modules/environment/networking: implement /etc/hosts options

AUTHORS

@@ -1,3 +1,4 @@
 Alexander Sosedkin <monk@unboiled.info>
 Tobias Happ <tobias.happ@gmx.de>
 Bruno Bigras <bigras.bruno@gmail.com>
+Evgeny Kurnevsky <kurnevsky@gmail.com>

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 ## Release 23.11 (unreleased)
 
+### New Options
+
+* New options `networking.hosts`, `networking.hostFiles` and
+  `networking.extraHosts` for `/etc/hosts` configuration.
+
 ## Release 23.05
 
 ### New Options

modules/environment/networking.nix

@@ -1,20 +1,91 @@
-# Copyright (c) 2019-2021, see AUTHORS. Licensed under MIT License, see LICENSE.
+# Copyright (c) 2019-2023, see AUTHORS. Licensed under MIT License, see LICENSE.
+
+# Inspired by
+# https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/config/networking.nix
+# (Copyright (c) 2003-2023 Eelco Dolstra and the Nixpkgs/NixOS contributors,
+#  licensed under MIT License as well)
 
 { config, lib, pkgs, ... }:
 
 with lib;
 
+let
+  cfg = config.networking;
+
+  localhostMultiple = any (elem "localhost") (attrValues (removeAttrs cfg.hosts [ "127.0.0.1" "::1" ]));
+in
+
 {
 
   ###### interface
 
-  options = { };
+  options = {
+
+    networking.hosts = lib.mkOption {
+      type = types.attrsOf (types.listOf types.str);
+      default = { };
+      example = literalExpression ''
+        {
+          "127.0.0.1" = [ "foo.bar.baz" ];
+          "192.168.0.2" = [ "fileserver.local" "nameserver.local" ];
+        };
+      '';
+      description = lib.mdDoc ''
+        Locally defined maps of hostnames to IP addresses.
+      '';
+    };
+
+    networking.hostFiles = lib.mkOption {
+      type = types.listOf types.path;
+      defaultText = literalMD "Hosts from {option}`networking.hosts` and {option}`networking.extraHosts`";
+      example = literalExpression ''[ "''${pkgs.my-blocklist-package}/share/my-blocklist/hosts" ]'';
+      description = lib.mdDoc ''
+        Files that should be concatenated together to form {file}`/etc/hosts`.
+      '';
+    };
+
+    networking.extraHosts = lib.mkOption {
+      type = types.lines;
+      default = "";
+      example = "192.168.0.1 lanlocalhost";
+      description = lib.mdDoc ''
+        Additional verbatim entries to be appended to {file}`/etc/hosts`.
+        For adding hosts from derivation results, use {option}`networking.hostFiles` instead.
+      '';
+    };
+
+  };
 
 
   ###### implementation
 
   config = {
 
+    assertions = [{
+      assertion = !localhostMultiple;
+      message = ''
+        `networking.hosts` maps "localhost" to something other than "127.0.0.1"
+        or "::1". This will break some applications. Please use
+        `networking.extraHosts` if you really want to add such a mapping.
+      '';
+    }];
+
+    networking.hostFiles =
+      let
+        localhostHosts = pkgs.writeText "localhost-hosts" ''
+          127.0.0.1 localhost
+          ::1 localhost
+        '';
+        stringHosts =
+          let
+            oneToString = set: ip: ip + " " + concatStringsSep " " set.${ip} + "\n";
+            allToString = set: concatMapStrings (oneToString set) (attrNames set);
+          in
+          pkgs.writeText "string-hosts" (allToString (filterAttrs (_: v: v != [ ]) cfg.hosts));
+        extraHosts = pkgs.writeText "extra-hosts" cfg.extraHosts;
+      in
+      mkBefore [ localhostHosts stringHosts extraHosts ];
+
     environment.etc = {
       # /etc/services: TCP/UDP port assignments.
       services.source = pkgs.iana-etc + "/etc/services";
@@ -23,10 +94,7 @@ with lib;
       protocols.source = pkgs.iana-etc + "/etc/protocols";
 
       # /etc/hosts: Hostname-to-IP mappings.
-      hosts.text = ''
-        127.0.0.1 localhost
-        ::1 localhost
-      '';
+      hosts.source = pkgs.concatText "hosts" cfg.hostFiles;
 
       "resolv.conf".text = ''
         nameserver 1.1.1.1

tests/on-device/config-flake-hosts-localhost.cfg.nix

@@ -0,0 +1,7 @@
+{ pkgs, ... }:
+
+{
+  system.stateVersion = "23.05";
+
+  networking.hosts."127.0.0.2" = [ "localhost" ];
+}

tests/on-device/config-flake-hosts.bats

@@ -0,0 +1,37 @@
+# Copyright (c) 2023, see AUTHORS. Licensed under MIT License, see LICENSE.
+
+load lib
+
+@test 'hosts can be configured' {
+  # set up / build / activate the configuration
+  cat "$ON_DEVICE_TESTS_DIR/config-flake-hosts.cfg.nix" \
+    > ~/.config/nixpkgs/nix-on-droid.nix
+  _sed "s|<<FLAKE_URL>>|$FLAKE_URL|g" \
+    "$ON_DEVICE_TESTS_DIR/config-flake.nix" \
+    > ~/.config/nixpkgs/flake.nix
+
+  nix-on-droid switch --flake ~/.config/nixpkgs#device
+
+  # check that /etc/hosts contains configured hosts
+  for entry in '::1 localhost' \
+               '127.0.0.1 localhost' \
+               '127.0.0.2 a b' \
+               '127.0.0.3 c' \
+               '127.0.0.4 d'
+  do
+    grep "$entry" /etc/hosts
+  done
+}
+
+@test 'hosts can not map localhost' {
+  # set up / build / activate the configuration
+  cat "$ON_DEVICE_TESTS_DIR/config-flake-hosts-localhost.cfg.nix" \
+    > ~/.config/nixpkgs/nix-on-droid.nix
+  _sed "s|<<FLAKE_URL>>|$FLAKE_URL|g" \
+    "$ON_DEVICE_TESTS_DIR/config-flake.nix" \
+    > ~/.config/nixpkgs/flake.nix
+
+  # check that networking.hosts can't map localhost
+  run nix-on-droid switch --flake ~/.config/nixpkgs#device
+  [ "$status" -eq 1 ]
+}

tests/on-device/config-flake-hosts.cfg.nix

@@ -0,0 +1,19 @@
+{ pkgs, ... }:
+
+{
+  system.stateVersion = "23.05";
+
+  networking = {
+    hosts."127.0.0.2" = [ "a" "b" ];
+
+    extraHosts = ''
+      127.0.0.3 c
+    '';
+
+    hostFiles = [
+      (pkgs.writeText "hosts" ''
+        127.0.0.4 d
+      '')
+    ];
+  };
+}