dhcpd.conf(5)                                                    dhcpd.conf(5)

       dhcpd.conf - dhcpd configuration file

       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

       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 parame-
       ters  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  sub-
       net  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-sub-
       net 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

       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 dynami-
       cally assigned, class declarations and conditional declarations may  be
       used  to  group  parameter  assignments based on information the client

       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,  sub-
       net  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.

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

       global parameters...

       subnet netmask {
         subnet-specific parameters...

       subnet netmask {
         subnet-specific parameters...

       subnet netmask {
         subnet-specific parameters...

       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  parame-
       ters  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;

       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 cor-
       respond 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 parame-
       ters 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 ter-
       minals  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  decla-
       rations 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; }

       The  pool  declaration  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 decla-

       subnet netmask {
         option routers;

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

         # Known clients get this pool.
         pool {
           option domain-name-servers ns1.example.com, ns2.example.com;
           max-lease-time 28800;
           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.

       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

       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 incor-
       rect 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

       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.  In this case,  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 immedi-
       ately allocated to the client.  If the address is available for alloca-
       tion 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  par-
       ticular  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  pos-
       sible, and there is no way to configure this behavior with version 3 of
       the ISC DHCP server.

       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  state-
       ments, 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.

       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

       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

       The failover protocol defines a primary server  role  and  a  secondary
       server  role.   There  are some differences in how primaries and secon-
       daries act, but most of the differences simply have to do with  provid-
       ing  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 con-
       figured  as  secondary,  and  it  doesn't  matter too much which one is

       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  con-
       tact with its partner will have expired.

       While the failed server is recovering, its partner remains in the part-
       ner-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  com-
       municated  with their partner, they both come up in this recovery state
       and follow the procedure we have just described.  In this case, no ser-
       vice will be provided to DHCP clients until MCLT has expired.

       In  order  to  configure failover, you need to write a peer declaration
       that configures the failover protocol, and you need to write peer  ref-
       erences  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  seg-
       ment.    You must not tell one server it's doing failover on a particu-
       lar 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 dec-
       laration 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" {
         address anthrax.rc.vix.com;
         port 519;
         peer address trantor.rc.vix.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 identi-
         fier.  Because this value is used as an identifier,  it  may  not  be

       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 sec-
         onds 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 connec-
         tion  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 com-
         municating.  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 index;

         The split statement specifies the split between the primary and  sec-
         ondary for the purposes of load balancing.  Whenever a client makes a
         DHCP request, the DHCP server runs a hash on the  client  identifica-
         tion,  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 255, of which
         the most reasonable is 128.

       The hba statement

         hba colon-separated-hex-list;

         The hba statement specifies the split between the  primary  and  sec-
         ondary  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:

         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:

           hba 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  conse-
         quently  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 bal-
         ancing 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.

       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  dan-
         gerous  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 satu-
         ration...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  sec-
         ondary 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  giv-
         ing 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 dif-
         ference 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  state-
         ment.  This parameter configures what used to be a 10% fixed value in
         previous versions: if lts is less than free+backup  *  max-lease-mis-
         balance 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, how-
         ever, 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  sys-
         tem 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  mul-
         tiplied  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 bal-
         ance 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

         Finally,  the  min-balance and max-balance make certain that a sched-
         uled 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, inclu-
         sive, 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  recom-
         mended  to  select a max-lease-ownership value that is lower than the
         value selected for the max-lease-misbalance value.   max-lease-owner-
         ship 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).

       Clients can be separated into classes, and treated differently  depend-
       ing 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.

       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";

       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" {

       In  addition  to classes, it is possible to declare subclasses.  A sub-
       class is a class with the same name as a regular class, but with a spe-
       cific  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 netmask {
         pool {
           allow members of "allocation-class-1";
         pool {
           allow members of "allocation-class-2";

       The data following the class name in the subclass declaration is a con-
       stant  value  to  use  in  matching the match expression for the class.
       When class matching is done, the server will evaluate the match expres-
       sion  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 sub-

       Subclasses  can  be declared with or without scope.  In the above exam-
       ple, 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  con-
       figuration 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.

       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.

       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 pro-
       vide 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  match-
       ing  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.

       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 exam-

       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.

       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 com-
       pliant so any DNS server supporting RFC 2136 should be able  to  accept
       updates from the DHCP server.

