# Postfix Stress-Dependent Configuration

## Overview

This document describes the symptoms of Postfix SMTP server overload. It presents permanent main.cf changes to avoid overload during normal operation, and temporary main.cf changes to cope with an unexpected burst of mail. This document makes specific suggestions for Postfix 2.5 and later which support stress-adaptive behavior, and for earlier Postfix versions that don't.

Topics covered in this document:

- Symptoms of Postfix SMTP server overload
- Automatic stress-adaptive behavior
- Service more SMTP clients at the same time
- Spend less time per SMTP client
- Disconnect suspicious SMTP clients
- Temporary measures for older Postfix releases
- Detecting support for stress-adaptive behavior
- Forcing stress-adaptive behavior on or off
- Other measures to off-load zombies
- Credits

## Symptoms of Postfix SMTP server overload

Under normal conditions, the Postfix SMTP server responds immediately when an SMTP client connects to it; the time to deliver mail is noticeable only with large messages. Performance degrades dramatically when the number of SMTP clients exceeds the number of Postfix SMTP server processes. When an SMTP client connects while all Postfix SMTP server processes are busy, the client must wait until a server process becomes available.

SMTP server overload may be caused by a surge of legitimate mail (example: a DNS registrar opens a new zone for registrations), by mistake (mail explosion caused by a forwarding loop) or by malice (worm outbreak, botnet, or other illegitimate activity).

Symptoms of Postfix SMTP server overload are:

- Remote SMTP clients experience a long delay before Postfix sends the "220 hostname.example.com ESMTP Postfix" greeting.

  - NOTE: Broken DNS configurations can also cause lengthy delays before Postfix sends "220 hostname.example.com ...". These delays also exist when Postfix is NOT overloaded.

  - NOTE: To avoid "overload" delays for end-user mail clients, enable the "submission" service entry in master.cf (present since Postfix 2.1), and tell users to connect to this instead of the public SMTP service.

- The Postfix SMTP server logs an increased number of "lost connection after CONNECT" events. This happens because remote SMTP clients disconnect before Postfix answers the connection.

  - NOTE: A portscan for open SMTP ports can also result in "lost connection ..." logfile messages.

- Postfix 2.3 and later logs a warning that all server ports are busy:

      Oct  3 20:39:27 spike postfix/master[28905]: warning: service "smtp"
       (25) has reached its process limit "30": new clients may experience
       noticeable delays
      Oct  3 20:39:27 spike postfix/master[28905]: warning: to avoid this
       condition, increase the process count in master.cf or reduce the
       service time per client
      Oct  3 20:39:27 spike postfix/master[28905]: warning: see
        https://www.postfix.org/STRESS_README.html for examples of
        stress-adapting configuration settings

Legitimate mail that doesn't get through during an episode of Postfix SMTP server overload is not necessarily lost. It should still arrive once the situation returns to normal, as long as the overload condition is temporary.

## Automatic stress-adaptive behavior

Postfix version 2.5 introduces automatic stress-adaptive behavior. It works as follows. When a "public" network service such as the SMTP server runs into an "all server ports are busy" condition, the Postfix master(8) daemon logs a warning, restarts the service (without interrupting existing network sessions), and runs the service with "-o stress=yes" on the server process command line:

>     80821  ??  S      0:00.24 smtpd -n smtp -t inet -u -c -o stress=yes

Normally, the Postfix master(8) daemon runs such a service with "-o stress=" on the command line (i.e. with an empty parameter value):

>     83326  ??  S      0:00.28 smtpd -n smtp -t inet -u -c -o stress=

You won't see "-o stress" command-line parameters with services that have local clients only. These include services internal to Postfix such as the queue manager, and services that listen on a loopback interface only, such as after-filter SMTP services.

The "stress" parameter value is the key to making main.cf parameter settings stress adaptive. The following settings are the default with Postfix 2.6 and later.

>     1 smtpd_timeout = ${stress?{10}:{300}}s
>     2 smtpd_hard_error_limit = ${stress?{1}:{20}}
>     3 smtpd_junk_command_limit = ${stress?{1}:{100}}
>     4 # Parameters added after Postfix 2.6:
>     5 smtpd_per_record_deadline = ${stress?{yes}:{no}}
>     6 smtpd_starttls_timeout = ${stress?{10}:{300}}s
>     7 address_verify_poll_count = ${stress?{1}:{3}}

Postfix versions before 3.0 use the older form ${stress?x}${stress:y} instead of the newer form ${stress?{x}:{y}}.

