OpenSMTPD filter-rewrite-from

This is a very simple OpenSMTPD filter for rewriting email address in both MAIL FROM command and From header in all ^[1] messages with the specified email address. It’s useful when you want to ensure that all the relayed messages will be “send from” some no-reply email address.

This filter is implemented in ~60 lines (LoC) of awk.

Requirements

• POSIX awk (e.g. from Busybox, GNU, BSD, …)

• OpenSMTPD 6.6.0 or higher

Installation

Just copy filter-rewrite-from to some directory, e.g. /usr/local/share/opensmtpd. If you have awk installed in a different directory than /usr/bin, adjust the shebang.

Configuration

The filter itself requires no configuration, only the desired From email address as the first argument.

It must be declared in smtpd.conf and attached to a listener:

filter "rewrite-from" proc-exec "/usr/local/share/opensmtpd/filter-rewrite-from no-reply@example.org"

listen on socket filter "rewrite-from"

See Also

• OpenSMTPD released and upcoming filters preview by Gilles Chehade

• July 2019 report: tons of smtpd work mostly by Gilles Chehade

License

This project is licensed under ISC License. For the full text of the license, see the LICENSE file.

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1. This implementation doesn’t support any conditions or different rewrite addresses based on the source. It’s really trivial, just for my current needs.