The Postfix postscreen(8) server performs triage on
multiple 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 deployment,
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
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.
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.
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 temporary whitelist but it cannot not hand off the
"live" connection 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.
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.
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.
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.
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.
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.
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 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 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.
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.
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.
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 perform a cache operation before it
is terminated by a built-in watchdog timer.
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.