add networking.hosts and .hostFiles from nixos

This code was taking nearly verbatim from
https://github.com/NixOS/nixpkgs/blob/nixos-unstable/nixos/modules/config/networking.nix

The few changes were related to making default /etc/hosts match the Apple's stock one. This implied
no 127.0.0.2 and forcing the IPV6 ::1 entry.

modules/networking/default.nix

@@ -21,6 +21,8 @@ let
       esac
     '') cfg.knownNetworkServices}
   '';
+
+  localhostMultiple = any (elem "localhost") (attrValues (removeAttrs cfg.hosts [ "127.0.0.1" "::1" ]));
 in
 
 {
@@ -94,9 +96,50 @@ in
       default = [];
       description = "The list of search paths used when resolving domain names.";
     };
+
+    networking.hosts = lib.mkOption {
+      type = types.attrsOf (types.listOf types.str);
+      example = literalExpression ''
+        {
+          "127.0.0.1" = [ "foo.bar.baz" ];
+          "192.168.0.2" = [ "fileserver.local" "nameserver.local" ];
+        };
+      '';
+      description = ''
+        Locally defined maps of hostnames to IP addresses.
+      '';
+      default = {};
+    };
+
+    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 = ''
+        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 = ''
+        Additional verbatim entries to be appended to {file}`/etc/hosts`.
+        For adding hosts from derivation results, use {option}`networking.hostFiles` instead.
+      '';
+    };
   };
 
   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.
+      '';
+    }];
 
     warnings = [
       (mkIf (cfg.knownNetworkServices == [] && cfg.dns != []) "networking.knownNetworkServices is empty, dns servers will not be configured.")
@@ -119,5 +162,32 @@ in
       ${setNetworkServices}
     '';
 
+    networking.hostFiles = let
+      # Note: localhostHosts has to appear first in /etc/hosts so that 127.0.0.1
+      # resolves back to "localhost" (as some applications assume) instead of
+      # the FQDN!
+      localhostHosts = pkgs.writeText "localhost-hosts" ''
+        ##
+        # Host Database
+        #
+        # localhost is used to configure the loopback interface
+        # when the system is booting.  Do not change this entry.
+        ##
+        127.0.0.1	localhost
+        255.255.255.255	broadcasthost
+        ::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.hosts = {
+      copy = true;
+      source = pkgs.concatText "hosts" cfg.hostFiles;
+    };
   };
 }

release.nix

@@ -109,6 +109,7 @@ let
     tests.launchd-daemons = makeTest ./tests/launchd-daemons.nix;
     tests.launchd-setenv = makeTest ./tests/launchd-setenv.nix;
     tests.networking-hostname = makeTest ./tests/networking-hostname.nix;
+    tests.networking-hosts = makeTest ./tests/networking-hosts.nix;
     tests.networking-networkservices = makeTest ./tests/networking-networkservices.nix;
     tests.nixpkgs-overlays = makeTest ./tests/nixpkgs-overlays.nix;
     tests.programs-ssh = makeTest ./tests/programs-ssh.nix;

tests/networking-hosts.nix

@@ -0,0 +1,16 @@
+{ config, pkgs, ... }:
+
+{
+  networking.hosts = {
+    "127.0.0.1" = [ "my.super.host" ];
+    "10.0.0.1" = [ "my.super.host" "my.other.host" ];
+  };
+
+  test = ''
+    echo checking /etc/hosts file >&2
+
+    file=${config.out}/etc/hosts
+    grep '127.0.0.1' $file
+    grep '10.0.0.1 my.super.host my.other.host' $file
+  '';
+}