How do I include another configuration file from configuration.nix?

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1

I’d like to author nftables in another file instead of embedding a string in my /etc/nixos/configuration.nix.
Why? I prefer to author some configuration directly to remove the cognitive overhead of thinking in nix or whatever other language; focus I guess.

Do you know how to do this or of an example doing so?

2

Option 1:

# configuration.nix:
{
  networking.nftables.tables.filter = {
    family = "inet";
    content = builtins.readFile ./rules.nft;
  };
  /* ... other options ... */
}

Option 2:

{
  networking.nftables.rulesetFile = ./rules.nft;
  /* ... other options ... */
}

Option 3:

# configuration.nix:
{
  imports = [ ./nftables-config.nix ];
  /* ... other options ... */
}

# nftables-config.nix
{
  networking.nftables.tables.filter = {
    family = "inet";
    content = ''
      # rules go here...
    '';
  };
}
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