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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