postfix-mta-sts-resolver

Daemon which provides TLS client policy for Postfix via socketmap, according to domain MTA-STS policy.

Current support of RFC 8461 is limited:

• MTA-STS policy overrides DANE TLS authentication against RFC 8461, 2.

• Daemon lacks some minor features:

  • Fetch error reporting.
  • Fetch ratelimit (but actual fetch rate partially restricted with cache_grace config option).

Server has configurable cache backend which allows to store cached STS policies in memory (internal), file (sqlite) or in Redis database (redis).

Requirements

• Postfix 3.4+ (or Postfix 2.10+ if missing Postfix SNI feature is tolerable. In that case you have to set zone option require_sni to false in MTA-STS daemon config)
• Python 3.5.3+ (see "Systems without Python 3.5+" below if you haven't one, or use Docker installation method)
• aiodns
• aiohttp
• aiosqlite
• redis-py
• PyYAML
• (optional) uvloop

All dependency packages installed automatically if this package is installed via pip.

Installation

Method 1. System-wide install from PyPI (recommended for humans)

Run:

sudo python3 -m pip install postfix-mta-sts-resolver[redis,sqlite,postgres]

If you don't need redis or sqlite support, you may omit one of them in square brackets. If you don't need any of them and you plan to use internal cache without persistence, you should also omit square brackets.

Package scripts shall be available in standard executable locations upon completion.

pip user install

All pip invocations can be run with --user option of pip installer. In this case superuser privileges are not required and package(s) are getting installed into user home directory. Usually, script executables will appear in ~/.local/bin.

Method 2. System-wide install from project source

Run in project directory:

sudo python3 -m pip install .[redis,sqlite,postgres]

If you don't need redis, sqlite or postgres support, you may omit one of them in square brackets. If you don't need any of them and you plan to use internal cache without persistence, you should also omit square brackets.

Package scripts shall be available in standard executable locations upon completion.

Method 3. Install into virtualenv

See "Building virtualenv"

Method 4. Docker

Run

docker volume create mta-sts-cache
docker run -d \
    --security-opt no-new-privileges \
    -v mta-sts-cache:/var/lib/mta-sts \
    -p 127.0.0.1:8461:8461 \
    --restart unless-stopped \
    --name postfix-mta-sts-resolver \
    yarmak/postfix-mta-sts-resolver

Daemon will be up and running, listening on local interface on port 8461. Default configuration baked into docker image uses SQLite for cache stored in persistent docker volume. You may override this configuration with your own config file by mapping it into container with option -v my_config.yml:/etc/mta-sta-daemon.yml.

Method 5. Snap Store

sudo snap install postfix-mta-sts-resolver

NOTE: in snap layout mta-sta-daemon program is named postfix-mta-sts-resolver.daemon and mta-sts-query is named postfix-mta-sts-resolver.query.

Common installation notes

See also contrib/README.md for RHEL/OEL/Centos and FreeBSD notes.

See contrib/ for example of systemd unit file suitable to run daemon under systemd control.

Running

This package provides two executables available after installation in respective locations.

mta-sts-query

mta-sts-query is a command line tool which fetches and outputs domain MTA-STS policies. Intended to be used for debug purposes.

Synopsis:

$ mta-sts-query --help
usage: mta-sts-query [-h] [-v {debug,info,warn,error,fatal}]
                     domain [known_version]

positional arguments:
  domain                domain to fetch MTA-STS policy from
  known_version         latest known version (default: None)

optional arguments:
  -h, --help            show this help message and exit
  -v {debug,info,warn,error,fatal}, --verbosity {debug,info,warn,error,fatal}
                        logging verbosity (default: warn)

mta-sts-daemon

mta-sts-daemon is a daemon which provides external TLS policy for Postfix SMTP client via socketmap interface.

You may find useful systemd unit file to run daemon in contrib/.

Synopsis:

$ mta-sts-daemon --help
usage: mta-sts-daemon [-h] [-v {debug,info,warn,error,fatal}] [-c FILE]
                      [-l FILE] [--disable-uvloop]

optional arguments:
  -h, --help            show this help message and exit
  -v {debug,info,warn,error,fatal}, --verbosity {debug,info,warn,error,fatal}
                        logging verbosity (default: info)
  -c FILE, --config FILE
                        config file location (default: /etc/mta-sts-
                        daemon.yml)
  -l FILE, --logfile FILE
                        log file location (default: None)
  --disable-uvloop      do not use uvloop even if it is available (default:
                        False)

Seamless restart/upgrade/reload and load balancing

By default mta-sts-daemon allows its multiple instances to share same port (on Linux/FreeBSD/Windows). Therefore, restart or upgrade of daemon can be performed seamlessly. Set of unit files for systemd in contrib/ directory implements "reload" by mean of running backup instance when main instance is getting restarted.

Also on Linux and FreeBSD, load distributed across all processes (with SO_REUSEPORT and SO_REUSEPORT_LB respectively).

MTA-STS Daemon configuration

See configuration man page and config_examples/ directory. Default config location is: /etc/mta-sts-daemon.yml, but it can be overridden with command line option -c FILE.

All options is self-explanatory, only exception is strict_testing option. If set to true, STS policy will be enforced even if domain announces testing MTA-STS mode. Useful for premature incorporation of MTA-STS against domains hesitating to go enforce. Please use with caution.

