linux - How can you have a TCP connection back to the same port?

Question

Machine is RHEL 5.3 (kernel 2.6.18).

Some times I notice in netstat that my application has connection, established TCP connection when Local Address and Foreign Address are same.

Here same problem reported by someone else too.

The symptoms are same as described in link - client connects to port X port of server running locally. After some time netstat shows that client has connection from 127.0.0.1:X to 127.0.0.1:X

How it's possible?

Edit 01

Simultaneous open is causing the problem (thanks a lot to Hasturkun). You can see it on classical TCP state diagram in transition from SYN_SENT state to SYNC_RECEIVED

Answer 1

This may be caused by TCP simultaneous connect (mentioned on this post to LKML, see also here).

It's possible for a program looping on trying to connect to a port within the dynamic local port range (which can be seen in /proc/sys/net/ipv4/ip_local_port_range),to succeed while the server is not listening on that port.

On a large enough number of attempts, the socket being used to connect may be bound to the same port being connected to, which succeeds due to previously mentioned simultaneous connect. You now magically have a client connected to itself