Postfix before-queue Milter support

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Introduction

Postfix implements support for the Sendmail version 8 Milter (mail filter)
protocol. This protocol is used by applications that run outside the MTA to
inspect SMTP events (CONNECT, DISCONNECT), SMTP commands (HELO, MAIL FROM,
etc.) as well as mail content (headers and body). All this happens before mail
is queued.

The reason for adding Milter support to Postfix is that there exists a large
collection of applications, not only to block unwanted mail, but also to verify
authenticity (examples: DomainKeys Identified Mail (DKIM), SenderID+SPF and
DomainKeys) or to digitally sign mail (examples: DomainKeys Identified Mail
(DKIM), DomainKeys). Having yet another Postfix-specific version of all that
software is a poor use of human and system resources.

The Milter protocol has evolved over time, and different Postfix versions
implement different feature sets. See the workarounds and limitations sections
at the end of this document for differences between Postfix and Sendmail
implementations.

This document provides information on the following topics:

  * How Milter applications plug into Postfix
  * Building Milter applications
  * Running Milter applications
  * Configuring Postfix
  * Workarounds
  * Limitations

How Milter applications plug into Postfix

The Postfix Milter implementation uses two different lists of mail filters: one
list of filters for SMTP mail only, and one list of filters for non-SMTP mail.
The two lists have different capabilities, which is unfortunate. Avoiding this
would require major restructuring of Postfix.

  * The SMTP-only filters handle mail that arrives via the Postfix smtpd(8)
    server. They are typically used to filter unwanted mail and to sign mail
    from authorized SMTP clients. You specify SMTP-only Milter applications
    with the smtpd_milters parameter as described in a later section. Mail that
    arrives via the Postfix smtpd(8) server is not filtered by the non-SMTP
    filters that are described next.

  * The non-SMTP filters handle mail that arrives via the Postfix sendmail(1)
    command-line or via the Postfix qmqpd(8) server. They are typically used to
    digitally sign mail only. Although non-SMTP filters can be used to filter
    unwanted mail, they have limitations compared to the SMTP-only filters. You
    specify non-SMTP Milter applications with the non_smtpd_milters parameter
    as described in a later section.

For those who are familiar with the Postfix architecture, the figure below
shows how Milter applications plug into Postfix. Names followed by a number are
Postfix commands or server programs, while unnumbered names inside shaded areas
represent Postfix queues. To avoid clutter, the path for local submission is
simplified (the OVERVIEW document has a more complete description of the
Postfix architecture).

               SMTP-only      non-SMTP
                 filters       filters

                     ^ |
                     | v
                                   ^ |
                                   | |
    Network ->   smtpd(8)          | |
                                   | v

                           \

    Network ->   qmqpd(8)  -> cleanup(8) -> incoming

                           /

                pickup(8)

                      :

    Local   -> sendmail(1)

Building Milter applications

Milter applications have been written in C, JAVA and Perl, but this document
deals with C applications only. For these, you need an object library that
implements the Sendmail 8 Milter protocol. Postfix currently does not provide
such a library, but Sendmail does.

  * The first option is to use a pre-compiled library. Some systems install the
    Sendmail libmilter library by default. With other systems, libmilter may be
    provided by a package (called "sendmail-devel" on some Linux systems).

    Once libmilter is installed, applications such as dkim-milter and sid-
    milter build out of the box without requiring any tinkering:

        $ gzcat dkim-milter-x.y.z.tar.gz | tar xf -
        $ cd dkim-milter-x.y.z
        $ make
        [...lots of output omitted...]

  * The other option is to build the libmilter library from Sendmail source
    code:

        $ gzcat sendmail-x.y.z.tar.gz | tar xf -
        $ cd sendmail-x.y.z/libmilter
        $ make
        [...lots of output omitted...]