Postfix configuration

SMTP client of your Postfix instance must be able to validate peer certificates. In order to achieve that, you have to ensure smtp_tls_CAfile or smtp_tls_CApath points to system CA bundle. Otherwise you'll get Unverified TLS connection even for peers with valid certificate, and delivery failures for MTA-STS-enabled destinations. Also note: even enabled tls_append_default_CA will not work alone if both smtp_tls_CAfile and smtp_tls_CApath are empty.

Once certificate validation is enabled and your Postfix log shows "Trusted TLS connection ... " for destinations with valid certificates signed by public CA, you may enable MTA-STS by adding following line to main.cf:

smtp_tls_policy_maps = socketmap:inet:127.0.0.1:8461:postfix

If your configuration already has some TLS policy maps, just add MTA-STS socketmap to list of configured maps accordingly to smtp_tls_policy_maps syntax. TLS policy tables are searched in the specified order until a match is found, so you may have table with local overrides of TLS policy prior to MTA-STS socketmap. This may be useful for skipping network lookup for well-known destinations or relaxing security for broken destinations, announcing MTA-STS support.

Reload Postfix after reconfiguration.

Warning: MTA-STS policy overrides DANE TLS authentication

Due to Postfix's limitations, a resolved MTA-STS policy overrides DANE TLS authentication (RFC 6698), because DANE is an internal feature of Postfix, and the postfix-mta-sts-resolver always responds with a (smtp_tls_policy_maps) lookup result secure for Secure server certificate verification.

• The resulting behaviour is against RFC 8461, 2:

  However, MTA-STS is designed not to interfere with DANE deployments when the two overlap; in particular, senders who implement MTA-STS validation MUST NOT allow MTA-STS Policy validation to override a failing DANE validation.

  Domains implementing both MTA-STS and DANE probably want DANE to be preferred:

  • DANE allows strict binding of certificates; the policy can authorize only a certain certificate or certificates from a certain CA. With MTA-STS, a certificate from any trusted CA is automatically trusted; RFC 8461, 10.1:

    SMTP MTA-STS relies on certificate validation via PKIX-based TLS identity checking [RFC6125]. Attackers who are able to obtain a valid certificate for the targeted recipient mail service (e.g., by compromising a CA) are thus able to circumvent STS authentication.

  • Based on DNSSEC, DANE not vulnerable to downgrade attack that could prevent policy discovery. MTA-STS security considerations acknowledges this weakness in RFC 8461, 10.2:

    Since MTA-STS uses DNS TXT records for policy discovery, an attacker who is able to block DNS responses can suppress the discovery of an MTA-STS Policy, making the Policy Domain appear not to have an MTA-STS Policy.

    Resistance to downgrade attacks of this nature -- due to the ability to authoritatively determine "lack of a record" even for non-participating recipients -- is a feature of DANE, due to its use of DNSSEC for policy discovery.

• The postfix-mta-sts-resolver does not intent to implement policy lookups for DANE, and responses other than secure with match= would not verify the TLS certificate as required by RFC 8461, 4,2.

If you wish to meet this requirement:

• List a DANE policy resolver responding with dane-only (for Mandatory DANE) before postfix-mta-sts-resolver in smtp_tls_policy_maps lookup table list.

• Alternatively, you could use a static lookup table for domains known to implement both MTA-STS & DANE, e.g.,

  smtp_tls_policy_maps = hash:/etc/postfix/tls_policy,socketmap:inet:127.0.0.1:8461:postfix
  

Operability check

Assuming default MTA-STA daemon configuration. Following command:

/usr/sbin/postmap -q dismail.de socketmap:inet:127.0.0.1:8461:postfix

should return something like:

secure match=mx1.dismail.de

Postfix log should show Verified TLS connection established to ... instead of Untrusted ... or Trusted TLS connection established to ... when mail is getting sent to MTA-STS-enabled domain.

Special cases of deployment

Systems without Python 3.5+

Some people may find convenient to install latest python from source into /opt directory. This way you can have separate python installation not interfering with system packages by any means. Download latest python source from python.org, unpack and run in unpacked source directory:

./configure --prefix=/opt --enable-optimizations && make -j $[ $(nproc) + 1 ] && make test && sudo make install

Python binaries will be available in /opt/bin, including pip3. You may install postfix-mta-sts-resolver using /opt/bin/pip3 without interference with any system packages:

sudo /opt/bin/pip3 install postfix-mta-sts-resolver[sqlite,redis]

Executable files of postfix-mta-sts-resolver will be available in /opt/bin/mta-sts-query and /opt/bin/mta-sts-daemon

Building virtualenv

Run make in project directory in order to build virtualenv. As result of it, new directory venv shall appear. venv contains interpreter and all required dependencies, i.e. encloses package with dependencies in separate environment. It is possible to specify alternative path where virtualenv directory shall be placed. Specify VENV variable for make command. Example:

make VENV=~/postfix-mta-sts-resolver

Such virtual environment can be moved to another machine of similar type (as far python interpreter is compatible with new environment). If virtualenv is placed into same location on new machine, application can be run this way:

venv/bin/mta-sts-daemon

Otherwise, some hacks required. First option - explicitly call virtualenv interpreter:

venv/bin/python venv/bin/mta-sts-daemon

Second option - specify new path in shebang of scripts installed in virtualenv. It is recommended to build virtualenv at same location which app shall occupy on target system.

Credits

Inspired by a postfix-user mailing list thread (subject: 'MTA-STS when?', first message: 2018-02-17).