       Two  DNS  update  schemes  are  currently  implemented,  and another is
       planned.  The two that are currently implemented  are  the  ad-hoc  DNS
       update mode and the interim DHCP-DNS interaction draft update mode.  In
       the future we plan to add a third mode which will be the  standard  DNS
       update  method based on the RFCS for DHCP-DNS interaction and DHCID The
       DHCP server must be configured to use one  of  the  two  currently-sup-
       ported  methods,  or  not to do dns updates.  This can be done with the
       ddns-update-style configuration parameter.

       The ad-hoc Dynamic DNS update scheme is now  deprecated  and  does  not
       work.   In future releases of the ISC DHCP server, this scheme will not
       likely be available.  The interim scheme works,  allows  for  failover,
       and  should  now  be  used.  The following description is left here for
       informational purposes only.

       The ad-hoc Dynamic DNS update scheme implemented in this version of the
       ISC  DHCP  server is a prototype design, which does not have much to do
       with the standard update method that is being standardized in the  IETF
       DHC  working  group, but rather implements some very basic, yet useful,
       update capabilities.  This mode does not work with the failover  proto-
       col  because  it  does not account for the possibility of two different
       DHCP servers updating the same set of DNS records.

       For the ad-hoc DNS update method, the client's FQDN is derived  in  two
       parts.   First,  the  hostname is determined.  Then, the domain name is
       determined, and appended to the hostname.

       The DHCP server determines the client's hostname by first looking for a
       ddns-hostname  configuration  option,  and using that if it is present.
       If no such option is present, the server looks for a valid hostname  in
       the  FQDN option sent by the client.  If one is found, it is used; oth-
       erwise, if the client sent a host-name option, that  is  used.   Other-
       wise,  if  there  is a host declaration that applies to the client, the
       name from that declaration will be used.  If none of these applies, the
       server will not have a hostname for the client, and will not be able to
       do a DNS update.

       The domain name is determined from  the  ddns-domainname  configuration
       option.  The default configuration for this option is:

         option server.ddns-domainname = config-option domain-name;

       So  if this configuration option is not configured to a different value
       (over-riding the above default), or if a  domain-name  option  has  not
       been  configured  for  the  client's  scope,  then  the server will not
       attempt to perform a DNS update.

       The client's fully-qualified domain name, derived as we have described,
       is  used  as  the  name  on  which an "A" record will be stored.  The A
       record will contain the IP address that the client was assigned in  its
       lease.   If  there is already an A record with the same name in the DNS
       server, no update of either the A or PTR records will occur - this pre-
       vents a client from claiming that its hostname is the name of some net-
       work  server.   For  example,  if  you   have   a   fileserver   called
       "fs.sneedville.edu", and the client claims its hostname is "fs", no DNS
       update will be done for that client,  and  an  error  message  will  be

       If  the  A record update succeeds, a PTR record update for the assigned
       IP address will be done, pointing to the  A  record.   This  update  is
       unconditional  - it will be done even if another PTR record of the same
       name exists.  Since the IP  address  has  been  assigned  to  the  DHCP
       server, this should be safe.

       Please note that the current implementation assumes clients only have a
       single network interface.  A client with two  network  interfaces  will
       see  unpredictable  behavior.   This  is  considered a bug, and will be
       fixed in a later release.  It may be helpful to enable  the  one-lease-
       per-client  parameter  so that roaming clients do not trigger this same

       The DHCP protocol normally involves a four-packet exchange - first  the
       client sends a DHCPDISCOVER message, then the server sends a DHCPOFFER,
       then the client sends a DHCPREQUEST, then the server sends  a  DHCPACK.
       In  the  current version of the server, the server will do a DNS update
       after it has received the DHCPREQUEST, and before it has sent the  DHC-
       PACK.   It  only  sends  the  DNS update if it has not sent one for the
       client's address before, in order to minimize the impact  on  the  DHCP

       When the client's lease expires, the DHCP server (if it is operating at
       the time, or when next it operates) will remove the client's A and  PTR
       records  from  the  DNS  database.  If the client releases its lease by
       sending a DHCPRELEASE message, the server will likewise  remove  the  A
       and PTR records.

