Returned UDP packets lacking port and fail to arrive

0
votes

Golang application with a client and server.

Server uses net.ListenUDP -- client also uses net.ListenUDP, connects to server and sends a packet with conn.WriteToUDP with the server address.

Server receives the packet with ReadFromUDP and grabs the return address. Using this return address, it then sends a large number of packets back to the client.

When running both client and server on local machine, this works perfectly. Using Wireshark I can inspect the UDP packets and see that they contain the source and destination ports - and in the application I can see that they arrive and my various checksum tests show the data is accurate.

I then moved the server off site to a remote machine. The application stops working. I can successfully send the first message from the client to server - this is received just fine. The server sends the response back 'toward' the client - but the client never receives them.

Using Wireshark, I can see that the packets do arrive back on the local machine with the correct IP address. It appears that my network router has performed NAT on the outgoing packets - and has correctly re-addressed response packets to the internal IP.. BUT there is no port.

So I have UDP packets arriving on the correct machine, but no port - so the client application does not receive them. Application times out on ReadFromUDP.

I don't know if it is relevant, but on local machine, Wireshark labels the packets as BT-uTP Utorrent packets. When they come in from remote server, this is what I see in Wireshark - note the lack of Port.

screenshot of wireshark data

Any thoughts how I can solve this. I didn't think this was a UDP hole punching problem because although I am establishing a connection across a NAT it is with a server not a peer.

1
socketsgonetworkingudp
@frog2000 is correct, the problem was packet fragmentation and nothing to do with hole punching. Making sure packet size did not exceed MTU fixed the problem. – richp10

1 Answers

5
votes

This packet is fragmented, You can see this under Internet Protocol Version 4 > Flags.

If you look at the frame as shown on the bottom of the picture you provided you should see the ports.

net.ListenUDP doesn't appear to support fragmentation at the socket level.

Do you have a PPPoe connection? You may need to reduce your packet size being sent by 8 bytes or change the MTU on the routers external interface of the remote side. You may also need to change the local routers MTU if it's on a PPPoe interface.