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TCP traffic SYN/ACK packets that contain window scaling options

asked 2022-03-18 23:40:44 +0000

tlm gravatar image

Hello, in your opinion how can I filter TCP traffic SYN/ACK packets that does contain window scaling options?

Can I use !(window_size_scalefactor == -2)?

(tcp.flags.syn==1 && tcp.flags.ack==1) && !(tcp.window_size_scalefactor == -2)

Window size scaling factor: -1 (unknown, start of session not captured) Window size scaling factor: -2 (no window scaling used)

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Is Window Scale Kind 3? Can I filter TCP traffic SYN/ACK packets that does contain window scaling options this way?

tcp.option_kind == 3 && tcp.flags.syn==1 && tcp.flags.ack==1

tlm gravatar imagetlm ( 2022-03-18 23:52:59 +0000 )edit

dfilter: Add bitwise masking of bits
When complete, you could streamline the flag check into tcp.flags & 0x012 == 0x012.
I'm not sure that's easier to read but more compact.

Chuckc gravatar imageChuckc ( 2022-03-22 00:44:00 +0000 )edit

@Chuckctcp.flags&18==18 is even more compact! ;-) I'm looking forward to this filter functionality in the next (major) release!

SYN-bit gravatar imageSYN-bit ( 2022-03-24 07:18:40 +0000 )edit

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answered 2022-03-19 00:50:18 +0000

Chuckc gravatar image

Check the Display Filter Reference for TCP fields.

tcp.options.wscale.shift is the option value in the packet.
tcp.options.wscale.multiplier is the Wirehark generated value for the multiplier.

Do you want to know if the options exists:

tcp.options.wscale.shift && tcp.flags.syn==1 && tcp.flags.ack==1

Or that it affects the window size:

(tcp.options.wscale.shift > 0) && tcp.flags.syn==1 && tcp.flags.ack==1
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Chuckc, I wanted know if the options exists. Thank you for confirming that.

tlm gravatar imagetlm ( 2022-03-19 02:00:21 +0000 )edit

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Asked: 2022-03-18 23:40:44 +0000

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Last updated: Mar 19 '22