What can be the reasons of connection refused errors?

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

1. The port is not open on the destination machine.

2. The port is open on the destination machine, but its backlog of pending connections is full.

3. A firewall between the client and server is blocking access (also check local firewalls).

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.

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 requests 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.

If you try to open a TCP connection to another host and see the error "Connection refused," it means that

1. You sent a TCP SYN packet to the other host.
2. Then you received a TCP RST packet in reply.

RST is a bit on the TCP packet which indicates that the connection should be reset. Usually it means that the other host has received your connection attempt and is actively refusing your TCP connection, but sometimes an intervening firewall may block your TCP SYN packet and send a TCP RST back to you.

See https://tools.ietf.org/html/rfc793 page 69:

SYN-RECEIVED STATE

If the RST bit is set

If this connection was initiated with a passive OPEN (i.e., came from the LISTEN state), then return this connection to LISTEN state and return. The user need not be informed. If this connection was initiated with an active OPEN (i.e., came from SYN-SENT state) then the connection was refused, signal the user "connection refused". In either case, all segments on the retransmission queue should be removed. And in the active OPEN case, enter the CLOSED state and delete the TCB, and return.