Add module for rsyslog. Although rsyslog is supposed to be a drop-in

replacement for sysklogd, it lacks some support for certain arguments
used in the default syslog module of NixOS.

modules/module-list.nix

@@ -86,6 +86,7 @@
   ./services/logging/logrotate.nix
   ./services/logging/logstash.nix
   ./services/logging/syslogd.nix
+  ./services/logging/rsyslogd.nix
   ./services/mail/dovecot.nix
   ./services/mail/freepops.nix
   ./services/mail/mail.nix

modules/services/logging/rsyslogd.nix

@@ -0,0 +1,105 @@
+{ config, pkgs, ... }:
+
+with pkgs.lib;
+
+let
+
+  cfg = config.services.rsyslogd;
+
+  syslogConf = pkgs.writeText "syslog.conf" ''
+    $ModLoad imuxsock
+    $SystemLogSocketName /run/systemd/journal/syslog
+
+    ${cfg.defaultConfig}
+    ${cfg.extraConfig}
+  '';
+
+  defaultConf = ''
+    # "local1" is used for dhcpd messages.
+    local1.*                     -/var/log/dhcpd
+
+    mail.*                       -/var/log/mail
+
+    *.=warning;*.=err            -/var/log/warn
+    *.crit                        /var/log/warn
+
+    *.*;mail.none;local1.none    -/var/log/messages
+  '';
+
+in
+
+{
+  ###### interface
+
+  options = {
+
+    services.rsyslogd = {
+
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether to enable syslogd.  Note that systemd also logs
+          syslog messages, so you normally don't need to run syslogd.
+        '';
+      };
+
+      defaultConfig = mkOption {
+        type = types.string;
+        default = defaultConf;
+        description = ''
+          The default <filename>syslog.conf</filename> file configures a
+          fairly standard setup of log files, which can be extended by
+          means of <varname>extraConfig</varname>.
+        '';
+      };
+
+      extraConfig = mkOption {
+        type = types.string;
+        default = "";
+        example = "news.* -/var/log/news";
+        description = ''
+          Additional text appended to <filename>syslog.conf</filename>,
+          i.e. the contents of <varname>defaultConfig</varname>.
+        '';
+      };
+
+      extraParams = mkOption {
+        type = types.listOf types.string;
+        default = [ ];
+        example = [ "-m 0" ];
+        description = ''
+          Additional parameters passed to <command>rsyslogd</command>.
+        '';
+      };
+
+    };
+
+  };
+
+
+  ###### implementation
+
+  config = mkIf cfg.enable {
+
+    environment.systemPackages = [ pkgs.rsyslog ];
+
+    systemd.services.syslog =
+      { description = "Syslog Daemon";
+
+        requires = [ "syslog.socket" ];
+
+        wantedBy = [ "multi-user.target" "syslog.target" ];
+
+        environment.TZ = config.time.timeZone;
+
+        serviceConfig =
+          { ExecStart = "${pkgs.rsyslog}/sbin/rsyslogd ${toString cfg.extraParams} -f ${syslogConf} -n";
+            # Prevent syslogd output looping back through journald.
+            StandardOutput = "null";
+          };
+      };
+
+  };
+
+}