TCP checksum calculation

TCP checksum calculation

[Rahul Jain]

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.

Thanks,
Rahul.

Re: TCP checksum calculation

[Stephen Hemminger]

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>