Postfix SMTP relay and access control

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Introduction

The Postfix SMTP server receives mail from the network and is exposed to the
big bad world of junk email and viruses. This document introduces the built-in
and external methods that control what SMTP mail Postfix will accept, what
mistakes to avoid, and how to test your configuration.

Topics covered in this document:

  * Relay control, junk mail control, and per-user policies
  * Restrictions that apply to all SMTP mail
  * Getting selective with SMTP access restriction lists
  * Delayed evaluation of SMTP access restriction lists
  * Dangerous use of smtpd_recipient_restrictions
  * SMTP access rule testing

Relay control, junk mail control, and per-user policies

In a distant past, the Internet was a friendly environment. Mail servers
happily forwarded mail on behalf of anyone towards any destination. On today's
Internet, spammers abuse servers that forward mail from arbitrary systems, and
abused systems end up on anti-spammer blacklists. See, for example, the
information on http://www.mail-abuse.org/ and other websites.

By default, Postfix has a moderately restrictive approach to mail relaying.
Postfix forwards mail only from clients in trusted networks, or to domains that
are configured as authorized relay destinations. For a description of the
default policy, see the smtpd_recipient_restrictions parameter in the postconf
(5) manual page, and the information that is referenced from there.

Most of the Postfix SMTP server access controls are targeted at stopping junk
email.

  * Protocol oriented: some SMTP server access controls block mail by being
    very strict with respect to the SMTP protocol; these catch poorly
    implemented and/or poorly configured junk email software, as well as email
    worms that come with their own non-standard SMTP client implementations.
    Protocol-oriented access controls become less useful over time as spammers
    and worm writers learn to read RFC documents.

  * Blacklist oriented: some SMTP server access controls query blacklists with
    known to be bad sites such as open mail relays, open web proxies, and home
    computers that have been compromised and that are under remote control by
    criminals. The effectiveness of these blacklists depends on how complete
    and how up to date they are.

  * Threshold oriented: some SMTP server access controls attempt to raise the
    bar by either making the client do more work (greylisting) or by asking for
    a second opinion (SPF and sender/recipient address verification). The
    greylisting and SPF policies are implemented externally, and are the
    subject of the SMTPD_POLICY_README document. Sender/recipient address
    verification is the subject of the ADDRESS_VERIFICATION_README document.

Unfortunately, all junk mail controls have the possibility of falsely rejecting
legitimate mail. This can be a problem for sites with many different types of
users. For some users it is unacceptable when any junk email slips through,
while for other users the world comes to an end when a single legitimate email
message is blocked. Because there is no single policy that is "right" for all
users, Postfix supports different SMTP access restrictions for different users.
This is described in the RESTRICTION_CLASS_README document.

Restrictions that apply to all SMTP mail

Besides the restrictions that can be made configurable per client or per user
as described in the next section, Postfix implements a few restrictions that
apply to all SMTP mail.

  * The built-in header_checks and body_checks content restrictions, as
    described in the BUILTIN_FILTER_README document. This happens while Postfix
    receives mail, before it is stored in the incoming queue.

  * The external before-queue content restrictions, as described in the
    SMTPD_PROXY_README document. This happens while Postfix receives mail,
    before it is stored in the incoming queue.

  * Requiring that the client sends the HELO or EHLO command before sending the
    MAIL FROM or ETRN command. This may cause problems with home-grown
    applications that send mail. For this reason, the requirement is disabled
    by default ("smtpd_helo_required = no").

  * Disallowing illegal syntax in MAIL FROM or RCPT TO commands. This may cause
    problems with home-grown applications that send mail, and with ancient PC
    mail clients. For this reason, the requirement is disabled by default
    ("strict_rfc821_envelopes = no").

      o Disallowing RFC 822 address syntax (example: "MAIL FROM: the dude
        <dude@example.com>").

      o Disallowing addresses that are not enclosed with <> (example: "MAIL
        FROM: dude@example.com").

  * Rejecting mail from a non-existent sender address. This form of egress
    filtering helps to slow down worms and other malware, but may cause
    problems with home-grown software that sends out mail software with an
    unreplyable address. For this reason the requirement is disabled by default
    ("smtpd_reject_unlisted_sender = no").

  * Rejecting mail for a non-existent recipient address. This form of ingress
    filtering helps to keep the mail queue free of undeliverable MAILER-DAEMON
    messages. This requirement is enabled by default
    ("smtpd_reject_unlisted_recipient = yes").

Getting selective with SMTP access restriction lists

Postfix allows you to specify lists of access restrictions for each stage of
the SMTP conversation. Individual restrictions are described in the postconf(5)
manual page.

Examples of simple restriction lists are:

/etc/postfix/main.cf:
    # Allow connections from trusted networks only.
    smtpd_client_restrictions = permit_mynetworks, reject

    # Don't talk to mail systems that don't know their own hostname.
    smtpd_helo_restrictions = reject_unknown_hostname

    # Don't accept mail from domains that don't exist.
    smtpd_sender_restrictions = reject_unknown_sender_domain

    # Whitelisting: local clients may specify any destination. Others may not.
    smtpd_recipient_restrictions = permit_mynetworks, reject_unauth_destination

Each restriction list is evaluated from left to right until some restriction
produces a result of PERMIT, REJECT or DEFER (try again later). The end of the
list is equivalent to a PERMIT result. By placing a PERMIT restriction before a
REJECT restriction you can make exceptions for specific clients or users. This
is called whitelisting; the last example above allows mail from local networks
but otherwise rejects mail to arbitrary destinations.

