policyd-spf man page

policyd-spf — pure-Python Postfix policy daemon for SPF checking

Version

2.0.0

Usage

NOTE: Depending on the packaging and distribution, the exact path to the executable may vary.

$ policyd-spf (Start using installed config file)

$ policyd-spf -h (Display usage message)

$ policyd-spf /etc/policyd-spf/policyd-spf.conf (Config file name to use)

Configuration options are described in the sample configuration file provided with the package (policyd-spf.conf.commented) and in policyd-spf.conf(5).  The provided setup.py installs an uncommented configuration file in  /etc/policyd-spf/.

Additionally, whitelisting certain IP addresses or IP addresses used by listed domains from SPF checks is supported.  Skipping SPF checks for local submission or trusted relays is also provided.  The sample configuration file and policyd-spf.conf(5) shows the format to use.

Other Documentation

This documentation assumes you have read Postfix's README_FILES/ SMTPD_POLICY_README and are generally familiar with Sender Policy Framework (SPF).  See RFC 7208 for details.

See man 5 policyd-spf.conf for configuration file information.

man 5 policyd-spf.peruser provides documentation on setting up and using different configuration options on a per user (mail reciepient) basis.

Synopsis

python-policyd-spf is a Postfix SMTPd policy daemon for SPF checking. It is implemented in pure Python and uses the pyspf module.  The SPF web site is http://www.openspf.org/.  The Postfix configuration must be changed to check SPF.  

Description

Logging is sent to syslogd.

Each time a Postfix SMTP server process is started it connects to the policy service socket and Postfix runs one instance of this Python script.  By default, a Postfix SMTP server process terminates after 100 seconds of idle time, or after serving 100 clients.  Thus, the cost of starting this Python script is smoothed over time

The default policy_time_limit is 1000 seconds.  This may be too short for some SMTP transactions to complete.  As recommended in SMTPD_POLICY_README, this should be extended to 3600 seconds.  To do so, set "policy_time_limit = 3600" in /etc/postfix/main.cf.

Messages that get a Fail SPF result will be rejected.  Messages that get a  Permerror are, by default, treated as if they had no SPF record.  Messages that get a Temperror result are, by default, treated as if they had no SPF  record, but can (and probably should) be deferred if otherwise permitted.   Messages that get other SPF results (Pass, None, Neutral, Softfail) will have  the SPF Received header prepended.  Note: Spamasassisn 3.2 and follow will use this header for spam scoring so there is no need to configure a separate SPF check in these Spamassassin versions.  See Spamassassin documentation for  details.

Default Mail From rejection/deferal criteria are, by design, conservative. Default HELO check actions are to reject mail with other than Pass/None. HELO records are much simpler than Mail From records and rejecting based on HELO  checking does not present a false positive risk.  These settings are a matter of local policy and should be adjusted to meet the requirements of site administrators.  See policyd-spf.conf(5) for configuration file details.

Logging

Policyd-spf will log messages to syslog about it's activities.  The "debugLevel" value in "policyd-spf.conf" can be increased to get additional information to be logged.  When set to a value of "0", only test results (SPF hits/misses)  are logged.  Results will be returned to Postfix and logged as a warning by  Postfix also.  For logging by this policy server, look for "policyd-spf" in your mail log files.

Testing the Policy Daemon

Testing the policy daemon

To test the policy daemon by hand, execute:

   policyd-spf

Each query is a bunch of attributes.  Order does not matter, and the daemon uses only a few of all the attributes shown below:

   request=smtpd_access_policy
   protocol_state=RCPT
   protocol_name=SMTP
   helo_name=some.domain.tld
   queue_id=8045F2AB23
   instance=12345.6789
   sender=foo@bar.tld
   recipient=bar@foo.tld
   client_address=1.2.3.4
   client_name=another.domain.tld
   [empty line]

The policy daemon will answer in the same style, with an attribute list followed by a empty line:

   action=dunno
   [empty line]

Postfix Integration

1. Add the following to /etc/postfix/master.cf:

       policyd-spf  unix  -       n       n       -       0       spawn
           user=nobody argv=/usr/bin/policyd-spf

   NOTE: Check the path to both the installed Python 3 interpreter and
         policyd-spf.  These vary from system to system.  To use non-default
         settings, you must also add the config file (see above and
         policyd-spf.conf(5) for details).  If you run other services with
         user nobody, create a dedicated user for this policy server and use
         that instead.  Python and Python 3 versions prior to 3.3 are not
         supported.

2. Configure the Postfix policy service in /etc/postfix/main.cf:

       smtpd_recipient_restrictions =
           ...
           reject_unauth_destination
           check_policy_service unix:private/policyd-spf
           ...
       policyd-spf_time_limit = 3600

   NOTE:  Specify check_policy_service AFTER reject_unauth_destination or
          else your system can become an open relay.

3. Reload Postfix.

Performance Considerations

The time to complete DNS lookups associated with SPF checks is the most significant factor in policy server performance.  Use of a capable local caching resolver is highly recommended.  Specifics of resolver setup are outside the scope of this document.

Long waits for DNS answers can cause performance bottlenecks.  There are two primary controls, policyd-spf.conf(5) has details, provided to affect DNS timing.  Lookup_Time controls the overall time, in seconds, allowed for an SPF check to complete.  Whitelist_Lookup_Time controls the amount of time allowed for individual DNS lookups associated with name based whitelist options.

Lookup_Time defaults to the 20 second value recommended by RFC 7208.  This is a conservative recommendation and often a lower limit works quite well. Delays associated with whitelisting related DNS lookups can be avoided by using IP based options instead.

A shorter time increases the likelihood that the result for some messages will be a temporary error rather then the actual correct result.  To avoid some reported interoperability issues with some greylisting implementations, TempError_Defer defaults to False.  With TempError_Defer set to True, such TempError messages will be deferred and when retried, the responses should be in the local DNS resolver's cache.

The combination of a low Lookup_Time setting, TempError_Defer = True, and no DNS name based whitelisting will maximize SPF checking throughput with only mimimal delays.  Consider a value of Lookup_Time = 5 seconds and monitor the system mail logs.  If TempError results are rare or non-existent, the value can be lowered.  If such result are common (more than 1% of mail), then the value should be raised.

See Also

policyd-spf.conf(5), policyd-spf.peruser(5), python-spf, <http://www.openspf.org>, RFC 7208

Authors

This version of policyd-spf (python) was written by Copyright © 2007-2016 Scott Kitterman <scott@kitterman.com>.  It is derived from Tumgreyspf, written by Sean Reifschneider, tummy.com, ltd <jafo@tummy.com>. Portions of the documentation were written by Meng Weng Wong <mengwong@pobox.com>.

This man-page was created by Scott Kitterman <scott@kitterman.com>.  It is licensed under the same terms as the program.