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POSTSCREEN(8postfix) POSTSCREEN(8postfix)
NAME
postscreen - Postfix zombie blocker
SYNOPSIS
postscreen [generic Postfix daemon options]
DESCRIPTION
The Postfix postscreen(8) server provides additional protection against mail server over-
load. 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 de-
lays 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 connec-
tion immediately to a Postfix SMTP server process. This minimizes the overhead for legiti-
mate 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 greet-
ing", 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 includ-
ing examples.
NOTE: Some postscreen(8) parameters implement stress-dependent behavior. This is sup-
ported only when the default parameter value is stress-dependent (that is, it looks like
${stress?{X}:{Y}}, or it is the $name of an smtpd parameter with a stress-dependent de-
fault). Other parameters always evaluate as if the stress parameter value is the empty
string.
COMPATIBILITY CONTROLS
postscreen_command_filter ($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 ($smtpd_reject_footer_maps)
Optional lookup table for information that is appended after a 4XX or 5XX
postscreen(8) server response.
Available in Postfix 3.6 and later:
respectful_logging (see 'postconf -d' output)
Avoid logging that implies white is better than black.
TROUBLE SHOOTING CONTROLS
postscreen_expansion_filter (see 'postconf -d' output)
List of characters that are permitted in postscreen_reject_footer attribute expan-
sions.
postscreen_reject_footer ($smtpd_reject_footer)
Optional information that is appended after a 4XX or 5XX postscreen(8) server re-
sponse.
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_up-
stream_proxy_protocol parameter.
PERMANENT ALLOW/DENYLIST TEST
This test is executed immediately after a remote SMTP client connects. If a client is per-
manently 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 com-
mand 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" greet-
ing. 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 com-
mands 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 com-
mands 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:$data_directory/postscreen_cache)
Persistent storage for the postscreen(8) server decisions.
postscreen_cache_retention_time (7d)
The amount of time that postscreen(8) will cache an expired temporary allowlist en-
try before it is removed.
postscreen_bare_newline_ttl (30d)
The amount of time that postscreen(8) will use the result from a successful "bare
newline" SMTP protocol test.
postscreen_dnsbl_max_ttl (${postscreen_dnsbl_ttl?{$postscreen_dnsbl_ttl}:{1}}h)
The maximum amount of time that postscreen(8) will use the result from a successful
DNS-based reputation test before a client IP address is required to pass that test
again.
postscreen_dnsbl_min_ttl (60s)
The minimum amount of time that postscreen(8) will use the result from a successful
DNS-based reputation test before a client IP address is required to pass that test
again.
postscreen_greet_ttl (1d)
The amount of time that postscreen(8) will use the result from a successful PRE-
GREET test.
postscreen_non_smtp_command_ttl (30d)
The amount of time that postscreen(8) will use the result from a successful
"non_smtp_command" SMTP protocol test.
postscreen_pipelining_ttl (30d)
The amount of time that postscreen(8) will use the result from a successful "pipe-
lining" SMTP protocol test.
RESOURCE CONTROLS
line_length_limit (2048)
Upon input, long lines are chopped up into pieces of at most this length; upon de-
livery, long lines are reconstructed.
postscreen_client_connection_count_limit ($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 re-
quire 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 incom-
ing 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 ex-
ample, "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(8postfix)
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On Apache
Under GNU General Public License
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