TCP port state check for an application.

Question

I have chat application developed in vb.net . It is used to chat between PC's which are connected in LAN network inside a office. It uses TCP/IP port 25025 to connect to another. The app works fine . But in some cases receiver won't get the chat message.

So I just run the netstat -an command in that pc and find so many tcp ports and its state. Below is a part of it (error case). I have shown only lines which has 25025 in it.

Proto  Local Address          Foreign Address        State
TCP    0.0.0.0:25025          0.0.0.0:0              LISTENING
TCP    192.168.1.79:25025     192.168.1.60:1320      TIME_WAIT
TCP    192.168.1.79:25025     192.168.1.60:1321      TIME_WAIT
TCP    192.168.1.79:58508     192.168.1.60:25025     TIME_WAIT
TCP    192.168.1.79:58509     192.168.1.60:25025     TIME_WAIT
TCP    192.168.1.79:58510     192.168.1.60:25025     TIME_WAIT
TCP    192.168.1.79:58511     192.168.1.60:25025     ESTABLISHED

Then i checked the same command where i didn't get any error with my app (proper working of my app). The output was,

TCP    192.168.1.60:25025     192.168.1.79:58511     ESTABLISHED

So how can i troubleshoot it? What does this so many port with 25025 indicate. In the error case i have lot of 25025 port as above shown. So please help me in understanding this and solve the problem.

Answer 1

Check to see whether the bind() call is succeeding or not. My guess is that when your application binds to the listening port it fails with the error "address already in use". The TIME_WAIT lines in the netstat output suggest this is so. But I'm guessing that your application isn't checking the return value from bind() and is blindly continuing assuming that the call succeeded. This would explain why it never receives anything.

You get "address already in use" if the socket has not completed its shutdown from an previous invocation of the application. Typically it takes about 4 minutes for the socket to be ready to be reused after it has been closed by the application, and in the meantime the state is TIME_WAIT.

You could use the SO_REUSEADDR socket option to avoid this TIME_WAIT period.