title: TCP: Server sends [RST, ACK] immediately after receiving [SYN] from Client
body: <p>Host_A tries to send some data to Host_B over TCP. Host_B is listening on port 8181. Both Host_A &amp; Host_B are Linux boxes (Red Hat Enterprise). The TCP layer is implemented using Java NIO API. </p>
  
  <p>Whatever Host_A sends, Host_B is unable to receive. Sniffing the data on wire using WireShark resulted in the following log:</p>
  
  <blockquote>
    <p>1) Host_A (33253) > Host_B (8181): [SYN] Seq=0 Win=5840 Len=0 MSS=1460 TSV=513413781 TSER=0 WS=7<br>
    2) Host_B (8181) > Host_A (33253): [RST, ACK] Seq=1 Ack=1 Win=0 Len=0  </p>
  </blockquote>
  
  <p>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.</p>
  
  <p>I am wondering under what circumstance does a TCP listener sends [RST,ACK] in response to a [SYN]?</p>