The syntax of ${name?{value}:{value}}, ${name?value} and ${name:value} is explained at the beginning of the postconf(5) manual page.

Translation:

- Line 1: under conditions of stress, use an smtpd_timeout value of 10 seconds instead of the default 300 seconds. Experience on the postfix-users list from a variety of sysadmins shows that reducing the "normal" smtpd_timeout to 60s is unlikely to affect legitimate clients. However, it is unlikely to become the Postfix default because it's not RFC compliant. Setting smtpd_timeout to 10s or even 5s under stress will still allow most legitimate clients to connect and send mail, but may delay mail from some clients. No mail should be lost, as long as this measure is used only temporarily.

- Line 2: under conditions of stress, use an smtpd_hard_error_limit of 1 instead of the default 20. This disconnects clients after a single error, giving other clients a chance to connect. However, this may cause significant delays with legitimate mail, such as a mailing list that contains a few no-longer-active user names that didn't bother to unsubscribe. No mail should be lost, as long as this measure is used only temporarily.

- Line 3: under conditions of stress, use an smtpd_junk_command_limit of 1 instead of the default 100. This prevents clients from keeping connections open by repeatedly sending HELO, EHLO, NOOP, RSET, VRFY or ETRN commands.

- Line 5: under conditions of stress, change the behavior of smtpd_timeout and smtpd_starttls_timeout, from a time limit per read or write system call, to a time limit to send or receive a complete record (an SMTP command line, SMTP response line, SMTP message content line, or TLS protocol message).

- Line 6: under conditions of stress, reduce the time limit for TLS protocol handshake messages to 10 seconds, from the default value of 300 seconds. See also the smtpd_timeout discussion above.

- Line 7: under conditions of stress, do not wait up to 6 seconds for the completion of an address verification probe. If the result is not already in the address verification cache, reply immediately with $unverified_recipient_tempfail_action or $unverified_sender_tempfail_action. No mail should be lost, as long as this measure is used only temporarily.

NOTE: Please keep in mind that the stress-adaptive feature is a fairly desperate measure to keep some legitimate mail flowing under overload conditions. If a site is reaching the SMTP server process limit when there isn't an attack or bot flood occurring, then either the process limit needs to be raised or more hardware needs to be added.

## Service more SMTP clients at the same time

This section and the ones that follow discuss permanent measures against mail server overload.

One measure to avoid the "all server processes busy" condition is to service more SMTP clients simultaneously. For this you need to increase the number of Postfix SMTP server processes. This will improve the responsiveness for remote SMTP clients, as long as the server machine has enough hardware and software resources to run the additional processes, and as long as the file system can keep up with the additional load.

- You increase the number of SMTP server processes either by increasing the default_process_limit in main.cf (line 3 below), or by increasing the SMTP server's "maxproc" field in master.cf (line 10 below). Either way, you need to issue a "postfix reload" command to make the change effective.

- Process limits above 1000 require Postfix version 2.4 or later, and an operating system that supports kernel-based event filters (BSD kqueue(2), Linux epoll(4), or Solaris /dev/poll).

- More processes use more memory. You can reduce the Postfix memory footprint by using cdb: lookup tables instead of Berkeley DB's hash: or btree: tables.

       1 /etc/postfix/main.cf:
       2     # Raise the global process limit, 100 since Postfix 2.0.
       3     default_process_limit = 200
       4
       5 /etc/postfix/master.cf:
       6     # =============================================================
       7     # service type  private unpriv  chroot  wakeup  maxproc command
       8     # =============================================================
       9     # Raise the SMTP service process limit only.
      10     smtp      inet  n       -       n       -       200     smtpd

- NOTE: older versions of the SMTPD_POLICY_README document contain a mistake: they configure a fixed number of policy daemon processes. When you raise the SMTP server's "maxproc" field in master.cf, SMTP server processes will report problems when connecting to policy server processes, because there aren't enough of them. Examples of errors are "connection refused" or "operation timed out".

  To fix, edit master.cf and specify a zero "maxproc" field in all policy server entries; see line 6 in the example below. Issue a "postfix reload" command to make the change effective.

