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 Post subject: TCP, UDP, and IP Checksum Offloading
PostPosted: Thu Jan 13, 2011 3:26 am 
Hello everyone. I'm not sure if this is the right place to post this but this information would have saved me 3 days of trying to find why I couldn't ping my virtual GNS Devices from my physical host. I stumbled on to the information from a wireshark wiki post. I hope it can help some others who have been scratching their heads over this. I think the WinPCAP folks should test for this as it's a real problem in a lot of software now. It's a modern feature called Checksum offloading.

I (like many others) couldn't ping my virtual cisco pix running in GNS3 from my host (Windows 7 32 bit). In XP I could. What I finally realized was that even wireshark was reporting bad checksums. After some heavy digging I ran into this wiki on the wireshark site Ladies and gentlemen TCP, UDP, and IP checksumming being offloaded to the nic card is killing us all. Cool feature but something that really messed up WinPCAP. If the OS is no longer doing the packet checksums it appears to all software using WinPCAP that the packet is bad and it just drops (I'm assuming that is what is happening). As soon as I disabled this feature on my laptop so that the OS does the checksumming again everything works great. SNORT also suffered from this issue which is why I'm heading to their support forum next. On my machine to turn off Checksum Offloading I did the following: In Windows 7, go to Control Panel->Network and Internet -> Network and Sharing Center, click Local Area Connection Link of the connection to change then choose 'Properties' button. Press the 'Configure...' button, choose the 'Advanced' tab to modify the "IP V4 Checksum Offload" and "TCP Checksum Offload (IPV4)" and "UDP Checksum Offload (IPV4)". I changed the setting from RX and TX Enable to Disabled so that the OS will compute the checksum. Note: I had to reboot my machine after the setting changes. Hope this works for you as it did for me!

 Post subject: Re: TCP, UDP, and IP Checksum Offloading
PostPosted: Mon Jan 24, 2011 11:05 pm 
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Jeremy, GNS3 Programmer & Benevolent Dictator for Life.

 Post subject: Re: TCP, UDP, and IP Checksum Offloading
PostPosted: Mon Jul 29, 2013 7:50 pm 

Joined: Thu Apr 04, 2013 10:37 pm
Posts: 22
I too am seeing a lot of these checksum errors in my captures however, my setup is a little different than what is described here.

I am running GNS3 0.8.4 on a Win7 64bit host. I built one lab with two Virtualbox VMs, one is WinXP 64bit and the other is Redhat Enterprise 6.3 64bit. I have these two VM's connected together in GNS3 with the built in switch. The ONLY network adapter that works for me is the Generic UDP adapter. That's the only adapter that will allow my VM's to ping each other (I've posted about this before but no responses to that post).

Anyway, from the XP host, I am trying to pull up a web site on the RH host. I can see the connection establish but the site won't pull up all the way.

I then did a capture and I am seeing a lot of these checksum errors. I can't help but assume the issues are related.

Header checksum: 0x0000 [incorrect, should be 0x4b3c (may be caused by "IP checksum offload"?)]

The odd part is however, the VM's are on their own network connected via the Generic UDP tunnel adapter and not at all using the adapters of my host. So, if that's the case, why am I seeing these checksum errors?

Is there something with GNS3 or VB causing these errors instead?

Also, for my network adapters in my host, I am not able to make the changes described in this thread. Those options are not there for my Intel NICs for some reason.

Has anyone seen this behavior before?

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