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SYN,ACK not followed by ACK response (noob question)

asked 2019-06-20 15:12:00 +0000

FenyX gravatar image

updated 2019-06-21 07:45:11 +0000

Hi everyone.

While attempting to diagnose an SFTP connection between a remote client and our server when the client was trying to establish a connection to the server, our sysadmin had a quick look to the Wireshark output and told us immediately that the issue was caused by the client and not the server (and it seemed so obvious to him!).

And as I love to understand things even if I've got a very limited network theoretical knowledge (I'm good at plugging cables on a patch panel, but hey I can't master everything! haha), I'd like to understand how he did identify that the issue came from the remote side 193.143.x.x. Could you please help to make it more clear to me?

link to screenshot

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answered 2019-06-20 15:24:14 +0000

grahamb gravatar image

Analysis by screenshot is always tricky, much better to share the capture.

Looks like the capture was taken at the server and the server respopnds to the income connection attempt SYN with a SYN, ACK, but the client fails to complete the TCP 3-way handshake with an ACK, instead the client resends the SYN.

The issue is that the server responses either don't reach or are not accepted by the client.

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Before all, thank you for taking the time to explain. As the black lines are packets coming from the server to the clients, my first wrong understanding was that server is the cause. But I may have been tricked by the checksum error and also by my lack of Wireshark mastering to be honest. My guess is that I have to search and learn more about networks and Wireshark basics so I will not be tricked anymore by my ignorance :) I think that I understand better now. Thanks for your help grahamb.

FenyX gravatar imageFenyX ( 2019-06-21 08:31:23 +0000 )edit

May I point you to this book by Chris Sanders, one of this years SharkFest keynote speakers.

Jaap gravatar imageJaap ( 2019-06-21 08:36:38 +0000 )edit

Thank you for this useful advice @Jaap .

FenyX gravatar imageFenyX ( 2019-06-21 11:07:09 +0000 )edit

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Asked: 2019-06-20 15:12:00 +0000

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Last updated: Jun 21 '19