#Ipv4 checksum offload software

I am running GNS3 0.8.4 on a Win7 64bit host. 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. Hope this works for you as it did for me! Note: I had to reboot my machine after the setting changes. I changed the setting from RX and TX Enable to Disabled so that the OS will compute the checksum. 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)'. 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. SNORT also suffered from this issue which is why I'm heading to their support forum next. As soon as I disabled this feature on my laptop so that the OS does the checksumming again everything works great.

#Ipv4 checksum offload software

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). Cool feature but something that really messed up WinPCAP. Ladies and gentlemen TCP, UDP, and IP checksumming being offloaded to the nic card is killing us all. After some heavy digging I ran into this wiki on the wireshark site. What I finally realized was that even wireshark was reporting bad checksums. I (like many others) couldn't ping my virtual cisco pix running in GNS3 from my host (Windows 7 32 bit). It's a modern feature called Checksum offloading. I think the WinPCAP folks should test for this as it's a real problem in a lot of software now. I hope it can help some others who have been scratching their heads over this. I stumbled on to the information from a wireshark wiki post. 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.