To: linux-kernel@vger.kernel.org
From: Stephen Hemminger <shemminger@osdl.org>
Subject: Re: TCP checksum calculation
Date: Thu, 20 Jan 2005 15:32:27 -0800
Organization: Open Source Development Lab
Message-ID: <20050120153227.69fd1d55@dxpl.pdx.osdl.net>
References: <Pine.GSO.4.58.0501201541510.13444@chrome.njit.edu>
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On Thu, 20 Jan 2005 15:52:34 -0500 (EST)
Rahul Jain <rbj2@oak.njit.edu> wrote:

> Hi,
> 
> I have written a module that changes IP addrs and TCP port values. After
> changing these fields, I am able to recalculate the IP checksum  within
> the module. To recalculate the TCP checksum, I wrote a new function in
> tcp_ipv4.c which is very similar to tcp_v4_send_check(). The only
> difference is that, my function does not use the sock parameter and gets
> the saddr and daddr from sk_buff. I call this function before the
> following piece of code in tcp_v4_rcv()
> 
> if ((skb->ip_summed != CHECKSUM_UNNECESSARY &&
>              tcp_v4_checksum_init(skb) < 0))
>                 goto bad_packet;
> 
> However I am still getting a bad tcp checksum error. Does anyone know what
> I am missing and point me in the right direction.
> 

Look at the netfilter code, in fact if you are changing values there may already
be a netfilter module to do what you want, and you could have saved the effort.

-- 
Stephen Hemminger	<shemminger@osdl.org>
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