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 Post‐
fix 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, while MUA clients submit mail via the submis‐
sion service on TCP port 587 which requires client authentication. Alternatively, a site could set up a dedi‐
cated, non-postscreen, "port 25" server that provides submission service and client authentication, but no MX
service.
postscreen(8) maintains a temporary whitelist for clients that have passed a number of tests. When an SMTP
client IP address is whitelisted, 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 every 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 mode 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 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).
BUGS
The postscreen(8) built-in SMTP protocol engine currently does not announce support for AUTH, XCLIENT or XFOR‐
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 ${stress?X}${stress: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.
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 WHITE/BLACKLIST TEST
This test is executed immediately after a remote SMTP client connects. If a client is permanently whitelisted,
the client will be handed off immediately to a Postfix SMTP server process.
postscreen_access_list (permit_mynetworks)
Permanent white/blacklist for remote SMTP client IP addresses.
postscreen_blacklist_action (ignore)
The action that postscreen(8) takes when a remote SMTP client is permanently blacklisted with the
postscreen_access_list parameter.
MAIL EXCHANGER POLICY TESTS
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 white/blacklist 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 Post‐
fix 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 (${stress?2}${stress:6}s)
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.
AFTER-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 RCPT TO commands until the client hangs
up. After this, the client 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)
postscreen_pipelining_action (enforce)
The action that postscreen(8) takes when a remote SMTP client sends multiple commands instead of send‐
ing 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:$data_directory/postscreen_cache)
Persistent storage for the postscreen(8) server decisions.
postscreen_cache_retention_time (7d)
The amount of time that postscreen(8) will cache an expired temporary whitelist entry before it is
removed.
postscreen_bare_newline_ttl (30d)
The amount of time that postscreen(8) will use the result from a successful "bare newline" SMTP proto‐
col test.
postscreen_dnsbl_ttl (1h)
The amount of time that postscreen(8) will use the result from a successful DNS blocklist test.
postscreen_greet_ttl (1d)
The amount of time that postscreen(8) will use the result from a successful PREGREET test.
postscreen_non_smtp_command_ttl (30d)
The amount of time that postscreen(8) will use the result from a successful "non_smtp_command" SMTP
protocol test.
postscreen_pipelining_ttl (30d)
The amount of time that postscreen(8) will use the result from a successful "pipelining" SMTP protocol
test.
RESOURCE CONTROLS
line_length_limit (2048)
Upon input, long lines are chopped up into pieces of at most this length; upon delivery, long lines are
reconstructed.
postscreen_client_connection_count_limit ($smtpd_client_connection_count_limit)
How many simultaneous connections any remote SMTP client is allowed to have with the postscreen(8) dae‐
mon.
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 (${stress?10}${stress:300}s)
The time limit to read an entire command line with postscreen(8)'s built-in SMTP protocol engine.
postscreen_post_queue_limit ($default_process_limit)
The number of clients that can be waiting for service from a real Postfix SMTP server process.
overrides the obsolete parameters postscreen_use_tls and postscreen_enforce_tls.
tlsproxy_service_name (tlsproxy)
The name of the tlsproxy(8) service entry in master.cf.
OBSOLETE STARTTLS SUPPORT CONTROLS
These parameters are supported for compatibility with smtpd(8) legacy parameters.
postscreen_use_tls ($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)
The mail system name that is prepended to the process name in syslog records, so that "smtpd" becomes,
for example, "postfix/smtpd".
SEE ALSO
smtpd(8), Postfix SMTP server
tlsproxy(8), Postfix TLS proxy server
dnsblog(8), DNS black/whitelist logger
syslogd(8), system logging
README FILES
Use "postconf readme_directory" or "postconf html_directory" to locate this information.
POSTSCREEN_README, Postfix Postscreen Howto
LICENSE
Yorktown Heights, NY 10598, USA
POSTSCREEN(8)
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