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https://github.com/zoni/postforward

Postfix SRS forwarding agent
https://github.com/zoni/postforward

mail mailer postfix postfix-helper sender-policy-framework spf srs

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Postfix SRS forwarding agent

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README 

        
Postforward

===========



*Postfix SRS forwarding agent.*



About

-----



Postforward is a mail forwarding utility which aims to compliment the

[Postfix Sender Rewriting Scheme daemon (PostSRSd)](https://github.com/roehling/postsrsd).



The downside of using PostSRSd is that all mail is naively rewritten, even

when no forwarding is actually performed. Such rewritten Return-Path

addresses may confuse sieve scripts and other mail filtering software.



This is where Postforward comes in. Instead of rewriting all incoming mail

regardless of final destination, mail systems may be configured to pipe

mail into Postforward only when forwarding needs to happen, leaving

non-forwarded mail unaltered by PostSRSd. Postforward will rewrite

envelope addresses for piped mail using PostSRSd itself and re-inject

these messages back into the queue, destined for the forwarding

recipient(s).



Project status

--------------



This software is actively maintained but considered feature-complete. No

changes or new features are planned except as required to fix any

potential issues that may come up in the future.



Installation

------------



I no longer provide pre-compiled binaries for small-time projects of mine

so you will have to build from sources yourself. If you have an up-to-date

[Go toolchain](https://golang.org/dl/) installed on your system this is as

simple as:



```sh

go get -d github.com/zoni/postforward

cd ~/go/src/github.com/zoni/postforward

make

```



This will create a binary called `postforward` which may be installed on

the target server(s). Go applications are statically linked by default so

no additional dependencies are needed.



The makefile also contains targets to build native packages for FreeBSD

and Debian-based operating systems (`make freebsd` and `make debian`

respectively). These require [fpm](https://github.com/jordansissel/fpm) to

be installed.



Configuration

-------------



Postforward relies on mail being delivered via stdin so this implies

delivery using Postfix's `local(8)` or `pipe(8)` delivery agents. One such

method may be achieved by configuring a pipe forward in `/etc/aliases`:



```

forwarder: "|/usr/local/bin/postforward [email protected]"

```



*(Note: when running PostSRSd on a different host or port, use the

`--srs-addr` flag to set the correct address here.)*



In `main.cf`, configure `recipient_canonical_maps` and

`recipient_canonical_classes` as

[recommended by PostSRSd](https://github.com/roehling/postsrsd#configuration)

but *do not* set `sender_canonical_maps` or `sender_canonical_classes`.



Beware that Postforward expects to be called for a single recipient at a

time (although it can *forward* to multiple recipients at once) so be sure

to set [transport_destination_recipient_limit](http://www.postfix.org/postconf.5.html#transport_destination_recipient_limit)

to 1 when using it with the `pipe(8)` daemon. See also

[SINGLE-RECIPIENT DELIVERY](http://www.postfix.org/pipe.8.html).



-----------------------------------------------------------------------------



The postfix `local(8)` delivery agent uses a highly sanitized environment

for executed processes for security reasons. Depending on your operating

system, the default `$PATH` setting may be too strict for postforward to

locate the `sendmail` binary (Debian/Ubuntu are known to have this issue).

If this is the case for you, a custom `$PATH` may be set by supplying the

`--path` argument. For example: `--path /usr/sbin:/sbin:/usr/bin:/bin`



-----------------------------------------------------------------------------



Note that in case of process errors, postfix bounces emails with the full

process argument string in the DSN message which could leak internal

information such as the forwarding address. This is default postfix

behavior for the local and pipe delivery agents.



If this is undesirable,

[local_delivery_status_filter](http://www.postfix.org/postconf.5.html#local_delivery_status_filter)

may be configured with a PCRE map such as the following to hide this

information (omit the `$2` in the final entry to also strip command

output):



```

/^(2\S+ deliver(s|ed) to file).+/    $1

/^(2\S+ deliver(s|ed) to command).+/ $1

/^(\S+ Command died with status \d+):.*(\. Command output:.*)/ $1$2

```



Performance

-----------



Using Postforward introduces additional overhead caused by forking of

processes which wouldn't happen with direct use of PostSRSd. Unless you

are forwarding very large volumes of mail this extra overhead is likely

negligible in relation to the total processing cost of a complete email

transaction.



Postforward takes care not to buffer entire messages in memory and is

therefore safe to use on very large emails.  Only message headers are

buffered in memory for processing, body content is streamed directly into

sendmail.



License

-------



Postforward is offered under the 2-Clause BSD license. See

[LICENSE.txt](LICENSE.txt) for the full license text.



Changelog

---------



See [CHANGES](CHANGES.md).