Postfix TLSRPT Howto

-------------------------------------------------------------------------------

Table of Contents

  * Introduction
  * Building Postfix with TLSRPT support
  * Turning on TLSRPT
  * TLSRPT Status logging
  * Delivering TLSRPT summaries via email
  * MTA-STS Support via smtp_tls_policy_maps
  * Limitations
  * Credits

Introduction

The TLSRPT protocol is defined in RFC 8460. With this, an email receiving
domain can publish a policy in DNS, and request daily summary reports for
successful and failed SMTP over TLS connections to that domain's MX hosts.
Support for TLSRPT was added in Postfix 3.10.

A policy example looks like this:

    _smtp._tls.example.com. IN TXT "v=TLSRPTv1; rua=mailto:smtp-tls-
    report@example.com"

Translation: email sending systems are requested to generate daily summaries of
successful and failed SMTP over TLS connections to domain example.com, and to
report those summaries via email to the specified address. Instead of mailto:,
a policy may specify an https: destination.

The high-level diagram shows how Postfix reports summaries to domains that
publish a TLSRPT policy.

    Postfix SMTP and     TLSRPT client      TLSRPT summary      Email or HTTP
       TLS client    -->    library     -->    generator    -->    delivery
        engines

When Postfix TLSRPT support is enabled (with "smtp_tlsrpt_enable = yes"):

  * The Postfix SMTP and TLS client engines will generate a "success" or
    "failure" event for each TLS handshake,

  * They will pass those events to an in-process TLSRPT client library that
    sends data over a local socket to

  * A TLSRPT report generator that produces daily summary reports.

The TLSRPT client library and report generator are maintained by sys4.

The Postfix implementation supports both DANE (Postfix built-in) and MTA-STS
(through an smtp_tls_policy_maps plug-in).

The Postfix smtp(8) client process implements the SMTP client engine. With
"smtp_tls_connection_reuse = no", the smtp(8) client process also implements
the TLS client engine. With "smtp_tls_connection_reuse = yes", the smtp(8)
client process delegates TLS processing to a Postfix tlsproxy(8) process.
Either way, Postfix will generate the exact same TLSRPT events.

Building Postfix with TLSRPT support

These instructions assume that you build Postfix from source code as described
in the INSTALL document. Some modification may be required if you build Postfix
from a vendor-specific source package.

The Postfix TLSRPT client builds on a TLSRPT client library whose source code
can be obtained from:

    https://github.com/sys4/tlsrpt

The library is typically installed as a header file in /usr/local/include/
tlsrpt.h and an object library in /usr/local/lib/libtlsrpt.a or /usr/local/lib/
libtlsrpt.so. The actual pathnames will depend on OS platform conventions.

In order to build Postfix with TLSRPT support, you will need to add compiler
options -DUSE_TLSRPT (to build with TLSRPT support), and -I (with the directory
containing the tlsrpt.h header file), and you will need to add linker options
to link with the TLSRPT client library, for example:

    make -f Makefile.init makefiles \
      "CCARGS=-DUSE_TLSRPT -I/usr/local/include" \
      "AUXLIBS=-L/usr/local/lib -ltlsrpt"

Then, just run 'make'.

    Note: if your build command line already has CCARGS or AUXLIBS settings,
    then simply append the above settings to the existing CCARGS or AUXLIBS
    values.

        make -f Makefile.init makefiles \
          "CCARGS=existing settings... -DUSE_TLSRPT -I/usr/local/include" \
          "AUXLIBS=existing settings... -L/usr/local/lib -ltlsrpt"

Turning on TLSRPT

After installing Postfix TLSRPT support, you can enable TLSRPT support in
main.cf like this:

    smtp_tlsrpt_enable = yes
    smtp_tlsrpt_socket_name = /path/to/socket

The smtp_tlsrpt_socket_name parameter specifies an absolute pathname, or a
pathname that is relative to $queue_directory.

    Note: the recommended socket location is still to be determined. A good
    socket location would be under the Postfix queue directory, for example:
    "smtp_tlsrpt_socket_name = run/tlsrpt/tlsrpt.sock". The advantage of using
    a relative name is that it will work equally well whether or not Postfix
    chroot is turned on.

Do not specify a location under a directory such as private or public that is
already used by Postfix programs. Only Postfix programs should create sockets
there.

TLSRPT Status logging

With TLSRPT support turned on, the Postfix TLSRPT client will not only report
an event to an invisible daily success/fail summary queue, but it will also log
a visible record to the mail logfile.

Below are a few examples of logging from a Postfix SMTP client or tlsproxy
daemon:

    TLSRPT: status=success, domain=example.com, receiving_mx=mail.example.com
    [ipaddr]

    TLSRPT: status=failure, domain=example.org, receiving_mx=mail.example.org
    [ipaddr],
        failure_type=starttls_not_supported

    TLSRPT: status=failure, domain=example.net, receiving_mx=mail.example.net
    [ipaddr],
        failure_type=validation_failure, failure_reason=self-signed_certificate

Notes:

  * Postfix logs and reports the TLSRPT status only for TLS handshakes on a new
    SMTP connection. There is no TLS handshake, and thus no TLSRPT status
    logging, when an SMTP connection is reused. Such connections have Postfix
    SMTP client logging like this:

    Verified TLS connection reused to mail.example.com[ipaddr]:25:
        TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)

    Untrusted TLS connection reused to mail.example.com[ipaddr]:25:
        TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)

  * By default, Postfix does not report the TLSRPT status for a TLS handshake
    that reuses a previously-negotiated TLS session (there would be no new
    information to report). Specify "smtp_tlsrpt_skip_reused_handshakes = no"
    to report the TLSRPT status for all TLS handshakes. This may be useful for
    troubleshooting.

