Question

Sometimes seeing SYN, ACK other times getting RST, ACK

Asked by: WebTeamRC

We are currently having intermittent trouble when trying to access a public website.  I have setup Wireshark to monitor traffic mirroring our port that connects us to the Internet.  This is the data I see.

When we are able to access the site I see the following
Source x.x.x.x destination y.y.y.y TCP http [SYN]
Source y.y.y.y destination x.x.x.x TCP http [SYN, ACK]
Source x.x.x.x destination y.y.y.y TCP http [ACK]
A correct 3 way handshake

When I am not able to access their site, I get
Source x.x.x.x destination y.y.y.y TCP http [SYN]
Source y.y.y.y destination x.x.x.x TCP http [RST, ACK]

This looks like a reset, where the port is closed.
They keep telling me the problem is on my end.  They seem to refuse to look at their side to perform any type of troubleshooting.  

Does anyone know what might explain this?

Thanks

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Asked On
2009-11-05 at 11:09:39ID24875632
Tags

Wireshark

,

Packet Sniffing

,

security

Topic

TCP/IP

Participating Experts
1
Points
500
Comments
3

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Answers

 

by: WebTeamRCPosted on 2009-11-05 at 12:29:21ID: 25753623

I think this is tougher than I first thought.

 

by: Otto_NPosted on 2009-11-06 at 04:40:54ID: 25758586

I assume that y.y.y.y is the website address?  If so, the website respond to the TCP initiation with a RESET command, and this cannot be generated by any device other than the server with IP y.y.y.y.  The sys-admins of y.y.y.y need to investigate the cause of this.

This is assuming that ther are no other TCP-modifying devices in the path (devices like proxy servers, load-balancers, and sometimes even firewalls).  But the y.y.y.y IP address should belong to this device.  So the y.y.y.y address is not always the IP address of the actual server or website.

I hope this information helps!

 

by: WebTeamRCPosted on 2009-11-06 at 04:51:48ID: 25758648

Yes. y is the website and x is my firewall IP.  I an monitoring the port of my Internet feed, outside my routers and firewall.

This is basically what  suspected.  Thanks

20120131-EE-VQP-002

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