c++ - TCP simultaneous open and self connect prevention

Question

TCP standard has "simultaneous open" feature.

The implication of the feature, client trying to connect to local port, when the port is from ephemeral range, can occasionally connect to itself (see here).

So client think it's connected to server, while it actually connected to itself. From other side, server can not open its server port, since it's occupied/stolen by client.

I'm using RHEL 5.3 and my clients constantly tries to connect to local server. Eventually client connects to itself.

I want to prevent the situation. I see two possible solutions to the problem:

1. Don't use ephemeral ports for server ports. Agree ephemeral port range and configure it on your machines (see ephemeral range)
2. Check connect() as somebody propose here.

What do you thinks? How do you handle the issue?

P.S. 1

Except of the solution, which I obviously looking for, I'd like you to share your real life experience with the problem.

When I found the cause of the problem, I was "astonished" on my work place people are not familiar with it. Polling server by connecting it periodically is IMHO common practice, so how it's that the problem is not commonly known.

Answer 1

When I stumbled into this I was flabbergasted. I could figure out that the outgoing port number accidentally matches the incoming port number, but not why the TCP handshake (SYN SYN-ACK ACK) would succeed (ask yourself: who is sending the ACK if there is nobody doing a listen() and accept()???)

Both Linux and FreeBSD show this behavior.

Anyway, one solution is to stay out of the high range of port numbers for servers.

I noticed that Darwin side-steps this issue by not allowing the outgoing port to be the same as the destination port. They must have been bitten by this as well...

An easy way to show this effect is as follows:

while true
do
    telnet 127.0.0.1 50000 
done

And wait for a minute or so and you will be chatting with yourself...

Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
hello?
hello?

Anyway, it makes good job interview material.