How to find the source ip and port of a client that wants to connect to a listening socket?

I am using a tcp socket on linux to listen for incoming tcp connections. the socket is blocking type.

I would like to find out what client is trying to connect to my socket before calling accept(). More specifically, I want to know the source IP and port.

Now, I have done some reading, and found people saying that this is not possible because the source IP address is not known (yet). I don't understand this. if a client notified the server that it wants to connect then it must have sent a SYN packet, which contains the information I want, right?

(source) Obtaining the source IP and port of an INADDR_ANY client socket before the TCP three-way handshake?

Perhaps they were referring to the client side.

As a server, I can select() or poll() the file descriptor to know how many clients want to connect, why can't I get their ip addresses and ports before accept()ing the connection?

I am sorry, my knowledge of networking is not so rich.

Thank you.

Solution

Thanks everyone for your answers. I figured out a workaround to my problem. so I figured I'd post it as a solution to my own question.

It is possible to use iptables and netfilter_queue under linux and filter out the SYN packets. One can then read the needed information from the packet and re-insert it into the stack without "accept"ing it!

I have tried this and it works. :)

Thanks, everyone, for your help.