    After building your own libmilter library, follow the installation
    instructions in the Milter application source distribution to specify the
    location of the libmilter include files and object library. Typically,
    these settings are configured in a file named sid-filter/Makefile.m4 or
    similar:

        APPENDDEF(`confINCDIRS', `-I/some/where/sendmail-x.y.z/include')
        APPENDDEF(`confLIBDIRS', `-L/some/where/sendmail-x.y.z/obj.systemtype/
        libmilter')

    Then build the Milter application.

Running Milter applications

To run a Milter application, see the documentation of the filter for options. A
typical command looks like this:

    # /some/where/dkim-filter -u userid -p inet:portnumber@localhost ...other
    options...

Please specify a userid value that isn't used for other applications (not
"postfix", not "www", etc.).

Configuring Postfix

Like Sendmail, Postfix has a lot of configuration options that control how it
talks to Milter applications. With the initial Postfix Milter protocol
implementation, many options are global, that is, they apply to all Milter
applications. Future Postfix versions may support per-Milter timeouts, per-
Milter error handling, etc.

Information in this section:

  * SMTP-Only Milter applications
  * Non-SMTP Milter applications
  * Milter error handling
  * Milter protocol version
  * Milter protocol timeouts
  * Sendmail macro emulation

SMTP-Only Milter applications

The SMTP-only Milter applications handle mail that arrives via the Postfix
smtpd(8) server. They are typically used to filter unwanted mail, and to sign
mail from authorized SMTP clients. Mail that arrives via the Postfix smtpd(8)
server is not filtered by the non-SMTP filters that are described in the next
section.

NOTE: Do not use the header_checks(5) IGNORE action to remove Postfix's own
Received: message header. This causes problems with mail signing filters.
Instead, keep Postfix's own Received: message header and use the header_checks
(5) REPLACE action to sanitize information.

You specify SMTP-only Milter applications (there can be more than one) with the
smtpd_milters parameter. Each Milter application is identified by the name of
its listening socket; other Milter configuration options will be discussed in
later sections. Milter applications are applied in the order as specified, and
the first Milter application that rejects a command will override the responses
from other Milter applications.

    /etc/postfix/main.cf:
        # Milters for mail that arrives via the smtpd(8) server.
        # See below for socket address syntax.
        smtpd_milters = inet:localhost:portnumber ...other filters...

The general syntax for listening sockets is as follows:

    unix:pathname
        Connect to the local UNIX-domain server that is bound to the specified
        pathname. If the smtpd(8) or cleanup(8) process runs chrooted, an
        absolute pathname is interpreted relative to the Postfix queue
        directory.

    inet:host:port
        Connect to the specified TCP port on the specified local or remote
        host. The host and port can be specified in numeric or symbolic form.

        NOTE: Postfix syntax differs from Milter syntax which has the form
        inet:port@host.

Non-SMTP Milter applications

The non-SMTP Milter applications handle mail that arrives via the Postfix
sendmail(1) command-line or via the Postfix qmqpd(8) server. They are typically
used to digitally sign mail. Although non-SMTP filters can be used to filter
unwanted mail, there are limitations as discussed later in this section. Mail
that arrives via the Postfix smtpd(8) server is not filtered by the non-SMTP
filters.

NOTE: Do not use the header_checks(5) IGNORE action to remove Postfix's own
Received: message header. This causes problems with mail signing filters.
Instead, keep Postfix's own Received: message header and use the header_checks
(5) REPLACE action to sanitize information.

You specify non-SMTP Milter applications with the non_smtpd_milters parameter.
This parameter uses the same syntax as the smtpd_milters parameter in the
previous section. As with the SMTP-only filters, you can specify more than one
Milter application; they are applied in the order as specified, and the first
Milter application that rejects a command will override the responses from the
other applications.

    /etc/postfix/main.cf:
        # Milters for non-SMTP mail.
        # See below for socket address syntax.
        non_smtpd_milters = inet:localhost:portnumber ...other filters...

