Updated: 2022/Sep/29
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POSTSCREEN(8) System Manager's Manual POSTSCREEN(8)
NAME
postscreen - Postfix zombie blocker
SYNOPSIS
postscreen [generic Postfix daemon options]
DESCRIPTION
The Postfix postscreen(8) server provides additional protection against
mail server overload. One postscreen(8) process handles multiple
inbound SMTP connections, and decides which clients may talk to a
Postfix SMTP server process. By keeping spambots away, postscreen(8)
leaves more SMTP server processes available for legitimate clients, and
delays the onset of server overload conditions.
This program should not be used on SMTP ports that receive mail from
end-user clients (MUAs). In a typical deployment, postscreen(8) handles
the MX service on TCP port 25, and smtpd(8) receives mail from MUAs on
the submission service (TCP port 587) which requires client
authentication. Alternatively, a site could set up a dedicated,
non-postscreen, "port 25" server that provides submission service and
client authentication, but no MX service.
postscreen(8) maintains a temporary allowlist for clients that have
passed a number of tests. When an SMTP client IP address is
allowlisted, postscreen(8) hands off the connection immediately to a
Postfix SMTP server process. This minimizes the overhead for legitimate
mail.
By default, postscreen(8) logs statistics and hands off each connection
to a Postfix SMTP server process, while excluding clients in mynetworks
from all tests (primarily, to avoid problems with non-standard SMTP
implementations in network appliances). This default mode blocks no
clients, and is useful for non-destructive testing.
In a typical production setting, postscreen(8) is configured to reject
mail from clients that fail one or more tests. postscreen(8) logs
rejected mail with the client address, helo, sender and recipient
information.
postscreen(8) is not an SMTP proxy; this is intentional. The purpose
is to keep spambots away from Postfix SMTP server processes, while
minimizing overhead for legitimate traffic.
SECURITY
The postscreen(8) server is moderately security-sensitive. It talks to
untrusted clients on the network. The process can be run chrooted at
fixed low privilege.
STANDARDS
RFC 821 (SMTP protocol)
RFC 1123 (Host requirements)
RFC 1652 (8bit-MIME transport)
RFC 1869 (SMTP service extensions)
RFC 1870 (Message Size Declaration)
RFC 1985 (ETRN command)
RFC 2034 (SMTP Enhanced Status Codes)
RFC 2821 (SMTP protocol)
Not: RFC 2920 (SMTP Pipelining)
RFC 3030 (CHUNKING without BINARYMIME)
RFC 3207 (STARTTLS command)
RFC 3461 (SMTP DSN Extension)
RFC 3463 (Enhanced Status Codes)
RFC 5321 (SMTP protocol, including multi-line 220 banners)
DIAGNOSTICS
Problems and transactions are logged to syslogd(8) or postlogd(8).
BUGS
The postscreen(8) built-in SMTP protocol engine currently does not
announce support for AUTH, XCLIENT or XFORWARD. If you need to make
these services available on port 25, then do not enable the optional
"after 220 server greeting" tests.
The optional "after 220 server greeting" tests may result in unexpected
delivery delays from senders that retry email delivery from a different
IP address. Reason: after passing these tests a new client must
disconnect, and reconnect from the same IP address before it can
deliver mail. See POSTSCREEN_README, section "Tests after the 220 SMTP
server greeting", for a discussion.
CONFIGURATION PARAMETERS
Changes to main.cf are not picked up automatically, as postscreen(8)
processes may run for several hours. Use the command "postfix reload"
after a configuration change.
The text below provides only a parameter summary. See postconf(5) for
more details including examples.
NOTE: Some postscreen(8) parameters implement stress-dependent
behavior. This is supported only when the default parameter value is
stress-dependent (that is, it looks like ${stress?{X}:{Y}}, or it is
the $name of an smtpd parameter with a stress-dependent default).
Other parameters always evaluate as if the stress parameter value is
the empty string.