      1 /etc/postfix/master.cf:
      2     # =============================================================
      3     # service type  private unpriv  chroot  wakeup  maxproc command
      4     # =============================================================
      5     # Disable the policy service process limit.
      6     policy    unix  -       n       n       -       0       spawn
      7         user=nobody argv=/some/where/policy-server

## Spend less time per SMTP client

When increasing the number of SMTP server processes is not practical, you can improve Postfix server responsiveness by eliminating delays. When Postfix spends less time per SMTP session, the same number of SMTP server processes can service more clients in a given amount of time.

- Eliminate non-functional RBL lookups (blocklists that are no longer in operation). These lookups can degrade performance. Postfix logs a warning when an RBL server does not respond.

- Eliminate redundant RBL lookups (people often use multiple Spamhaus RBLs that include each other). To find out whether RBLs include other RBLs, look up the websites that document the RBL's policies.

- Eliminate header_checks and body_checks, and keep just a few emergency patterns to block the latest worm explosion or backscatter mail. See BACKSCATTER_README for examples of the latter.

- Group your header_checks and body_checks patterns to avoid unnecessary pattern matching operations:

       1  /etc/postfix/header_checks:
       2      if /^Subject:/
       3      /^Subject: virus found in mail from you/ reject
       4      /^Subject: ..other../ reject
       5      endif
       6
       7      if /^Received:/
       8      /^Received: from (postfix\.org) / reject forged client name in received header: $1
       9      /^Received: from ..other../ reject ....
      10      endif

## Disconnect suspicious SMTP clients

Under conditions of overload you can improve Postfix SMTP server responsiveness by hanging up on suspicious clients, so that other clients get a chance to talk to Postfix.

- Use "521" SMTP reply codes (Postfix 2.6 and later) or "421" (Postfix 2.3-2.5) to hang up on clients that that match botnet-related RBLs (see next bullet) or that match selected non-RBL restrictions such as SMTP access maps. The Postfix SMTP server will reject mail and disconnect without waiting for the remote SMTP client to send a QUIT command.

- To hang up connections from denylisted zombies, you can set specific Postfix SMTP server reject codes for specific RBLs, and for individual responses from specific RBLs. We'll use zen.spamhaus.org as an example; by the time you read this document, details may have changed. Right now, their documents say that a response of 127.0.0.10 or 127.0.0.11 indicates a dynamic client IP address, which means that the machine is probably running a bot of some kind. To give a 521 response instead of the default 554 response, use something like:

       1  /etc/postfix/main.cf:
       2      smtpd_client_restrictions =
       3         permit_mynetworks
       4         reject_rbl_client zen.spamhaus.org=127.0.0.10
       5         reject_rbl_client zen.spamhaus.org=127.0.0.11
       6         reject_rbl_client zen.spamhaus.org
       7
       8      rbl_reply_maps = hash:/etc/postfix/rbl_reply_maps
       9
      10  /etc/postfix/rbl_reply_maps:
      11      # With Postfix 2.3-2.5 use "421" to hang up connections.
      12      zen.spamhaus.org=127.0.0.10 521 4.7.1 Service unavailable;
      13       $rbl_class [$rbl_what] blocked using
      14       $rbl_domain${rbl_reason?; $rbl_reason}
      15
      16      zen.spamhaus.org=127.0.0.11 521 4.7.1 Service unavailable;
      17       $rbl_class [$rbl_what] blocked using
      18       $rbl_domain${rbl_reason?; $rbl_reason}

  Although the above example shows three RBL lookups (lines 4-6), Postfix will only do a single DNS query, so it does not affect the performance.

- With Postfix 2.3-2.5, use reply code 421 (521 will not cause Postfix to disconnect). The down-side of replying with 421 is that it works only for zombies and other malware. If the client is running a real MTA, then it may connect again several times until the mail expires in its queue. When this is a problem, stick with the default 554 reply, and use "smtpd_hard_error_limit = 1" as described below.

- You can automatically turn on the above overload measure with Postfix 2.5 and later, or with earlier releases that contain the stress-adaptive behavior source code patch from the mirrors listed at https://www.postfix.org/download.html. Simply replace line above 8 with:

       8      rbl_reply_maps = ${stress?hash:/etc/postfix/rbl_reply_maps}

More information about automatic stress-adaptive behavior is in section "Automatic stress-adaptive behavior".