       The  interim  DNS  update  scheme  operates mostly according to several
       drafts considered by the IETF.  While the drafts have since become RFCs
       the code was written before they were finalized and there are some dif-
       ferences between our code and the final RFCs.  We plan  to  update  our
       code,  probably adding a standard DNS update option, at some time.  The
       basic framework is similar with the main material difference being that
       a  DHCID  RR was assigned in the RFCs whereas our code continues to use
       an experimental TXT record.  The format  of  the  TXT  record  bears  a
       resemblance  to  the  DHCID  RR  but it is not equivalent (MD5 vs SHA1,
       field length differences etc).  The standard RFCs are:

                            RFC 4701 (updated by RF5494)
                                      RFC 4702
                                      RFC 4703

       And the corresponding drafts were:


       Because our implementation is slightly different than the standard,  we
       will briefly document the operation of this update style here.

       The  first  point  to understand about this style of DNS update is that
       unlike the ad-hoc style, 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 from either the fqdn option (if present)  or  the
       hostname  option (if present).  It will use its own domain name for the
       client, just as in the ad-hoc update scheme.  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.

       Also, if the use-host-decl-names configuration option is enabled,  then
       the  host  declaration's hostname will be used in place of the hostname
       option, and the same rules will apply as described above.

       The other difference between the ad-hoc scheme and the  interim  scheme
       is that with the interim scheme, a method is used that allows more than
       one DHCP server to update the DNS database without accidentally  delet-
       ing  A  records  that shouldn't be deleted nor failing to add A records
       that should be added.  The scheme works as follows:

       When the DHCP server issues a client a new lease,  it  creates  a  text
       string  that  is an MD5 hash over the DHCP client's identification (see
       draft-ietf-dnsext-dhcid-rr-??.txt for details).  The update adds  an  A
       record  with  the name the server chose and a TXT 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 TXT record in the same name as the new A record, and that TXT
       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  interim  DNS  update  scheme  is  called  interim for two reasons.
       First, it does not quite follow the RFCs.  The  RFCs  call  for  a  new
       DHCID RRtype while he interim DNS update scheme uses a TXT record.  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.

       In  addition to these differences, 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.

       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 previ-

       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  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. {
         key DHCP_UPDATER;

       zone 17.127.10.in-addr.arpa. {
         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 state-
       ment 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  cur-
       rently 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 8 and 9
       distributions  come  with  a  program for generating secret keys called
       dnssec-keygen.  The version that comes with BIND 9 is likely to produce
       a  substantially more random key, so we recommend you use that one even
       if you are not using BIND 9 as your DNS server.  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

       If  you  are  using the BIND 8 dnskeygen program, the following command
       will generate a key as seen above:

            dnskeygen -H 128 -u -c -n DHCP_UPDATER

       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.

       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.  Events are used to implement DNS updates, so you should not
       define your own event handlers if you are using the built-in DNS update

       The built-in version of the DNS update mechanism is in  a  text  string
       towards  the  top  of  server/dhcpd.c.   If  you want to use events for
       things other than DNS updates, and you also want DNS updates, you  will
       have  to  start  out by copying this code into your dhcpd.conf file and
       modifying it.

       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

       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  state-
       ment.   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  sub-
       net.  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  net-
       mask,  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 sub-

       The subnet6-number should be an IPv6 network identifier,  specified  as

       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 omit-

       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

       The  temporary  variant makes the prefix (by default on 64 bits) avail-
       able 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-

       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

       The host statement

        host hostname {
          [ parameters ]
          [ declarations ]

       The host declaration provides a scope in which to provide configuration
       information about a specific client, and also provides a way to  assign
       a  client a fixed address.  The host declaration provides a way for the
       DHCP server to identify a DHCP or BOOTP  client,  and  also  a  way  to
       assign the client a static IP address.

       If  it  is  desirable to be able to boot a DHCP or BOOTP client on more
       than one subnet with fixed 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.

       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 state-
       ments, 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 match-
       ing 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

       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-identi-
       fier 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.

       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  key-
       words  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 con-
       trol 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.

       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 dynam-
       ically 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

       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.  Unfortu-
       nately, a malicious or buggy client can,  using  DHCPDECLINE  messages,
       completely  exhaust the DHCP server's allocation pool.  The server will
       reclaim these leases, but while the client is running through the pool,
       it  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 DHCPDE-
       CLINE  messages.  If it is set to deny or ignore in a particular scope,
       the DHCP server will not respond to DHCPDECLINE messages.

       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.  This is
       only relevant when doing interim DNS updates.   See  the  documentation
       under the heading THE INTERIM DNS UPDATE SCHEME for details.

       The leasequery keyword

        allow leasequery;
        deny leasequery;

       The leasequery flag tells the DHCP server whether or not to answer DHC-
       PLEASEQUERY 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  pack-

       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  DHCPDIS-
       COVER  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

       When a DHCPREQUEST message is processed, address allocation simply con-
       sists  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:


       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.


