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ERIC BRUHN

Network Applications and Network Applications Lab

TCPdump Flags

 

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Flag Symbol

TCP Flag

Flag Meaning

.

Placeholder

If the connection does not have a SYN, FIN, RESET, or PUSH flag set, a placeholder will be found after the destination port.

ack

ACK

This flag is used generally to acknowledge the receipt of data from the sender. This may be piggybacked with other flags.

F

FIN

This flag indicates the sender's intention to gracefully terminate the sending host's connection to the receiving host.

P

PUSH

This flag immediately "pushes" data from the sending host to the receiving host's application software. There is no waiting for the buffer to fill up.

R

RESET

This flag indicates the sender's intention to immediately abort the existing connection with the receiving host.

S

SYN

This is a session establishment request, which is the first part of any TCP connection.

urg

URGENT

This flag indicates that there is "urgent" data that should take precedence over other data. An example of this is pressing Ctrl + C to abort an FTP download.

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