Flags TCP IP for Pentesters

TCP Header

Flags TCP IP

TCP is a connection-oriented protocol, running on Layer 4 of the OSI Model. In order to achieve is base functionality TCP have some options on its packet header to verify the connection state, flow control and guaranty reliability. The header field responsible for control is the TCP Flags, they have 9 bits of reserved space and each flag is 1 bit of information.

TCP Synchronization Flag (Syn)

Synchronization (SYN) is used to initiate the connection negotiation 3-Way Handshake between two hosts. Syn flag is responsible to synchronize the sequence numbers between hosts.

TCP Syn Flag
TCP Syn Flag

After receiving a Syn Flag the other host will reply with a Syn-Ack

TCP Acknowledgement Flag (Ack)

Acknowledgement (ACK) is used to acknowledge the received packet contains a valid acknowledgement number.

TCP ACK Flag
TCP ACK Flag

TCP Reset Flag (Rst)

TCP RST Flags
TCP RST Flags

TCP Finish Flag (Fin)

Differences between Reset and Finish Flags

TCP Urgent Flag (Urg)

TCP Push Flag (Psh)

Differences between Urgent and Push Flags

Flags TCP IP

Source: Netgate Docs

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