# Enable SRS (Sender Rewriting Scheme) support

Attention

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Attention

- Please read Known Issues before applying SRS support.
- If you deploy iRedMail server with the iRedMail Easy platform, you can simply enable or disable it by toggling on/off a checkbox on the mail server profile page, then apply the change.

## What SRS is

It's recommended to read links below to understand what SRS is:

- WikiPedia: Sender Rewriting Scheme
- The original SRS paper (PDF)

## Upgrade iRedAPD to 2.6 or later release

We implemented SRS support since iRedAPD-2.6, please make sure you're running 2.6 or later release. You can check its version by running command below:

    ls -dl /opt/iredapd

To upgrade iRedAPD, please follow this tutorial: Upgrade iRedAPD.

## Enable SRS

SRS is not enabled by default since iRedAPD-3.3, you need to generate a secret string and restart iredapd service to enable it.

- Generate a secret string:

    $ echo $RANDOM$RANDOM$RANDOM$RANDOM$RANDOM$RANDOM | md5sum
    9d3e3fbb52ea136033fc2c40a5340f86 -

- Update parameter `srs_secrets` in `/opt/iredapd/settings.py`:

    srs_secrets = ["9d3e3fbb52ea136033fc2c40a5340f86"]

- Restart iRedAPD service:

    service iredapd restart

iRedAPD will listen to 3 network ports (all on `127.0.0.1`) by default:

- `7777`: for general smtp access policy, including greylisting, throttling, white/blacklisting, etc.
- `7778`: for SRS forward rewriting.
- `7779`: for SRS reverse rewriting.

Warning

Server hostname is used as srs domain (the mail domain name in rewritten addresses) by default, it's configureable by updating parameter `srs_domain =` in iRedAPD config file `/opt/iredapd/settings.py`. You are free to use a separated (sub-)domain name as srs domain, for example, if you own domain name `example.com`, you can use `srs.example.com` as srs domain.

The srs domain must be resolveable by DNS query and pointed to your iRedMail server. MX type DNS record is the best option, although it works with just A type DNS record (because MTA falls back to A if no MX record).

## Test SRS

You can use `telnet` or netcat (command `nc`) to test it. We use `nc` for example here.

Connect to port `7778` first:

    nc localhost 7778

Then type command:

    get user@gmail.com

Since `gmail.com` is an external domain, you should get a rewritten address returned like this:

    200 SRS0=XsrM=R5=gmail.com=a@<HOSTNAME>

The placholder `<HOSTNAME>` will be replaced by your server hostname.

Then try with your mail domain name (replace `mydomain.com` below by your real mail domain name):

    get user@mydomain.com

You should get this:

    500 Domain is a local mail domain, bypassed.

If you get same/similar output, the SRS feature is working fine.

## Enable SRS integration in Postfix

Please add 4 new parameters in Postfix config file `/etc/postfix/main.cf` (on Linux/OpenBSD) or `/usr/local/etc/postfix/main.cf` (on FreeBSD):

    sender_canonical_maps = tcp:127.0.0.1:7778
    sender_canonical_classes = envelope_sender
    recipient_canonical_maps = tcp:127.0.0.1:7779
    recipient_canonical_classes= envelope_recipient,header_recipient

Restarting or reloading Postfix service is required. After restarted/reloaded, please monitor its log file (`/var/log/maillog`) and pay close attention to the sender address.

## Known Issues

- Sender addresses will always be rewritten even if the mail is not forwarded at all, except locally hosted mail domains. This is because Postfix sends minimal info to iRedAPD via the tcp table lookup, and it performs address rewritten at the very beginning before any routing decision is made.
- If sender address was rewritten, SpamAssassin will check SPF against the domain name specified in iRedAPD parameter `srs_domain` (which is server hostname by default), if you don't have SPF DNS record for `srs_domain`, SpamAssassin may tag a score of the matched `SPF_FAIL` rule.
- Postfix will rewrite the address in the `Return-Path:` header, if you have any sieve rules based on `Return-Path:`, it MAY not work anymore. In this case, you need to update your sieve rules to match the rewritten address.
- Postfix logs rewritten addresses in its log file, so it may confuse you while troubleshooting.
- Amavisd stores rewritten addresses in its SQL database, so the `Top 10 Senders` and `Top 10 Recipients` in iRedAdmin-Pro Dashboard page may not work well.