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TCP Client Self-Connect (2013)

13 days ago
http://sgros.blogspot.com/2013/08/tcp-client-self-connect.html
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  • #TCP
  • #Networking
  • #Linux
  • A telnet client can connect to itself when the source port matches the destination port (50000 in this example).
  • This behavior is due to the TCP specification supporting 'simultaneous open', allowing a connection to be established even if it's the same endpoint.
  • The Linux kernel assigns ephemeral ports sequentially, which can lead to a client attempting to connect to its own port.
  • The TCP state machine transitions through SYN_SENT, SYN_RECEIVED, and finally ESTABLISHED during a self-connect scenario.
  • Self-connect requires specific conditions: using ephemeral ports for servers, client and server on the same IP, and occurring during the initial handshake phase.
  • Avoid using ephemeral ports for servers to prevent unpredictable behavior.
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