TCP Connect error 115 Operation in Progress What is the Cause?

Question

My application creats a TCP connection, This is working normaly. But in one network server has many IP say

• 174.X.X.X
• 54.x.x.x like this

Answer 1

This seems to be the behaviour of connect():

If the connection cannot be established immediately and O_NONBLOCK is set for the file descriptor for the socket, connect() shall fail and set errno to [EINPROGRESS], but the connection request shall not be aborted, and the connection shall be established asynchronously. Subsequent calls to connect() for the same socket, before the connection is established, shall fail and set errno to [EALREADY].

Answer 2

Based on your information:

• You are trying to do a connect() to 54.x.x.x
• The socket is non-blocking
• Connection timeout is 60 sec

First, if you look into your /usr/include/asm-generic/errno.h you'll see the following:

#define EINPROGRESS     115     /* Operation now in progress */

It means an existing operation on the socket is in progress. Since, you said you are doing a connect() call, lets do a man connect:

EINPROGRESS

The socket is nonblocking and the connection cannot be completed 
immediately. It is possible to select(2) or poll(2) for completion by
selecting the socket for writing. After select(2) indicates
writability, use getsockopt(2) to read the SO_ERROR option at level
SOL_SOCKET to determine whether connect() completed successfully
(SO_ERROR is zero) or unsuccessfully (SO_ERROR is one of the usual
error codes listed here, explaining the reason for the failure).

So, the best guess would be that the TCP 3-way handshake (your connect() call to 54.x.x.x IP address) is taking longer than expected to complete. Since the connect() operation is already in progress, any subsequent operation on the socket is resulting into EINPROGRESS error code. As suggested in the man page, try to use select() or poll() to check if your socket is ready to use (to perform read() or write() calls).

You can pin-point what is preventing your TCP handshake to complete by capturing and analyzing the traffic to/from your own machine and 54.x.x.x. The best tool to help you with this is called WireShark. Good luck.