feat(src): Add option to set the port for replies to the multicast

README.md

@@ -190,6 +190,13 @@ below for details.
  * `--metadata-timeout TIMEOUT`
      Set the timeout for HTTP-based metadata exchange. Default is 2.0 seconds.
 
+ * `--reply-port PORT`
+     Set the source port for outgoing multicast messages, so that replies will
+     use this as the destination port.
+     This is useful for firewalls that do not detect incoming unicast replies
+     to a multicast as part of the flow, so the port needs to be fixed in order
+     to be allowed manually.
+
  * `-s`, `--shortlog`
 
      Use a shorter logging format that only includes the level and message.

man/wsdd.8

@@ -54,6 +54,13 @@ For IPv6, only link local addresses are actually considered as noted above.
 \fB\-\-metadata-timeout\ \fITIMEOUT\fR
 Set the timeout for HTTP-based metadata exchange. Default is 2.0 seconds.
 .TP
+\fB\-\-reply-port\ \fPORT\fR
+Set the source port for outgoing multicast messages, so that replies will
+use this as the destination port.
+This is useful for firewalls that do not detect incoming unicast replies
+to a multicast as part of the flow, so the port needs to be fixed in order
+to be allowed manually.
+.TP
 \fB\-s\fR, \fB\-\-shortlog\fR
 Use a shorter logging format that only includes the level and message.
 This is useful in cases where the logging mechanism, like systemd on Linux,

src/wsdd.py

@@ -263,6 +263,13 @@ def init_v6(self) -> None:
         self.mc_send_socket.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_MULTICAST_HOPS, args.hoplimit)
         self.mc_send_socket.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_MULTICAST_IF, idx)
 
+        # bind multicast socket to interface address and a user-provided port (or random if unspecified)
+        # this allows not-so-smart firewalls to whitelist another port to allow incoming replies
+        try:
+            self.mc_send_socket.bind((str(self.address), args.reply_port, 0, idx))
+        except OSError:
+            logger.error('specified port {} already in use for {}'.format(args.reply_port, str(self.address)))
+
         self.listen_address = (self.address.address_str, WSD_HTTP_PORT, 0, idx)
 
     def init_v4(self) -> None:
@@ -292,6 +299,13 @@ def init_v4(self) -> None:
         self.mc_send_socket.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_LOOP, struct.pack('B', 0))
         self.mc_send_socket.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, struct.pack('B', args.hoplimit))
 
+        # bind multicast socket to interface address and a user-provided port (or random if unspecified)
+        # this allows not-so-smart firewalls to whitelist another port to allow incoming replies
+        try:
+            self.mc_send_socket.bind((self.address.address_str, args.reply_port))
+        except OSError:
+            logger.error('specified port {} already in use for {}'.format(args.reply_port, self.address.address_str))
+
         self.listen_address = (self.address.address_str, WSD_HTTP_PORT)
 
     def add_handler(self, socket: socket.socket, handler: INetworkPacketHandler) -> None:
@@ -1862,6 +1876,11 @@ def parse_args() -> None:
         '--metadata-timeout',
         help='set timeout for HTTP-based metadata exchange',
         default=2.0)
+    parser.add_argument(
+        '--reply-port',
+        help='recieve replies to multicast on this port',
+        type=int,
+        default=0)
 
     args = parser.parse_args(sys.argv[1:])