PostSRSd

About

PostSRSd provides the Sender Rewriting Scheme (SRS) via TCP-based lookup tables for Postfix. SRS is needed if your mail server acts as forwarder.

Sender Rewriting Scheme Crash Course

Imagine your server receives a mail from alice@example.com that is to be forwarded. If example.com uses the Sender Policy Framework to indicate that all legit mails originate from their server, your forwarded mail might be bounced, because you have no permission to send on behalf of example.com. The solution is that you map the address to your own domain, e.g. SRS0+xxxx=yy=example.com=alice@yourdomain.org (forward SRS). If the mail is bounced later and a notification arrives, you can extract the original address from the rewritten one (reverse SRS) and return the notification to the sender. You might notice that the reverse SRS can be abused to turn your server into an open relay. For this reason, xxxx and yy are a cryptographic signature and a time stamp. If the signature does not match, the address is forged and the mail can be discarded.

Building

PostSRSd requires a POSIX compatible system and CMake to build. Optionally, help2man is used to create a manual page.

For convenience, a Makefile fragment is provided which calls CMake with the recommended command line options. Just run make.

Alternatively, you can control many aspects of the build manually:

mkdir build
cd build
cmake .. <options>
make
make install

The CMake script defines a number of options in addition to the standard CMake flags. Use -D<option>=<value> to override the defaults.

• GENERATE_SRS_SECRET (default: ON). Generate a random secret on install.
• USE_APPARMOR (default: OFF): Install an AppArmor profile for the daemon.
• INIT_FLAVOR (default: auto-detect). Select the appriopriate startup script type. Must be one of (systemd, upstart,sysv-lsb,sysv-redhat) or none.
• CHROOT_DIR (default: ${CMAKE_INSTALL_PREFIX}/lib/postsrsd). Chroot jail for the daemon.
• SYSCONF_DIR (default: /etc). Location of system configuration files.
• CONFIG_DIR (default: ${SYSCONF_DIR}/default). Install destination for the postsrsd settings.
• DOC_DIR (default: ${CMAKE_INSTALL_PREFIX}/share/doc/postsrsd). Install destination for documentation files.

Installing

Run make install as root to install the daemon and the configuration files.

Configuration

The configuration is located in /etc/default/postsrsd by default. You must store at least one secret key in /etc/postsrsd.secret. The installer tries to generate one from /dev/urandom. Be careful that no one can guess your secret, because anyone who knows it can use your mail server as open relay! Each line of /etc/postsrsd.secret is used as secret. The first secret is used for signing and verification, the others for verification only.

PostSRSd exposes its functionality via two TCP lookup tables. The recommended Postfix configuration is to add the following fragment to your main.cf:

sender_canonical_maps = tcp:127.0.0.1:10001
sender_canonical_classes = envelope_sender
recipient_canonical_maps = tcp:127.0.0.1:10002
recipient_canonical_classes= envelope_recipient,header_recipient

This will transparently rewrite incoming and outgoing envelope addresses, and additionally undo SRS rewrites in the To: header of bounce notifications and vacation autoreplies.

Run service postsrsd start and postfix reload as root, or reboot.

Known Issues

• Due to the way PostSRSd is integrated with Postfix, sender addresses will always be rewritten even if the mail is not forwarded at all. This is because the canonical maps are read by the cleanup daemon, which processes mails at the very beginning before any routing decision is made.

• The Postfix package in CentOS 6 lacks the required support for TCP dictionaries. Please upgrade your distribution or build Postfix yourself.