There's one small complication when using Milter applications for non-SMTP
mail: there is no SMTP session. To keep Milter applications happy, the Postfix
cleanup(8) server actually has to simulate the SMTP client CONNECT and
DISCONNECT events, and the SMTP client EHLO, MAIL FROM, RCPT TO and DATA
commands.

  * When new mail arrives via the sendmail(1) command line, the Postfix cleanup
    (8) server pretends that the mail arrives with ESMTP from "localhost" with
    IP address "127.0.0.1". The result is very similar to what happens with
    command line submissions in Sendmail version 8.12 and later, although
    Sendmail uses a different mechanism to achieve this result.

  * When new mail arrives via the qmqpd(8) server, the Postfix cleanup(8)
    server pretends that the mail arrives with ESMTP, and uses the QMQPD client
    hostname and IP address.

  * When old mail is re-injected into the queue with "postsuper -r", the
    Postfix cleanup(8) server uses the same client information that was used
    when the mail arrived as new mail.

This generally works as expected, with only one exception: non-SMTP filters
must not REJECT or TEMPFAIL simulated RCPT TO commands. When a
non_smtpd_milters application REJECTs or TEMPFAILs a recipient, Postfix will
report a configuration error, and mail will stay in the queue.

None of this is a problem for mail filters that digitally sign mail.

Milter error handling

The milter_default_action parameter specifies how Postfix handles Milter
application errors. The default action is to respond with a temporary error
status, so that the client will try again later. Specify "accept" if you want
to receive mail as if the filter does not exist, and "reject" to reject mail
with a permanent status. The "quarantine" action is like "accept" but freezes
the message in the "hold" queue, and is available with Postfix 2.6 or later.

    /etc/postfix/main.cf:
        # What to do in case of errors? Specify accept, reject, tempfail,
        # or quarantine (Postfix 2.6 or later).
        milter_default_action = tempfail

Milter protocol version

As Postfix is not built with the Sendmail libmilter library, you may need to
configure the Milter protocol version that Postfix should use. The default
version is 6 (before Postfix 2.6 the default version is 2).

    /etc/postfix/main.cf:
        # Postfix >= 2.6
        milter_protocol = 6
        # 2.3 <= Postfix <= 2.5
        milter_protocol = 2

If the Postfix milter_protocol setting specifies a too low version, the
libmilter library will log an error message like this:

    application name: st_optionneg[xxxxx]: 0xyy does not fulfill action
    requirements 0xzz

The remedy is to increase the Postfix milter_protocol version number. See,
however, the limitations section below for features that aren't supported by
Postfix.

With Postfix 2.7 and earlier, if the Postfix milter_protocol setting specifies
a too high version, the libmilter library simply hangs up without logging a
warning, and you see a Postfix warning message like one of the following:

    warning: milter inet:host:port: can't read packet header: Unknown error : 0
    warning: milter inet:host:port: can't read packet header: Success
    warning: milter inet:host:port: can't read SMFIC_DATA reply packet header:
    No such file or directory

The remedy is to lower the Postfix milter_protocol version number. Postfix 2.8
and later will automatically turn off protocol features that the application's
libmilter library does not expect.

Milter protocol timeouts

Postfix uses different time limits at different Milter protocol stages. The
table shows the timeout settings and the corresponding protocol stages (EOH =
end of headers; EOM = end of message).

                                                                      
    |Postfix parameter     |Time limit|Milter protocol stage          |
    |                      |          |                               |
    |milter_connect_timeout|30s       |CONNECT                        |
    |                      |          |                               |
    |milter_command_timeout|30s       |HELO, MAIL, RCPT, DATA, UNKNOWN|
    |                      |          |                               |
    |milter_content_timeout|300s      |HEADER, EOH, BODY, EOM         |
    |                      |          |                               |

Beware: 30s may be too short for Milter applications that do lots of DNS
lookups. However, if you increase the above timeouts too much, remote SMTP
clients may hang up and mail may be delivered multiple times. This is an
inherent problem with before-queue filtering.