## Temporary measures for older Postfix releases

See the section "Automatic stress-adaptive behavior" if you are running Postfix version 2.5 or later, or if you have applied the source code patch for stress-adaptive behavior from the mirrors listed at https://www.postfix.org/download.html.

The following measures can be applied temporarily during overload. They still allow most legitimate clients to connect and send mail, but may affect some legitimate clients.

- Reduce smtpd_timeout (default: 300s). Experience on the postfix-users list from a variety of sysadmins shows that reducing the "normal" smtpd_timeout to 60s is unlikely to affect legitimate clients. However, it is unlikely to become the Postfix default because it's not RFC compliant. Setting smtpd_timeout to 10s (line 2 below) or even 5s under stress will still allow most legitimate clients to connect and send mail, but may delay mail from some clients. No mail should be lost, as long as this measure is used only temporarily.

- Reduce smtpd_hard_error_limit (default: 20). Setting this to 1 under stress (line 3 below) helps by disconnecting clients after a single error, giving other clients a chance to connect. However, this may cause significant delays with legitimate mail, such as a mailing list that contains a few no-longer-active user names that didn't bother to unsubscribe. No mail should be lost, as long as this measure is used only temporarily.

- Use an smtpd_junk_command_limit of 1 instead of the default 100. This prevents clients from keeping idle connections open by repeatedly sending NOOP or RSET commands.

>     1  /etc/postfix/main.cf:
>     2      smtpd_timeout = 10
>     3      smtpd_hard_error_limit = 1
>     4      smtpd_junk_command_limit = 1

With these measures, no mail should be lost, as long as these measures are used only temporarily. The next section of this document introduces a way to automate this process.

## Detecting support for stress-adaptive behavior

To find out if your Postfix installation supports stress-adaptive behavior, use the "ps" command, and look for the smtpd processes. Postfix has stress-adaptive support when you see "-o stress=" or "-o stress=yes" command-line options. Remember that Postfix never enables stress-adaptive behavior on servers that listen on local addresses only.

The following example is for FreeBSD or Linux. On Solaris, HP-UX and other System-V flavors, use "ps -ef" instead of "ps ax".

>     $ ps ax|grep smtpd
>     83326  ??  S      0:00.28 smtpd -n smtp -t inet -u -c -o stress=
>     84345  ??  Ss     0:00.11 /usr/bin/perl /usr/libexec/postfix/smtpd-policy.pl

You can't use postconf(1) to detect stress-adaptive support. The postconf(1) command ignores the existence of the stress parameter in main.cf, because the parameter has no effect there. Command-line "-o parameter" settings always take precedence over main.cf parameter settings.

If you configure stress-adaptive behavior in main.cf when it isn't supported, nothing bad will happen. The processes will run as if the stress parameter always has an empty value.

## Forcing stress-adaptive behavior on or off

You can manually force stress-adaptive behavior on, by adding a "-o stress=yes" command-line option in master.cf. This can be useful for testing overrides on the SMTP service. Issue "postfix reload" to make the change effective.

Note: setting the stress parameter in main.cf has no effect for services that accept remote connections.

>     1 /etc/postfix/master.cf:
>     2     # =============================================================
>     3     # service type  private unpriv  chroot  wakeup  maxproc command
>     4     # =============================================================
>     5     #
>     6     smtp      inet  n       -       n       -       -       smtpd
>     7         -o stress=yes
>     8         -o . . .

To permanently force stress-adaptive behavior off with a specific service, specify "-o stress=" on its master.cf command line. This may be desirable for the "submission" service. Issue "postfix reload" to make the change effective.

Note: setting the stress parameter in main.cf has no effect for services that accept remote connections.

>     1 /etc/postfix/master.cf:
>     2     # =============================================================
>     3     # service type  private unpriv  chroot  wakeup  maxproc command
>     4     # =============================================================
>     5     #
>     6     submission inet n       -       n       -       -       smtpd
>     7         -o stress=
>     8         -o . . .

## Other measures to off-load zombies

The postscreen(8) daemon, introduced with Postfix 2.8, provides additional protection against mail server overload. 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 delays the onset of server overload conditions.

## Credits

- Thanks to the postfix-users mailing list members for sharing early experiences with the stress-adaptive feature.
- The RBL example and several other paragraphs of text were adapted from postfix-users postings by Noel Jones.
- Wietse implemented stress-adaptive behavior as the smallest possible patch while he should be working on other things.