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

        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

       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 sec-
         onds. 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 there-
         fore 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 config-
         ured  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


         not authoritative;

         The DHCP server will normally assume that the configuration  informa-
         tion  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 DHCP-
         NAK 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

         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 net-
         works 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 oth-

       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 dec-
         larations 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 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  set-
         ting up the client's A and PTR records.  If no ddns-hostname is spec-
         ified in scope, then the server will derive  the  hostname  automati-
         cally,  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-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,  then  the  reversed  IP  address  is  So a
         client with that IP address would, by default, be given a PTR  record

       The ddns-update-style parameter

         ddns-update-style style;

         The  style  parameter  must  be  one of ad-hoc, 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

       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  expi-
         ration  time.   This is used for both DHCPv4 and DHCPv6 leases (it is
         also known as the "valid lifetime" in DHCPv6).  The default is  43200

       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
         28.  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).

         Please  note  that  as  delayed-ack  is  currently  experimental, the
         delayed-ack feature is not  compiled  in  by  default,  but  must  be
         enabled at compile time with './configure --enable-delayed-ack'.

       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  and  ddns-update-style  is  set  to
         interim.   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 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 Uni-
         versal 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 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 state-
         ment 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 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  state-
         ment.   hardware-type  must be the name of a physical hardware inter-
         face 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;

         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.

       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 should be 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.  Furthermore, it has no effect if  overridden
         by the -lf flag or the PATH_DHCPD_DB environment variable.

       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 dhcpv6-lease-file-name statement

         dhcpv6-lease-file-name name;

         Name  is  the name of the lease file to use if and only if the server
         is running in DHCPv6 mode.  By default, this is  DBDIR/dhcpd6.leases.
         This  statement, like lease-file-name, must appear in the outer scope
         of the configuration file.  It has no effect if overridden by the -lf
         flag  or  the  PATH_DHCPD6_DB environment variable.  If dhcpv6-lease-
         file-name is not specified, but lease-file-name is, the latter  value
         will be used.

       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  net-
         works...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  com-
         pile-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 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  sys-

         In  addition  to setting this value, you may need to modify your sys-
         log.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  how to change the default log facility.  When this parame-
         ter is used, the DHCP server prints its startup message a second time
         after parsing the configuration file, so that the log will be as com-
         plete as possible.

       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-

       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 sec-
         onds.   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-allocat-
         able 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 DHCPRE-
         QUEST, 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 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.  It has no effect if  overridden  by
         the -pf flag or the 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.  It has no effect if overridden by  the
            -pf   flag   or  the  PATH_DHCPD6_PID  environment  variable.   If
            dhcpv6-pid-file-name is not specified, but pid-file-name  is,  the
            latter value will be used.

         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.

            This ping check introduces a default one-second delay in  respond-
            ing  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 parame-
            ter can be used to control checking - if its value  is  false,  no
            ping check is done.

         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.

         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 the renew+rebind timers, or 3/4
            the default lease time if none were specified.

         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 configura-
            tion value.  Should the DHCP  Relay  happen  to  be  addressed  as
  ,  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 gener-
            ally 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 particu-
            lar 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 equiva-
            lent to using the server-identifier statement.

         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

            If you choose EN, you must include the enterprise number  and  the

            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 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 allo-
            cated 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 RENEW-
            ING state.  This works around a problem with relay agent  informa-
            tion 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.

         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 leav-
            ing  this  option enabled, which is the default.  This option only
            affects the behavior of the interim DNS update scheme, and has  no
            effect  on the ad-hoc DNS update scheme.  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 statement  -  that
            is,  the client is being given a static assignment.  This can only
            work with the interim DNS update scheme.  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  perfor-
            mance  impact in environments that place heavy demands on the DHCP

         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 pro-
            vided 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.fugue.com;

            is equivalent to

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

            An option host-name statement within a host declaration will over-
            ride 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

         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.

       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 evalua-
       tion to an option, define the option as follows:

         my-parameter = expression ;

       For example:

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

       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'.

       DHCP  option  statements  are  documented in the dhcp-options(5) manual

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

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

       dhcpd.conf(5) was written by Ted Lemon  under  a  contract  with  Vixie
       Labs.   Funding  for this project was provided by Internet Systems Con-
       sortium.  Information about Internet Systems Consortium can be found at