Python socket.connect hangs sometimes

Question

I'm learning to use sockets in python and something weird is happening.

I call socket.connect in a try block, and typically it either completes and I have a new socket connection, or it raises the exception. Sometimes, however, it just hangs.

I don't understand why sometimes it returns (even without connecting!) and other times it just hangs. What makes it hang?

I am using blocking sockets (non-blocking don't seem to work for connect...), so I've added a timeout, but I'd prefer connect to finish without needing to timeout.

Perhaps, when it doesn't hang, it receives a response that tells it the requested ip/port is not available, and when it does hang there is just no response from the other end?

I'm on OSX10.8 using python2.7

Answer 1

When connect() hangs it is usually because you connect to an address that is behind a firewall and the firewall just drops your packets with no response. It keeps trying to connect for around 2 minutes on Linux and then times out and return an error.

Answer 2

Firewall may be the explanation behind this unexpected response. Rather than supposing the remote firewall accepts connection, using timeout is the best option. Since, making a connection is a swift process and within a network, it won't take longer time. So, give a proper timeout so that you can tell that the host is either down or dropping packets.