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Why will a TCP Server send a FIN immediately after accepting a connection?

From the ethreal packet capture, I see the following behaviour which appears quite strange to me:

Client --> Server  [SYN]
Server --> Client  [SYN, ACK]
Client --> Server  [ACK]
Server --> Client  [FIN, ACK]
Client --> Server  [ACK]
Client --> Server  [TCP Segment of a reassembled PDU] (I don't know what this means)
Server --> Client  [RST]

Any ideas as to why this could be happening?

Also, the Server Port is 6000. Could that cause any problem?

My other doubts:

  1. Why is there a FIN, ACK? Shouldn't it be only FIN? What is the meaning of the ACK in that message?
  2. Shouldn't there be a FIN from Client also?

EDIT: After some more analysis, I found if the number of file descriptors have exceeded the limit then a FIN is sent by the Server. But, in this case it doesn't appear that the file descriptors have exceeded the limit. For what other scenarios can this happen?

Solution

  • FIN usually means the other side called shutdown(..) on the socket.