COMPATIBILITY CONTROLS
postscreen_command_filter ($smtpd_command_filter)
A mechanism to transform commands from remote SMTP clients.
postscreen_discard_ehlo_keyword_address_maps
($smtpd_discard_ehlo_keyword_address_maps)
Lookup tables, indexed by the remote SMTP client address, with
case insensitive lists of EHLO keywords (pipelining, starttls,
auth, etc.) that the postscreen(8) server will not send in the
EHLO response to a remote SMTP client.
postscreen_discard_ehlo_keywords ($smtpd_discard_ehlo_keywords)
A case insensitive list of EHLO keywords (pipelining, starttls,
auth, etc.) that the postscreen(8) server will not send in the
EHLO response to a remote SMTP client.
Available in Postfix version 3.1 and later:
dns_ncache_ttl_fix_enable (no)
Enable a workaround for future libc incompatibility.
Available in Postfix version 3.4 and later:
postscreen_reject_footer_maps ($smtpd_reject_footer_maps)
Optional lookup table for information that is appended after a
4XX or 5XX postscreen(8) server response.
Available in Postfix 3.6 and later:
respectful_logging (see 'postconf -d' output)
Avoid logging that implies white is better than black.
TROUBLE SHOOTING CONTROLS
postscreen_expansion_filter (see 'postconf -d' output)
List of characters that are permitted in
postscreen_reject_footer attribute expansions.
postscreen_reject_footer ($smtpd_reject_footer)
Optional information that is appended after a 4XX or 5XX
postscreen(8) server response.
soft_bounce (no)
Safety net to keep mail queued that would otherwise be returned
to the sender.
BEFORE-POSTSCREEN PROXY AGENT
Available in Postfix version 2.10 and later:
postscreen_upstream_proxy_protocol (empty)
The name of the proxy protocol used by an optional
before-postscreen proxy agent.
postscreen_upstream_proxy_timeout (5s)
The time limit for the proxy protocol specified with the
postscreen_upstream_proxy_protocol parameter.
PERMANENT ALLOW/DENYLIST TEST
This test is executed immediately after a remote SMTP client connects.
If a client is permanently allowlisted, the client will be handed off
immediately to a Postfix SMTP server process.
postscreen_access_list (permit_mynetworks)
Permanent allow/denylist for remote SMTP client IP addresses.
postscreen_blacklist_action (ignore)
Renamed to postscreen_denylist_action in Postfix 3.6.
MAIL EXCHANGER POLICY TESTS
When postscreen(8) is configured to monitor all primary and backup MX
addresses, it can refuse to allowlist clients that connect to a backup
MX address only. For small sites, this requires configuring primary and
backup MX addresses on the same MTA. Larger sites would have to share
the postscreen(8) cache between primary and backup MTAs, which would
introduce a common point of failure.
postscreen_whitelist_interfaces (static:all)
Renamed to postscreen_allowlist_interfaces in Postfix 3.6.
BEFORE 220 GREETING TESTS
These tests are executed before the remote SMTP client receives the
"220 servername" greeting. If no tests remain after the successful
completion of this phase, the client will be handed off immediately to
a Postfix SMTP server process.
dnsblog_service_name (dnsblog)
The name of the dnsblog(8) service entry in master.cf.
postscreen_dnsbl_action (ignore)
The action that postscreen(8) takes when a remote SMTP client's
combined DNSBL score is equal to or greater than a threshold (as
defined with the postscreen_dnsbl_sites and
postscreen_dnsbl_threshold parameters).
postscreen_dnsbl_reply_map (empty)
A mapping from actual DNSBL domain name which includes a secret
password, to the DNSBL domain name that postscreen will reply
with when it rejects mail.
postscreen_dnsbl_sites (empty)
Optional list of DNS allow/denylist domains, filters and weight
factors.
postscreen_dnsbl_threshold (1)
The inclusive lower bound for blocking a remote SMTP client,
based on its combined DNSBL score as defined with the
postscreen_dnsbl_sites parameter.
postscreen_greet_action (ignore)
The action that postscreen(8) takes when a remote SMTP client
speaks before its turn within the time specified with the
postscreen_greet_wait parameter.
postscreen_greet_banner ($smtpd_banner)
The text in the optional "220-text..." server response that
postscreen(8) sends ahead of the real Postfix SMTP server's "220
text..." response, in an attempt to confuse bad SMTP clients so
that they speak before their turn (pre-greet).
postscreen_greet_wait (normal: 6s, overload: 2s)
The amount of time that postscreen(8) will wait for an SMTP
client to send a command before its turn, and for DNS blocklist
lookup results to arrive (default: up to 2 seconds under stress,
up to 6 seconds otherwise).
smtpd_service_name (smtpd)
The internal service that postscreen(8) hands off allowed
connections to.
