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How do i filter TCP connection with [SYN] and [SYN, ACK ] without [ACK ] response?


I want to identify SYN FLOOD attacks in my Packet trace (TCP) file by applying a Wireshark filter command that is capable of filtering out TCP connections that completed only 2WAY handshake without [ACK ] response. But I don't the command to use.

Thank in anticipation

asked 24 Apr '17, 00:34

moronto's gravatar image

accept rate: 0%

One Answer:


That's not an easy task because Wireshark can't filter on packet dependencies between multiple packets without some tricks. What I would do is try this filter:

(tcp.flags==0x12) and not tcp.analysis.initial_rtt

"tcp.flags==0x12" looks for SYN/ACK packets (you could also use "tcp.flags.syn==1 and tcp.flags.ack==1", or, if you want SYN and SYN/ACK, use "tcp.flags.syn==1 or (tcp.flags.syn==1 and tcp.flags.ack==1)".

The trick is using "not tcp.analysis.initial_rtt", because that checks if Wireshark calculcated the initial round trip time for the conversation - and that's something it only does if the handshake is complete. So if the field is missing, and the SYN/ACK was seen, you have a half open connection (assuming the SYN is there). Note that the filter is not checking for an actual iRTT value, which it would do with a double equal operator (e.g. "tcp.analysis.initial_rtt==0.12345"), but if the field exists at all.

answered 24 Apr '17, 00:46

Jasper's gravatar image

Jasper ♦♦
accept rate: 18%

edited 24 Apr '17, 00:49

Thanks Jasper, your comment really solved the problem.

(24 Apr '17, 08:54) moronto

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(24 Apr '17, 10:05) grahamb ♦