tcpdump mailing list archives
Re: false checksum failure reports
From: Rick Jones <rick.jones2 () hp com>
Date: Tue, 06 Nov 2007 17:54:26 -0800
Harley Stenzel wrote:
On Nov 6, 2007 2:03 PM, Rick Jones <rick.jones2 () hp com> wrote:Any thoughts as to how to deal with false checksum failure reports for outbound traffic being sniffed on a system with ChecKsum Offload (CKO)? It seems that linux has a flag they can set when capturing the packet that would tell us, not sure what other platforms might haveLove it. It would be very nice to know if a packet's checksum will be calculated in a CKO card.
These are some of the specifics courtesy of folks over in the linux netdev mailing list:
The thing to check is "TP_STATUS_CSUMNOTREADY". When using mmap(), it will be provided in the descriptor. When using recvmsg() it will be provided via a PACKET_AUXDATA control message when enabled via the PACKET_AUXDATA socket option.
I have tried to take a quick first look at the pcap code for linux but wasn't in the correct frame of mind and so got lost rather quickly.
Without a corresponding change to the binary file format (as in find a spare bit somewhere) the change would initially be limited to "live" tracing. Not a complete solution, but a step in the right direction.
Meanwhile, making certain that the docs/manpage etc call-out that tracing on a CKO capable system/NIC will result in false checksum failure reports for outbound traffic would be goodness. I suppose I should have checked if that was already there before typing the previous sentence, but there you go :)
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Current thread:
- false checksum failure reports Rick Jones (Nov 06)
- Re: false checksum failure reports Harley Stenzel (Nov 06)
- Re: false checksum failure reports Rick Jones (Nov 06)
- Re: false checksum failure reports ronnie sahlberg (Nov 06)
- Re: false checksum failure reports Rick Jones (Nov 07)
- Re: false checksum failure reports Rick Jones (Nov 06)
- Re: false checksum failure reports Harley Stenzel (Nov 06)