The table below summarizes the purpose of each SMTP access restriction list.
All lists use the exact same syntax; they differ only in the time of evaluation
and in the effect of a REJECT or DEFER result.

                                                                           
    |Restriction list name       |Status  |Effect of REJECT or DEFER result|
    |                            |        |                                |
    |smtpd_client_restrictions   |Optional|Reject all client commands      |
    |                            |        |                                |
    |smtpd_helo_restrictions     |Optional|Reject HELO/EHLO information    |
    |                            |        |                                |
    |smtpd_sender_restrictions   |Optional|Reject MAIL FROM information    |
    |                            |        |                                |
    |smtpd_recipient_restrictions|Required|Reject RCPT TO information      |
    |                            |        |                                |
    |smtpd_data_restrictions     |Optional|Reject DATA command             |
    |                            |        |                                |
    |smtpd_etrn_restrictions     |Optional|Reject ETRN command             |
    |                            |        |                                |

Delayed evaluation of SMTP access restriction lists

Early Postfix versions evaluated SMTP access restrictions lists as early as
possible. The client restriction list was evaluated before Postfix sent the
"220 $myhostname..." greeting banner to the SMTP client, the helo restriction
list was evaluated before Postfix replied to the HELO (EHLO) command, the
sender restriction list was evaluated before Postfix replied to the MAIL FROM
command, and so on. This approach turned out to be difficult to use.

Current Postfix versions postpone the evaluation of client, helo and sender
restriction lists until the RCPT TO or ETRN command. This behavior is
controlled by the smtpd_delay_reject parameter. Restriction lists are still
evaluated in the proper order of (client, helo, etrn) or (client, helo, sender,
recipient, data) restrictions. When a restriction list (example: client)
evaluates to REJECT or DEFER the other restriction lists (example: helo,
sender, etc.) are skipped.

Around the time that smtpd_delay_reject was introduced, Postfix was also
changed to support mixed restriction lists that combine information about the
client, helo, sender and recipient or etrn command.

Benefits of delayed restriction evaluation, and of restriction mixing:

  * Some SMTP clients do not expect a negative reply early in the SMTP session.
    When the bad news is postponed until the RCPT TO reply, the client goes
    away as it is supposed to, instead of hanging around until a timeout
    happens, or worse, going into an endless connect-reject-connect loop.

  * Postfix can log more useful information. For example, when Postfix rejects
    a client name or address and delays the action until the RCPT TO command,
    it can log the sender and the recipient address. This is more useful than
    logging only the client hostname and IP address and not knowing whose mail
    was being blocked.

  * Mixing is needed for complex whitelisting policies. For example, in order
    to reject local sender addresses in mail from non-local clients, you need
    to be able to mix restrictions on client information with restrictions on
    sender information in the same restriction list. Without this ability, many
    per-user access restrictions would be impossible to express.

Dangerous use of smtpd_recipient_restrictions

By now the reader may wonder why we need smtpd client, helo or sender
restrictions, when their evaluation is postponed until the RCPT TO or ETRN
command. Some people recommend placing ALL the access restrictions in the
smtpd_recipient_restrictions list. Unfortunately, this can result in too
permissive access. How is this possible?

The purpose of the smtpd_recipient_restrictions feature is to control how
Postfix replies to the RCPT TO command. If the restriction list evaluates to
REJECT or DEFER, the recipient address is rejected; no surprises here. If the
result is PERMIT, then the recipient address is accepted. And this is where
surprises can happen.

Here is an example that shows when a PERMIT result can result in too much
access permission:

1 /etc/postfix/main.cf:
2     smtpd_recipient_restrictions =
3         permit_mynetworks
4         check_helo_access hash:/etc/postfix/helo_access
5         reject_unknown_hostname
6         reject_unauth_destination
7
8 /etc/postfix/helo_access:
9     localhost.localdomain PERMIT

Line 5 rejects mail from hosts that don't specify a proper hostname in the HELO
command. Lines 4 and 9 make an exception to allow mail from some machine that
announces itself with "HELO localhost.localdomain".

The problem with this configuration is that smtpd_recipient_restrictions
evaluates to PERMIT for EVERY host that announces itself as
"localhost.localdomain", making Postfix an open relay for all such hosts.

In order to avoid surprises like these with smtpd_recipient_restrictions, you
should place non-recipient restrictions AFTER the reject_unauth_destination
restriction, not before. In the above example, the HELO based restrictions
should be placed AFTER reject_unauth_destination, or better, the HELO based
restrictions should be placed under smtpd_helo_restrictions where they can do
no harm.

SMTP access rule testing

Postfix has several features that aid in SMTP access rule testing:

soft_bounce
    This is a safety net that changes SMTP server REJECT actions into DEFER
    (try again later) actions. This keeps mail queued that would otherwise be
    returned to the sender. Specify "soft_bounce = yes" in the main.cf file to
    prevent the Postfix SMTP server from rejecting mail permanently, by
    changing all 5xx SMTP reply codes into 4xx.

warn_if_reject
    This is a different safety net that changes SMTP server REJECT actions into
    warnings. Instead of rejecting a command, Postfix logs what it would
    reject. Specify "warn_if_reject" in an SMTP access restriction list, before
    the restriction that you want to test without actually rejecting mail.

XCLIENT
    With this Postfix 2.1 feature, authorized SMTP clients can impersonate
    other systems, so that you can do realistic SMTP access rule tests.
    Examples of how to impersonate other systems for access rule testing are
    given at the end of the XCLIENT_README document.

