameter determines from what option
             space vendor options are taken.  The use of this configuration
             parameter is illustrated in the dhcp-options(5) manual page, in
             the VENDOR ENCAPSULATED OPTIONS section.

SETTING PARAMETER VALUES USING EXPRESSIONS
       Sometimes it's helpful to be able to set the value of a DHCP server
       parameter based on some value that the client has sent.  To do this,
       you can use expression evaluation.  The dhcp-eval(5) manual page
       describes how to write expressions.  To assign the result of an
       evaluation to an option, define the option as follows:

         my-parameter = expression ;

       For example:

         ddns-hostname = binary-to-ascii (16, 8, "-",
                                          substring (hardware, 1, 6));

RESERVED LEASES
       It's often useful to allocate a single address to a single client, in
       approximate perpetuity.  Host statements with fixed-address clauses
       exist to a certain extent to serve this purpose, but because host
       statements are intended to approximate 'static configuration', they
       suffer from not being referenced in a littany of other Server Services,
       such as dynamic DNS, failover, 'on events' and so forth.

       If a standard dynamic lease, as from any range statement, is marked
       'reserved', then the server will only allocate this lease to the client
       it is identified by (be that by client identifier or hardware address).

       In practice, this means that the lease follows the normal state engine,
       enters ACTIVE state when the client is bound to it, expires, or is
       released, and any events or services that would normally be supplied
       during these events are processed normally, as with any other dynamic
       lease.  The only difference is that failover servers treat reserved
       leases as special when they enter the FREE or BACKUP states - each
       server applies the lease into the state it may allocate from - and the
       leases are not placed on the queue for allocation to other clients.
       Instead they may only be 'found' by client identity.  The result is
       that the lease is only offered to the returning client.

       Care should probably be taken to ensure that the client only has one
       lease within a given subnet that it is identified by.

       Leases may be set 'reserved' either through OMAPI, or through the
       'infinite-is-reserved' configuration option (if this is applicable to
       your environment and mixture of clients).

       It should also be noted that leases marked 'reserved' are effectively
       treated the same as leases marked 'bootp'.

REFERENCE: OPTION STATEMENTS
       DHCP option statements are documented in the dhcp-options(5) manual
       page.

REFERENCE: EXPRESSIONS
       Expressions used in DHCP option statements and elsewhere are documented
       in the dhcp-eval(5) manual page.

SEE ALSO
       dhcpd(8), dhcpd.leases(5), dhcp-options(5), dhcp-eval(5), RFC2132,
       RFC2131.

AUTHOR
       dhcpd.conf(5) is maintained by ISC.  Information about Internet Systems
       Consortium can be found at https://www.isc.org.



                                                                 dhcpd.conf(5)