What can be the reasons of connection refused errors?

Question

Hello,

I'm trying to write a server program in C, using another client, I get this error when I try to connect through port 2080 for example.

connection refused

What can be the reasons of this error?

Answer 1

There could be many reasons, but the most common are:

1. Firewall blocking access between the machines (also check local firewalls)
2. Port not open on the destination machine

After checking for firewalls and that the port is open, use telnet to connect to the ip/port to test connectivity. This removes any potential issues from your application.

Answer 2

Connection refused means that the port you are trying to connect to is not actually open.

So either you are connecting to the wrong IP address, or to the wrong port, or the server is listening on the wrong port, or is not actually running.

A common mistake is not specifying the port number when binding or connecting in network byte order...

Answer 3

Check at the server side that it is listening at the port 2080. First try to confirm it on the server machine by issuing telnet to that port:

telnet localhost 2080

If it is listening, it is able to respond.

Answer 4

The error means the OS of the listening socket recognized the inbound connection request but chose to intentionally reject it. Assuming an intermediate firewall is not getting in the way, there are only two reasons (that I know of) for the OS to reject an inbound connection request. One reason has already been mentioned several times - the listening port being connected to is not open. There is another reason that has not been mentioned yet - the listening port is actually open and actively being used, but its backlog of queued inbound connection requeusts has reached its maximum so there is no room available for the inbound connection request to be queued at that moment. The server code has not called accept() enough times yet to finish clearing out available slots for new queue items. Wait a moment or so and try the connection again. Unfortunately, there is no way to differentiate between "the port is not open at all" and "the port is open but too busy right now". They both use the same generic error code.