From owner-freebsd-hackers  Thu Sep 20 20:45:48 2001
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Date: Thu, 20 Sep 2001 22:40:26 -0500
From: Matt Finlay <matt@ipperformance.com>
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To: Les Biffle <les@safety.net>, davep@afterswish.com,
	Alfred Perlstein <bright@mu.org>, hackers@FreeBSD.ORG
Subject: Re: sendto not sending what I asked...
References: <200109210250.f8L2oO615274@ns3.safety.net> <3BAAB547.C065FF97@ipperformance.com>
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come to think of it...  perhaps you are not calculating the tcp checksum
correctly because of the tcp/ip pseudo header.  i no longer have the code that
was posted, but thinking back i believe this might be the case.

Matt Finlay wrote:

> Les Biffle wrote:
>
> > >       pkt.ipbit.ip_sum=in_cksum((unsigned short*)(void*)&pkt,20);
> > >       pkt.tcpbit.th_sum=tcpsum(&pkt.ipbit);
> >
> > You need to do the TCP checksum first.  It changes the payload (the TCP
> > checksum).  The IP checksum includes all data, so must be done after the
> > payload changes are done.
>
> i think this is incorrect.  the IP checksum only includes the IP header and
> any IP options.  therefore order should not matter.
>
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