CODEWITHSUNDEEP
socketsnetwork-programmingtcpudpport
Ali Yekta
Ali Yekta

Reputation: 45

UDP packets lost (LAN-Netcat) , switching server-client sides packets are OK

I was trying to run a game that uses UDP socket on LAN , but could not see the other computer in my network, so i tried somethings to see what can give rise to this issue. The running OS's are:

PC1: Linux Ubuntu
PC2: Windows 10

The issue is that when PC2 is listening , the PC1 can send and receive packets:

$PC2> ncat -ul 8888
$PC1> ncat -u PC2 8888
OK

Send and receive works OK, However when client/server is switched, the packets are lost. 

$PC1> ncat -ul 8888
$PC2> ncat -u PC1 8888

On a side note, on PC2 when trying to send the packet with Pythons socket.sendto function the return value is not -1 and is the size of the packet. Also turning of firewall on PC2 doesn't help.

This is weird problem since one-way works fine but switching the client/server the packets are loss.

Upvotes: 1

Views: 249

Answers (1)

artem
artem

Reputation: 36

One of the hosts (PC1) might be behind a NAT, while the other (PC2) is in a public network. It should be clear if it is the case, if you provide ip addresses of the hosts. Presence of NAT would explain why switching client and server breaks.

When a packet is sent from PC1 to PC2, NAT would add an ip translation record of the form (PC1 internal ip address, port1) -- (PC1 public ip address, port2), and translate internal address to public and vice versa whenever needed.

When PC1 becomes a server, there is no way to reach it from PC2 through the NAT. Unless you configure port forwarding of the NAT or use some third-party NAT discovery services.

NAT port forwarding works in the following manner. If a port forward rule (PC1 public ip address, port2) -- (PC1 internal ip address, port1) is specified, all packets send to (PC1 public ip address, port2) are forwarded to (PC1 internal ip address, port1).

Upvotes: 2

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