Sendmail macro emulation

Postfix emulates a limited number of Sendmail macros, as shown in the table.
Some macro values depend on whether a recipient is rejected (rejected
recipients are available on request by the Milter application). Different
macros are available at different Milter protocol stages (EOH = end-of-header,
EOM = end-of-message); their availability is not always the same as in
Sendmail. See the workarounds section below for solutions.

                                                                              
    |Sendmail macro      |Milter protocol stage    |Description               |
    |                    |                         |                          |
    |i                   |DATA, EOH, EOM           |Queue ID, also Postfix    |
    |                    |                         |queue file name           |
    |                    |                         |                          |
    |j                   |Always                   |Value of myhostname       |
    |                    |                         |                          |
    |_                   |Always                   |The validated client name |
    |                    |                         |and address               |
    |                    |                         |                          |
    |{auth_authen}       |MAIL, DATA, EOH, EOM     |SASL login name           |
    |                    |                         |                          |
    |{auth_author}       |MAIL, DATA, EOH, EOM     |SASL sender               |
    |                    |                         |                          |
    |{auth_type}         |MAIL, DATA, EOH, EOM     |SASL login method         |
    |                    |                         |                          |
    |{client_addr}       |Always                   |Client IP address         |
    |                    |                         |                          |
    |                    |                         |Connection concurrency for|
    |                    |                         |this client (zero if the  |
    |{client_connections}|CONNECT                  |client is excluded from   |
    |                    |                         |all smtpd_client_*        |
    |                    |                         |limits).                  |
    |                    |                         |                          |
    |                    |                         |Client hostname           |
    |                    |                         |When address -> name      |
    |{client_name}       |Always                   |lookup or name -> address |
    |                    |                         |verification fails:       |
    |                    |                         |"unknown"                 |
    |                    |                         |                          |
    |{client_port}       |Always (Postfix >=2.5)   |Client TCP port           |
    |                    |                         |                          |
    |                    |                         |Client name from address -|
    |{client_ptr}        |CONNECT, HELO, MAIL, DATA|> name lookup             |
    |                    |                         |When address -> name      |
    |                    |                         |lookup fails: "unknown"   |
    |                    |                         |                          |
    |{cert_issuer}       |HELO, MAIL, DATA, EOH,   |TLS client certificate    |
    |                    |EOM                      |issuer                    |
    |                    |                         |                          |
    |{cert_subject}      |HELO, MAIL, DATA, EOH,   |TLS client certificate    |
    |                    |EOM                      |subject                   |
    |                    |                         |                          |
    |{cipher_bits}       |HELO, MAIL, DATA, EOH,   |TLS session key size      |
    |                    |EOM                      |                          |
    |                    |                         |                          |
    |{cipher}            |HELO, MAIL, DATA, EOH,   |TLS cipher                |
    |                    |EOM                      |                          |
    |                    |                         |                          |
    |{daemon_name}       |Always                   |value of                  |
    |                    |                         |milter_macro_daemon_name  |
    |                    |                         |                          |
    |{mail_addr}         |MAIL                     |Sender address            |
    |                    |                         |                          |
    |{mail_host}         |MAIL (Postfix >= 2.6,    |Sender next-hop           |
    |                    |only with smtpd_milters) |destination               |
    |                    |                         |                          |
    |{mail_mailer}       |MAIL (Postfix >= 2.6,    |Sender mail delivery      |
    |                    |only with smtpd_milters) |transport                 |
    |                    |                         |                          |
    |                    |                         |Recipient address         |
    |{rcpt_addr}         |RCPT                     |With rejected recipient:  |
    |                    |                         |descriptive text          |
    |                    |                         |                          |
    |                    |                         |Recipient next-hop        |
    |{rcpt_host}         |RCPT (Postfix >= 2.6,    |destination               |
    |                    |only with smtpd_milters) |With rejected recipient:  |
    |                    |                         |enhanced status code      |
    |                    |                         |                          |
    |                    |                         |Recipient mail delivery   |
    |{rcpt_mailer}       |RCPT (Postfix >= 2.6,    |transport                 |
    |                    |only with smtpd_milters) |With rejected recipient:  |
    |                    |                         |"error"                   |
    |                    |                         |                          |
    |{tls_version}       |HELO, MAIL, DATA, EOH,   |TLS protocol version      |
    |                    |EOM                      |                          |
    |                    |                         |                          |
    |v                   |Always                   |value of milter_macro_v   |
    |                    |                         |                          |

