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---
title = 'Postfix on a NixOS null client with external catch-all'
date = 2020-09-11T18:49:00+02:00
[taxonomies]
tags = ["nixos","infrastructure"]
---
I wanted to set up a server so that any local email (e.g. generated by cron jobs/systemd timers) would be forwarded to an external address, regardless of the user. I also wanted the from address to keep the system hostname whilst not allowing any external use of the mailserver.
It took me a while to figure out how to this, so I thought I'd share my method.
Here's the config that can be used to do this on any NixOS host, after redefining the first two variables.
```txt,linenos,hl_lines=2-3
services.postfix = let
localUser = "example-user";
forwardingAddress = "user@external.domain";
in
{
enable = true;
destination = [];
domain = config.networking.domain;
virtual = ''
@${config.networking.hostName}.${config.networking.domain} ${localUser}
${localUser} ${forwardingAddress}
'';
config = {
inet_interfaces = "loopback-only";
};
};
```
Emails to any user without a domain part are all sent to the forwarding address with a clear *from* address (e.g. `System administrator <root@host.example.com>`).
## Background
First, the basic setup for a null client can be found in the [postfix documentation][0]. The example config would be translated into NixOS like so:
```txt
services.postfix = {
enable = true;
destination = [];
domain = config.networking.domain;
origin = config.networking.domain;
relayHost = config.networking.domain;
lookupMX = true;
config = {
inet_interfaces = "loopback-only";
};
};
```
However, this rewrites user\@hostname.example.com to user\@example.com (due to `origin` on line 5). I wanted to be able to see which host a mail concerns.
[0]: http://www.postfix.org/STANDARD_CONFIGURATION_README.html#null_client
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