POSTSCREEN(8) POSTSCREEN(8)
NAME
postscreen - Postfix zombie blocker
SYNOPSIS
postscreen [generic Postfix daemon options]
DESCRIPTION
The Postfix postscreen(8) server performs triage on multi-
ple inbound SMTP connections at the same time. While a
single postscreen(8) process keeps spambots away from
Postfix SMTP server processes, more Postfix SMTP server
processes remain available for legitimate clients.
This program should not be used on SMTP ports that receive
mail from end-user clients (MUAs). In a typical deploy-
ment, postscreen(8) is used on the "port 25" service,
while MUA clients submit mail via the submission service,
or via a "port 25" server that provides no MX service
(i.e. a dedicated server that provides submission service
on port 25).
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 config-
ured 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 XFORWARD.
Support for AUTH may be added in the future. In the mean
time, if you need to make these services available on port
25, then do not enable the optional "after 220 server
greeting" tests, and do not use DNSBLs that reject traffic
from dial-up and residential networks.
The optional "after 220 server greeting" tests involve
postscreen(8)'s built-in SMTP protocol engine. When these
tests succeed, postscreen(8) adds the client to the tempo-
rary whitelist but it cannot not hand off the "live" con-
nection to a Postfix SMTP server process in the middle of
a session. Instead, postscreen(8) defers attempts to
deliver mail with a 4XX status, and waits for the client
to disconnect. The next time a good client connects, it
will be allowed to talk to a Postfix SMTP server process
to deliver mail. postscreen(8) mitigates the impact of
this limitation by giving such tests a long expiration
time.
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 ${stress?X}${stress:Y}, or it is the $name of
an smtpd parameter with a stress-dependent default).
Other parameters always evaluate as if the stress parame-
ter value is the empty string.
COMPATIBILITY CONTROLS
postscreen_command_filter ($smtpd_command_filter)
A mechanism to transform commands from remote SMTP
clients.
postscreen_discard_ehlo_keyword_address_maps ($smtpd_dis-
card_ehlo_keyword_address_maps)
Lookup tables, indexed by the remote SMTP client
address, with case insensitive lists of EHLO key-
words (pipelining, starttls, auth, etc.) that the
postscreen(8) server will not send in the EHLO
response to a remote SMTP client.
postscreen_discard_ehlo_keywords ($smtpd_discard_ehlo_key-
words)
A case insensitive list of EHLO keywords (pipelin-
ing, 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.
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
When a remote SMTP client is not on the permanent access
list, postscreen(8) can implement a number of whitelist
tests before it grants the client a temporary whitelist
status to talk to a Postfix SMTP server process.
By listening on both primary and backup MX addresses,
postscreen(8) can deny the temporary whitelist status to
clients that connect only to backup MX hosts.
postscreen_whitelist_interfaces (static:all)
A list of local postscreen(8) server IP addresses
where a non-whitelisted remote SMTP client can
obtain postscreen(8)'s temporary whitelist status.
BEFORE-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 mas-
ter.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_thresh-
old 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, fil-
ters 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 (${stress?2}${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)
List of commands that the postscreen(8) server con-
siders 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_direc-
tory/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
($smtpd_client_connection_count_limit)
How many simultaneous connections any remote SMTP
client is allowed to have with the postscreen(8)
daemon.
postscreen_command_count_limit (20)
The limit on the total number of commands per SMTP
session for postscreen(8)'s built-in SMTP protocol
engine.
postscreen_command_time_limit (${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 ser-
vice from a real Postfix SMTP server process.
postscreen_pre_queue_limit ($default_process_limit)
The number of non-whitelisted clients that can be
waiting for a decision whether they will receive
service from a real Postfix SMTP server process.
postscreen_watchdog_timeout (10s)
How much time a postscreen(8) process may take to
respond to a remote SMTP client command or to per-
form a cache operation before it is terminated by a
built-in watchdog timer.
STARTTLS CONTROLS
postscreen_tls_security_level ($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 mas-
ter.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 com-
mands.
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
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
POSTSCREEN(8)