Postfix sends specific sets of macros at different Milter protocol stages. The
sets are configured with the parameters as described in the table (EOH = end of
headers; EOM = end of message). The protocol version is a number that Postfix
sends at the beginning of the Milter protocol handshake.

As of Sendmail 8.14.0, Milter applications can specify what macros they want to
receive at different Milter protocol stages. An application-specified list
takes precedence over a Postfix-specified list.

                                                                        
    |Postfix parameter            |Milter protocol|Milter protocol stage|
    |                             |version        |                     |
    |                             |               |                     |
    |milter_connect_macros        |2 or higher    |CONNECT              |
    |                             |               |                     |
    |milter_helo_macros           |2 or higher    |HELO/EHLO            |
    |                             |               |                     |
    |milter_mail_macros           |2 or higher    |MAIL FROM            |
    |                             |               |                     |
    |milter_rcpt_macros           |2 or higher    |RCPT TO              |
    |                             |               |                     |
    |milter_data_macros           |4 or higher    |DATA                 |
    |                             |               |                     |
    |milter_end_of_header_macros  |6 or higher    |EOH                  |
    |                             |               |                     |
    |milter_end_of_data_macros    |2 or higher    |EOM                  |
    |                             |               |                     |
    |milter_unknown_command_macros|3 or higher    |unknown command      |
    |                             |               |                     |

Workarounds

  * To avoid breaking DKIM etc. signatures with an SMTP-based content filter,
    update the before-filter SMTP client in master.cf, and add a line with "-
    o disable_mime_output_conversion=yes" (note: no spaces around the "="). For
    details, see the advanced content filter example.

    /etc/postfix/master.cf:
        # =============================================================
        # service type  private unpriv  chroot  wakeup  maxproc command
        #               (yes)   (yes)   (yes)   (never) (100)
        # =============================================================
        scan      unix  -       -       n       -       10      smtp
            -o smtp_send_xforward_command=yes
            -o disable_mime_output_conversion=yes
            -o smtp_generic_maps=

  * Some Milter applications use the "{if_addr}" macro to recognize local mail;
    this macro does not exist in Postfix. Workaround: use the "{client_addr}"
    macro instead.

  * Some Milter applications log a warning that looks like this:

        sid-filter[36540]: WARNING: sendmail symbol 'i' not available

    And they may insert an ugly message header with "unknown-msgid" like this:

        X-SenderID: Sendmail Sender-ID Filter vx.y.z host.example.com <unknown-
        msgid>

    The problem is that Milter applications expect that the queue ID is known
    before the MTA accepts the MAIL FROM (sender) command. Postfix does not
    choose a queue ID, which is used as the queue file name, until after it
    accepts the first valid RCPT TO (recipient) command.

    If you experience the ugly header problem, see if a recent version of the
    Milter application fixes it. For example, current versions of dkim-filter
    and dk-filter already have code that looks up the Postfix queue ID at a
    later protocol stage, and sid-filter version 1.0.0 no longer includes the
    queue ID in the message header.

