What are TCP Flags

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References to TCP flags may be found in various places on pfSense, primarily in the firewall logs. These are control bits that indicate different connection states or information about how a packet should be handled.

They may be abbreviated even more to the first letter of the flag.

A list from Wikipedia:

  • CWR – Congestion Window Reduced (CWR) flag is set by the sending host to indicate that it received a TCP segment with the ECE flag set (added to header by RFC 3168).
  • ECE (ECN-Echo) – indicate that the TCP peer is ECN capable during 3-way handshake (added to header by RFC 3168).
  • URG – indicates that the URGent pointer field is significant
  • ACK – indicates that the ACKnowledgment field is significant (Sometimes abbreviated by tcpdump as ".")
  • PSH – Push function
  • RST – Reset the connection (Seen on rejected connections)
  • SYN – Synchronize sequence numbers (Seen on new connections)
  • FIN – No more data from sender (Seen after a connection is closed)