TCP Flags Explained

TCP Flags

TCP has six flags that can help you troubleshoot a connection. The flags are:

U – URG
A – ACK
P – PSH
R – RST
S – SYN
F – FIN

When using tcpdump command to troubleshoot network connections, you can view TCP conversations with these flags as follows:

# tcpdump 'tcp[13] & 32 != 0' #URG
# tcpdump 'tcp[13] & 16 != 0' #ACK
# tcpdump 'tcp[13] & 8  != 0' #PSH
# tcpdump 'tcp[13] & 4  != 0' #RST
# tcpdump 'tcp[13] & 2  != 0' #SYN
# tcpdump 'tcp[13] & 0  != 0' #FIN

Another way of expressing the values is ‘tcp-fin, tcp-syn, tcp-rst, tcp-push, tcp-act, tcp-urg.’.

# tcpdump 'tcp[tcpflags] & tcp-urg != 0' #URG
# tcpdump 'tcp[tcpflags] & tcp-ack != 0' #ACK
# tcpdump 'tcp[tcpflags] & tcp-push != 0' #PSH
# tcpdump 'tcp[tcpflags] & tcp-rst != 0' #RST
# tcpdump 'tcp[tcpflags] & tcp-sync != 0' #SYN
# tcpdump 'tcp[tcpflags] & tcp-fin != 0' #FIN

A mnemonic to remember the above is ‘Unskilled Attackers Pester Real Security Folks’.

URG flag is used to indicate that the packet should be prioritized over other packets for processing.
This flag is not used often. I can only think of telnet that uses it.

#  tcpdump 'tcp[13] & 32 != 0'
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on br0, link-type EN10MB (Ethernet), capture size 65535 bytes
18:29:47.741814 IP destination > source.39697: Flags [P.U], seq 2342494158:2342494159, ack 1232430662, win 31, urg 1, options [nop,nop,TS val 877959588 ecr 703327131], length 1
18:29:51.293145 IP destination.telnet > source.39697: Flags [P.U], seq 12:13, ack 5, win 31, urg 1, options [nop,nop,TS val 877963140 ecr 703330673], length 1

SYN is used for starting a connection.
ACK is used to acknowledge packets received.
PSH is used to ask the receiving end not to buffer packets, but to process them as soon as they are received.
RST is used to denote that no service is listening on the given port.

# tcpdump -n -v 'tcp[tcpflags] & (tcp-rst) != 0'
tcpdump: listening on br0, link-type EN10MB (Ethernet), capture size 65535 bytes
18:44:11.087487 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 40)
    destination.ms-sql-s > source.x11: Flags [R.], cksum 0xb12d (correct), seq 0, ack 1435238401, win 0, length 0

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