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PostSRSd provides the Sender Rewriting Scheme (SRS) via TCP-based lookup tables for Postfix. SRS is needed if your mail server acts as forwarder.
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.
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.USE_SELINUX
(default: OFF
): Install an SELinux policy module 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.SYSD_UNIT_DIR
(default: ${SYSCONF_DIR}/systemd/system
). Install
destination for systemd startup files.Run make install
as root to install the daemon and the configuration
files.
The configuration is located in /etc/default/postsrsd
by default. On many
systems, the default configuration will work out-of-the-box, but please take
the two minutes to check the settings for yourself. Also, please make sure
that Postfix has the correct domain name configured, i.e.
postconf -h mydomain
returns the correct value.
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:localhost:10001
sender_canonical_classes = envelope_sender
recipient_canonical_maps = tcp:localhost: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.
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.
Exim configuration examples can be found in README.exim.md