Postfix TLSRPT Howto

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TOC

  * Introduction
  * Building Postfix with TLSRPT support
  * Using TLSRPT
  * 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 to request daily summary reports for
successful and failed TLS connections to that domain. Support for TLSRPT was
added in Postfix 3.10.

When Postfix TLSRPT support is enabled (with "smtp_tlsrpt_enable = yes"), the
Postfix SMTP and TLS clients engines will generate "success" and "failure"
events, and will pass those events to a TLSRPT client library and report
generator that 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 high-level diagram shows how Postfix events are reported to domains that
publish a TLSRPT policy.

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

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 options,
    then simply append the above options to the existing CCARGS or AUXLIBS
    values.

Using 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.

A good socket location would be under $queue_directory/run/tlsrpt or
$queue_directory/var/run/tlsrpt. These can then be configured in Postfix as a
relative pathname (run/tlsrpt/tlsrpt.sock or var/run/tlsrpt/tlsrpt.sock) so
that the same name will work with and without Postfix chroot support. Do not
specify a location under directory that is already used by Postfix programs.
Only Postfix programs should create sockets there.

MTA-STS Support via smtp_tls_policy_maps

Postfix supports MTA-STS though an smtp_tls_policy_maps policy plugin. Postfix
TLSRPT support expects a response with the usual security level and matching
requirements, plus any applicable 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),
    but it will not use most attributes except ttl and policy_failure.

    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 https://
datatracker.ietf.org/doc/html/rfc8460#section-4.5.

  * 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: testing },
        ...

    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=mx1.example.com, mx_host_pattern=mx2.example.com, ...

  * 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 following limitations exist primarily because some errors may be reported
by a Postfix smtp(8) client process, and some errors by a Postfix tlsproxy(8)
process. It is too difficult to propagate TLSRPT client library state between
processes.

The Postfix TLSRPT client reports one final status (either 'success' or
'failure') for each MTA that it is able to connect to. It cannot report a final
status 'success' with some recoverable 'failure'. Specifically:

  * The Postfix TLSRPT client can report either successful TLS policy
    compliance, or an unrecoverable failure that prevents TLS policy compliance
    (examples: all TLSA records are unusable, or some PKI error during
    certificate verification).

  * The Postfix TLSRPT client must not be used to report a potentially
    recoverable failure such as a non-parsable TLSA record, because some other
    TLSA record for the same host may still allow successful TLS policy
    compliance.

Credits

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