    To fix the ugly message header, you will need to add code that looks up the
    Postfix queue ID at some later point in time. The example below adds the
    lookup after the end-of-message.

      o Edit the filter source file (typically named xxx-filter/xxx-filter.c or
        similar).

      o Look up the mlfi_eom() function and add code near the top shown as bold
        text below:

        dfc = cc->cctx_msg;
        assert(dfc != NULL);

        /* Determine the job ID for logging. */
        if (dfc->mctx_jobid == 0 || strcmp(dfc->mctx_jobid, JOBIDUNKNOWN) == 0)
        {
                char *jobid = smfi_getsymval(ctx, "i");
                if (jobid != 0)
                        dfc->mctx_jobid = jobid;
        }

    NOTES:

      o Different mail filters use slightly different names for variables. If
        the above code does not compile, look elsewhere in the mail filter
        source file for code that looks up the "i" macro value, and copy that
        code.

      o This change fixes only the ugly message header, but not the WARNING
        message. Fortunately, many Milters log that message only once.

Limitations

This section lists limitations of the Postfix Milter implementation. Some
limitations will be removed as the implementation is extended over time. Of
course the usual limitations of before-queue filtering will always apply. See
the CONTENT_INSPECTION_README document for a discussion.

  * The Milter protocol has evolved over time. Therefore, different Postfix
    versions implement different feature sets.

                                                                             
    |Postfix|Supported Milter requests                                       |
    |       |                                                                |
    |   2.6 |All Milter requests of Sendmail 8.14.0 (see notes below).       |
    |       |                                                                |
    |       |All Milter requests of Sendmail 8.14.0, except:                 |
    |       |SMFIP_RCPT_REJ (report rejected recipients to the mail filter), |
    |   2.5 |SMFIR_CHGFROM (replace sender, with optional ESMTP parameters), |
    |       |SMFIR_ADDRCPT_PAR (add recipient, with optional ESMTP           |
    |       |parameters).                                                    |
    |       |                                                                |
    |   2.4 |All Milter requests of Sendmail 8.13.0.                         |
    |       |                                                                |
    |   2.3 |All Milter requests of Sendmail 8.13.0, except:                 |
    |       |SMFIR_REPLBODY (replace message body).                          |
    |       |                                                                |

  * For Milter applications that are written in C, you need to use the Sendmail
    libmilter library.

  * Postfix has TWO sets of mail filters: filters that are used for SMTP mail
    only (specified with the smtpd_milters parameter), and filters for non-SMTP
    mail (specified with the non_smtpd_milters parameter). The non-SMTP filters
    are primarily for local submissions.

    When mail is filtered by non_smtpd_milters, the Postfix cleanup(8) server
    has to simulate SMTP client requests. This works as expected, with only one
    exception: non_smtpd_milters must not REJECT or TEMPFAIL simulated RCPT TO
    commands. When this rule is violated, Postfix will report a configuration
    error, and mail will stay in the queue.

  * Postfix currently does not apply content filters to mail that is forwarded
    or aliased internally, or to mail that is generated internally such as
    bounces or Postmaster notifications. This may be a problem when you want to
    apply a signing Milter to such mail.

  * When you use the before-queue content filter for incoming SMTP mail (see
    SMTPD_PROXY_README), Milter applications have access only to the SMTP
    command information; they have no access to the message header or body, and
    cannot make modifications to the message or to the envelope.

  * Postfix 2.6 ignores the optional ESMTP parameters in requests to replace
    the sender (SMFIR_CHGFROM) or to append a recipient (SMFIR_ADDRCPT_PAR).
    Postfix logs a warning message when a Milter application supplies such
    ESMTP parameters:

    warning: queue-id: cleanup_chg_from: ignoring ESMTP arguments "whatever"
    warning: queue-id: cleanup_add_rcpt: ignoring ESMTP arguments "whatever"

  * Postfix 2.3 does not implement requests to replace the message body. Milter
    applications log a warning message when they need this unsupported
    operation:

    st_optionneg[134563840]: 0x3d does not fulfill action requirements 0x1e

    The solution is to use Postfix version 2.4 or later.

  * Most Milter configuration options are global. Future Postfix versions may
    support per-Milter timeouts, per-Milter error handling, etc.