  * Postfix logging for certificate verification failures may differ between
    new or reused TLS sessions.

      o New TLS session:

        TLSRPT: status=failure, domain=example.org,
        receiving_mx=mail.example.org[ipaddr],
            failure_type=validation_failure, failure_reason=self-
        signed_certificate

      o Reused TLS session:

        mail.example.org[ipaddr]:25: re-using session with untrusted peer
            credential, look for details earlier in the log
        TLSRPT: status=failure, domain=example.org,
        receiving_mx=mail.example.org[ipaddr],
            failure_type=certificate_not_trusted

    The logging may differ because a reused TLS session does not have the
    details for why TLS authentication failed.

Delivering TLSRPT summaries via email

RFC 8460 Section 3 specifies that an MTA must not enforce TLS security when
sending failure reports via email. However, Postfix currently has no way to
request that TLS enforcement will be disabled when submitting an email message.

Options:

  * Do nothing. When TLS security enforcement is required, a persistent
    enforcement failure will delay the delivery of a TLSRPT summary until the
    problem is addressed, or until the message expires in the mail queue. Keep
    in mind that TLSRPT is not a real-time monitoring service; it takes on
    average 12 hours before a failure is reported through TLSRPT.

  * Exclude the sender of TLSRPT summaries from TLS enforcement. Implement the
    configuration below on outbound MTA instances (replace noreply-smtp-tls-
    reporting@example.com with your actual report generator's sender address):

    /etc/postfix/main.cf:
        # Limitation: this setting is overruled with transport_maps.
        sender_dependent_default_transport_maps = inline:{
            { noreply-smtp-tls-reporting@example.com = allow-plaintext } }

    /etc/postfix/master.cf:
        # service name    type    private unpriv  chroot  wakeup  maxproc
    command
        allow-plaintext   unix    -       -       n       -       -       smtp
            -o { smtp_tls_security_level = may }
            -o { smtp_tls_policy_maps = static:may }

MTA-STS Support via smtp_tls_policy_maps

Postfix supports MTA-STS though an smtp_tls_policy_maps policy plugin, which
replies with a TLS security level and optional matching requirements. Postfix
3.10 and later optionally also accept the name=value attributes described
below. Specify { name = value } when a value may contain whitespace.

    Note 1: Postfix 3.10 and later will accept these attributes in an MTA-STS
    response even if TLSRPT support is disabled (at build time or run time).
    With TLSRPT support turned off, Postfix will use the ttl and policy_failure
    attributes, and will ignore the attributes that are used only for TLSRPT.

    Note 2: It is an error to specify these attributes for a non-STS policy.

The examples in the table apply to the MTA-STS policy example given in RFC 8461
Section 3.2:

    version: STSv1
    mode: enforce
    mx: mail.example.com
    mx: *.example.net
    mx: backupmx.example.com
    max_age: 604800

A policy response may contain line breaks.

  * policy_type=type

    Specify sts or no-policy-found.

  * policy_domain=name

    The domain that the MTA-STS policy applies to.

  * policy_ttl=time

    How long (in seconds) a Postfix SMTP client process will cache the MTA-STS
    plugin response.

  * { policy_string = value }

    Specify one policy_string instance for each MTA-STS policy feature,
    enclosed inside "{" and "}" to protect whitespace in attribute values.

    Example:

        { policy_string = version: STSv1 } { policy_string = mode: enforce }
        ...

    This form ignores whitespace after the opening "{", around the "=", and
    before the closing "}".

  * mx_host_pattern=pattern

    Specify one mx_host_pattern instance for each "mx:" feature in the MTA-STS
    policy.

    Example:

        mx_host_pattern=mail.example.com mx_host_pattern=*.example.net ...

  * policy_failure=type

    If specified, forces MTA-STS policy enforcement to fail with the indicated
    error, even if a server certificate would satisfy conventional PKI
    constraints.

    Valid errors are sts-policy-fetch-error, sts-policy-invalid, sts-webpki-
    invalid, or the less informative validation-failure.

    Example:

        policy_failure=sts-webpki-invalid

Limitations

The Postfix TLSRPT implementation reports only TLS handshake success or
failure. It does not report failure to connect, or connections that break
before or after a TLS handshake.

The Postfix TLSRPT implementation reports at most one final TLS handshake
status (either 'success' or 'failure') per SMTP connection. Postfix TLSRPT will
not report a recoverable failure and then later report a final status of
'success' for that same connection. The reason is that it's too complicated to
filter TLS errors and to report error details from the TLS engine back to the
SMTP protocol engine. It just is not how Postfix works internally.

Credits

  * The TLSRPT client library and report generator are implemented and
    maintained by sys4.
  * Wietse Venema implemented the integration with Postfix.

