nixos/dnscache: add module

with improvements suggested by Jörg Thalheim <joerg@thalheim.io>

nixos/modules/module-list.nix

@@ -419,6 +419,7 @@
   ./services/networking/ddclient.nix
   ./services/networking/dhcpcd.nix
   ./services/networking/dhcpd.nix
+  ./services/networking/dnscache.nix
   ./services/networking/dnschain.nix
   ./services/networking/dnscrypt-proxy.nix
   ./services/networking/dnscrypt-wrapper.nix

nixos/modules/services/networking/dnscache.nix

@@ -0,0 +1,86 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.dnscache;
+
+  dnscache-root = pkgs.runCommand "dnscache-root" {} ''
+    mkdir -p $out/{servers,ip}
+
+    ${concatMapStrings (ip: ''
+      echo > "$out/ip/"${lib.escapeShellArg ip}
+    '') cfg.clientIps}
+
+    ${concatStrings (mapAttrsToList (host: ips: ''
+      ${concatMapStrings (ip: ''
+        echo ${lib.escapeShellArg ip} > "$out/servers/"${lib.escapeShellArg host}
+      '') ips}
+    '') cfg.domainServers)}
+
+    # djbdns contains an outdated list of root servers;
+    # if one was not provided in config, provide a current list
+    if [ ! -e servers/@ ]; then
+      awk '/^.?.ROOT-SERVERS.NET/ { print $4 }' ${pkgs.dns-root-data}/root.hints > $out/servers/@
+    fi
+  '';
+
+in {
+
+  ###### interface
+
+  options = {
+    services.dnscache = {
+      enable = mkOption {
+        default = false;
+        type = types.bool;
+        description = "Whether to run the dnscache caching dns server";
+      };
+
+      ip = mkOption {
+        default = "0.0.0.0";
+        type = types.str;
+        description = "IP address on which to listen for connections";
+      };
+
+      clientIps = mkOption {
+        default = [ "127.0.0.1" ];
+        type = types.listOf types.str;
+        description = "client IP addresses (or prefixes) from which to accept connections";
+        example = ["192.168" "172.23.75.82"];
+      };
+
+      domainServers = mkOption {
+        default = { };
+        type = types.attrsOf (types.listOf types.str);
+        description = "table of {hostname: server} pairs to use as authoritative servers for hosts (and subhosts)";
+        example = {
+          "example.com" = ["8.8.8.8" "8.8.4.4"];
+        };
+      };
+    };
+  };
+
+  ###### implementation
+
+  config = mkIf config.services.dnscache.enable {
+    environment.systemPackages = [ pkgs.djbdns ];
+    users.extraUsers.dnscache = {};
+
+    systemd.services.dnscache = {
+      description = "djbdns dnscache server";
+      wantedBy = [ "multi-user.target" ];
+      path = with pkgs; [ bash daemontools djbdns ];
+      preStart = ''
+        rm -rf /var/lib/dnscache
+        dnscache-conf dnscache dnscache /var/lib/dnscache ${config.services.dnscache.ip}
+        rm -rf /var/lib/dnscache/root
+        ln -sf ${dnscache-root} /var/lib/dnscache/root
+      '';
+      script = ''
+        cd /var/lib/dnscache/
+        exec ./run
+      '';
+    };
+  };
+}