Available in Postfix version 2.11 and later:
postscreen_dnsbl_whitelist_threshold (0)
Renamed to postscreen_dnsbl_allowlist_threshold in Postfix 3.6.
Available in Postfix version 3.0 and later:
postscreen_dnsbl_timeout (10s)
The time limit for DNSBL or DNSWL lookups.
Available in Postfix version 3.6 and later:
postscreen_denylist_action (ignore)
The action that postscreen(8) takes when a remote SMTP client is
permanently denylisted with the postscreen_access_list
parameter.
postscreen_allowlist_interfaces (static:all)
A list of local postscreen(8) server IP addresses where a
non-allowlisted remote SMTP client can obtain postscreen(8)'s
temporary allowlist status.
postscreen_dnsbl_allowlist_threshold (0)
Allow a remote SMTP client to skip "before" and "after 220
greeting" protocol tests, based on its combined DNSBL score as
defined with the postscreen_dnsbl_sites parameter.
AFTER 220 GREETING TESTS
These tests are executed after the remote SMTP client receives the "220
servername" greeting. If a client passes all tests during this phase,
it will receive a 4XX response to all RCPT TO commands. After the
client reconnects, it will be allowed to talk directly to a Postfix
SMTP server process.
postscreen_bare_newline_action (ignore)
The action that postscreen(8) takes when a remote SMTP client
sends a bare newline character, that is, a newline not preceded
by carriage return.
postscreen_bare_newline_enable (no)
Enable "bare newline" SMTP protocol tests in the postscreen(8)
server.
postscreen_disable_vrfy_command ($disable_vrfy_command)
Disable the SMTP VRFY command in the postscreen(8) daemon.
postscreen_forbidden_commands ($smtpd_forbidden_commands)
List of commands that the postscreen(8) server considers in
violation of the SMTP protocol.
postscreen_helo_required ($smtpd_helo_required)
Require that a remote SMTP client sends HELO or EHLO before
commencing a MAIL transaction.
postscreen_non_smtp_command_action (drop)
The action that postscreen(8) takes when a remote SMTP client
sends non-SMTP commands as specified with the
postscreen_forbidden_commands parameter.
postscreen_non_smtp_command_enable (no)
Enable "non-SMTP command" tests in the postscreen(8) server.
postscreen_pipelining_action (enforce)
The action that postscreen(8) takes when a remote SMTP client
sends multiple commands instead of sending one command and
waiting for the server to respond.
postscreen_pipelining_enable (no)
Enable "pipelining" SMTP protocol tests in the postscreen(8)
server.
CACHE CONTROLS
postscreen_cache_cleanup_interval (12h)
The amount of time between postscreen(8) cache cleanup runs.
postscreen_cache_map (btree:$data_directory/postscreen_cache)
Persistent storage for the postscreen(8) server decisions.
postscreen_cache_retention_time (7d)
The amount of time that postscreen(8) will cache an expired
temporary allowlist entry before it is removed.
postscreen_bare_newline_ttl (30d)
The amount of time that postscreen(8) will use the result from a
successful "bare newline" SMTP protocol test.
postscreen_dnsbl_max_ttl
(${postscreen_dnsbl_ttl?{$postscreen_dnsbl_ttl}:{1}}h)
The maximum amount of time that postscreen(8) will use the
result from a successful DNS-based reputation test before a
client IP address is required to pass that test again.
postscreen_dnsbl_min_ttl (60s)
The minimum amount of time that postscreen(8) will use the
result from a successful DNS-based reputation test before a
client IP address is required to pass that test again.
postscreen_greet_ttl (1d)
The amount of time that postscreen(8) will use the result from a
successful PREGREET test.
postscreen_non_smtp_command_ttl (30d)
The amount of time that postscreen(8) will use the result from a
successful "non_smtp_command" SMTP protocol test.
postscreen_pipelining_ttl (30d)
The amount of time that postscreen(8) will use the result from a
successful "pipelining" SMTP protocol test.
