[SOLVED] TCP: Server sends [RST, ACK] immediately after receiving [SYN] from Client

Issue

Host_A tries to send some data to Host_B over TCP. Host_B is listening on port 8181. Both Host_A & Host_B are Linux boxes (Red Hat Enterprise). The TCP layer is implemented using Java NIO API.

Whatever Host_A sends, Host_B is unable to receive. Sniffing the data on wire using WireShark resulted in the following log:

1) Host_A (33253) > Host_B (8181): [SYN] Seq=0 Win=5840 Len=0 MSS=1460 TSV=513413781 TSER=0 WS=7
2) Host_B (8181) > Host_A (33253): [RST, ACK] Seq=1 Ack=1 Win=0 Len=0

The logs show that Host_A sends a [SYN] flag to Host_B in order to establish connection. But instead of [SYN, ACK] Host_B responds with an [RST, ACK] which resets/closes the connection. This behavior is observed always.

I am wondering under what circumstance does a TCP listener sends [RST,ACK] in response to a [SYN]?

Solution

RST, ACK means the port is closed. You sure Host_B is listening on the right IP/interface?

Also check your firewall for a -j REJECT --reject-with tcp-reset

Answered By - Erik

Answer Checked By - Dawn Plyler (WPSolving Volunteer)