nixos/mosquitto: add missing listener option bind_interface

we expose it under settings instead of at the listener toplevel because
mosquitto seems to pick the addresses it will listen on
nondeterministically from the set of addresses configured on the
interface being bound to. encouraging its use by putting it into the
toplevel options for a listener seems inadvisable.

nixos/modules/services/networking/mosquitto.nix

@@ -199,6 +199,7 @@ let
     allow_anonymous = 1;
     allow_zero_length_clientid = 1;
     auto_id_prefix = 1;
+    bind_interface = 1;
     cafile = 1;
     capath = 1;
     certfile = 1;
@@ -629,9 +630,10 @@ in
                ]));
         RemoveIPC = true;
         RestrictAddressFamilies = [
-          "AF_UNIX"  # for sd_notify() call
+          "AF_UNIX"
           "AF_INET"
           "AF_INET6"
+          "AF_NETLINK"
         ];
         RestrictNamespaces = true;
         RestrictRealtime = true;

nixos/tests/mosquitto.nix

@@ -4,6 +4,7 @@ let
   port = 1888;
   tlsPort = 1889;
   anonPort = 1890;
+  bindTestPort = 1891;
   password = "VERY_secret";
   hashedPassword = "$7$101$/WJc4Mp+I+uYE9sR$o7z9rD1EYXHPwEP5GqQj6A7k4W1yVbePlb8TqNcuOLV9WNCiDgwHOB0JHC1WCtdkssqTBduBNUnUGd6kmZvDSw==";
   topic = "test/foo";
@@ -125,6 +126,10 @@ in {
               };
             };
           }
+          {
+            settings.bind_interface = "eth0";
+            port = bindTestPort;
+          }
         ];
       };
     };
@@ -134,6 +139,8 @@ in {
   };
 
   testScript = ''
+    import json
+
     def mosquitto_cmd(binary, user, topic, port):
         return (
             "mosquitto_{} "
@@ -162,6 +169,27 @@ in {
     start_all()
     server.wait_for_unit("mosquitto.service")
 
+    with subtest("bind_interface"):
+        addrs = dict()
+        for iface in json.loads(server.succeed("ip -json address show")):
+            for addr in iface['addr_info']:
+                # don't want to deal with multihoming here
+                assert addr['local'] not in addrs
+                addrs[addr['local']] = (iface['ifname'], addr['family'])
+
+        # mosquitto grabs *one* random address per type for bind_interface
+        (has4, has6) = (False, False)
+        for line in server.succeed("ss -HlptnO sport = ${toString bindTestPort}").splitlines():
+            items = line.split()
+            if "mosquitto" not in items[5]: continue
+            listener = items[3].rsplit(':', maxsplit=1)[0].strip('[]')
+            assert listener in addrs
+            assert addrs[listener][0] == "eth0"
+            has4 |= addrs[listener][1] == 'inet'
+            has6 |= addrs[listener][1] == 'inet6'
+        assert has4
+        assert has6
+
     with subtest("check passwords"):
         client1.succeed(publish("-m test", "password_store"))
         client1.succeed(publish("-m test", "password_file"))