RESOURCE CONTROLS
line_length_limit (2048)
Upon input, long lines are chopped up into pieces of at most
this length; upon delivery, long lines are reconstructed.
postscreen_client_connection_count_limit
($smtpd_client_connection_count_limit)
How many simultaneous connections any remote SMTP client is
allowed to have with the postscreen(8) daemon.
postscreen_command_count_limit (20)
The limit on the total number of commands per SMTP session for
postscreen(8)'s built-in SMTP protocol engine.
postscreen_command_time_limit (normal: 300s, overload: 10s)
The time limit to read an entire command line with
postscreen(8)'s built-in SMTP protocol engine.
postscreen_post_queue_limit ($default_process_limit)
The number of clients that can be waiting for service from a
real Postfix SMTP server process.
postscreen_pre_queue_limit ($default_process_limit)
The number of non-allowlisted clients that can be waiting for a
decision whether they will receive service from a real Postfix
SMTP server process.
postscreen_watchdog_timeout (10s)
How much time a postscreen(8) process may take to respond to a
remote SMTP client command or to perform a cache operation
before it is terminated by a built-in watchdog timer.
STARTTLS CONTROLS
postscreen_tls_security_level ($smtpd_tls_security_level)
The SMTP TLS security level for the postscreen(8) server; when a
non-empty value is specified, this overrides the obsolete
parameters postscreen_use_tls and postscreen_enforce_tls.
tlsproxy_service_name (tlsproxy)
The name of the tlsproxy(8) service entry in master.cf.
OBSOLETE STARTTLS SUPPORT CONTROLS
These parameters are supported for compatibility with smtpd(8) legacy
parameters.
postscreen_use_tls ($smtpd_use_tls)
Opportunistic TLS: announce STARTTLS support to remote SMTP
clients, but do not require that clients use TLS encryption.
postscreen_enforce_tls ($smtpd_enforce_tls)
Mandatory TLS: announce STARTTLS support to remote SMTP clients,
and require that clients use TLS encryption.
MISCELLANEOUS CONTROLS
config_directory (see 'postconf -d' output)
The default location of the Postfix main.cf and master.cf
configuration files.
delay_logging_resolution_limit (2)
The maximal number of digits after the decimal point when
logging sub-second delay values.
command_directory (see 'postconf -d' output)
The location of all postfix administrative commands.
max_idle (100s)
The maximum amount of time that an idle Postfix daemon process
waits for an incoming connection before terminating voluntarily.
process_id (read-only)
The process ID of a Postfix command or daemon process.
process_name (read-only)
The process name of a Postfix command or daemon process.
syslog_facility (mail)
The syslog facility of Postfix logging.
syslog_name (see 'postconf -d' output)
A prefix that is prepended to the process name in syslog
records, so that, for example, "smtpd" becomes "prefix/smtpd".
Available in Postfix 3.3 and later:
service_name (read-only)
The master.cf service name of a Postfix daemon process.
Available in Postfix 3.5 and later:
info_log_address_format (external)
The email address form that will be used in non-debug logging
(info, warning, etc.).
SEE ALSO
smtpd(8), Postfix SMTP server
tlsproxy(8), Postfix TLS proxy server
dnsblog(8), DNS allow/denylist logger
postlogd(8), Postfix logging
syslogd(8), system logging
README FILES
Use "postconf readme_directory" or "postconf html_directory" to locate
this information.
POSTSCREEN_README, Postfix Postscreen Howto
LICENSE
The Secure Mailer license must be distributed with this software.
HISTORY
This service was introduced with Postfix version 2.8.
Many ideas in postscreen(8) were explored in earlier work by Michael
Tokarev, in OpenBSD spamd, and in MailChannels Traffic Control.
AUTHOR(S)
Wietse Venema
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA
Wietse Venema
Google, Inc.
111 8th Avenue
New York, NY 10011, USA
POSTSCREEN(8)