Re: checksums on http transfers?

On Wed, 7 Nov 2001, Kevin Pilch-Bisson wrote:

> My ISP performs NAT somewhere in its network, which requires
> re-caculating the tcp checksum of packets as they pass through. It seems
> that the problem is that they weren't _verifying_ the original checksum
> before recalculating, therefore allowing corrupted packets to be received
> by my machine

Just a minor detail, but NATs don't mess with TCP, they mess with IP. So they
change the IP headers and that is separately checksummed (and very commonly
ignored). Thus, the TCP checksum and data shall remain unmodified even after
having passed through a NAT.

IPv6 doesn't even have a checksum for the IP header, as both the underlying
layer (mostly ethernet) and the upper layer (mostly TCP) usually already
checksum the data.

So, I don't think the NAT explains how your machine could accept broken TCP
packets.

-- 
      Daniel Stenberg - http://daniel.haxx.se - +46-705-44 31 77
   ech`echo xiun|tr nu oc|sed 'sx\([sx]\)\([xoi]\)xo un\2\1 is xg'`ol
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