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    SubjectRE: Cursed Checksums
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    Perhaps there is a way to recompute IP header checksums before they get into
    the interface? As I outlined, I have found a way to manipulate IP source
    address before the packet is flushed to system, but a means of recalculating
    the IP header checksum after that manipulation should be found. Because even
    if I ignore IP header CRC in one system, all other boxes connected to this
    machine has to be patched the same. That is impossible anyway.

    Only if I could find a way to recalculate the checksum in IP headers by
    doing a simple hack to the kernel, everything would be alright.

    -----Original Message-----
    From: linux-kernel-owner@vger.kernel.org
    [mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of Alan Cox
    Sent: Sunday, August 22, 2004 1:40 PM
    To: Josan Kadett
    Cc: Linux Kernel Mailing List
    Subject: Re: Cursed Checksums

    It depends on your hardware. With modern network cards we do the
    checksum processing in hardware. For older setups passing a packet
    through a Linux box won't directly help as the ttl recomputation is done
    without recalculation from scratch.

    We also have a pile of paths for checksumming including copy/checksum
    rolled into one so it isn't easy to remove there.

    I'd take up the issue with the vendor of the broken object. If its
    something like an internal prototype you need to test then you'll
    probably have to write a user space application using raw sockets to
    communicate with it and do the fixups/passthrough in use space. Pretty
    horrible either way.

    Alan


    Alan
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