Re: tcp checksum calculation incorrect - off by 2
Re: tcp checksum calculation incorrect - off by 2
- Subject: Re: tcp checksum calculation incorrect - off by 2
- From: Stephane Sudre <email@hidden>
- Date: Tue, 17 Jul 2007 13:14:10 +0200
On 16 juil. 07, at 15:46, Raj Hariginadoni wrote:
Hi Folks,
I am looking at a driver code for OS X 10.3 where the checksum is
being calculated on an inbound syn-ack. First by zeroing out the
checksum, then calculating the checksum of the pseudo header and
finally calculating the checksum. The checksum functions used are
available in the xnu sources.
Here is the snippet
[...]
tcplen=26
checksum before =7182
final checksum=7180
tcplen=26
checksum before =2d87
final checksum=2d85
tcplen=26
checksum before =5ab0
final checksum=5aae
I have also observed that the checksum is calculated correctly when
the length of the tcp segment in the packet is 40.
tcplen=40
checksum before =d1cf
final checksum=d1cf
tcplen=40
checksum before =e237
final checksum=e237
Can someone guide me in solving this problem?
Don't take my word for it, but a quick search on google shows that most
of the sample codes do something like this:
tcph->th_sum = in_pseudo(iph->ip_src.s_addr,
iph->ip_dst.s_addr,htons(IPPROTO_TCP + tcplen));
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