Checksum of ip headers : is it changed when switching source and dest addresses ?

Hello,

I am not familiar with the details of the checksum algorithm of ip
headers;
My question is this:

Suppose that I switch source and dest addresses in an IP header of an
IP packet - should the checksum of the ip header be changed ?
Or is the checksum algorithm indifferent to which is the IP src address
and
IP dest address in the ip header ?

Regards,
Mark

(E-Mail Removed) wrote:

> Suppose that I switch source and dest addresses in an IP header of an
> IP packet - should the checksum of the ip header be changed ?
> Or is the checksum algorithm indifferent to which is the IP src address
> and
> IP dest address in the ip header ?

What is the I here? Is this in a driver? In a user-space program?
Normally, the program that sends the actual packet will compute the
checksum, so you shouldn't need to change it no matter what it is and
whether it's right or not. Unless you're writing part of a network
stack, in which case you should definitely already know this stuff.

DS

On Fri, 29 Sep 2006 00:25:47 -0700, David Schwartz rearranged some
electrons to form:

>
> (E-Mail Removed) wrote:
>
>> Suppose that I switch source and dest addresses in an IP header of an
>> IP packet - should the checksum of the ip header be changed ?
>> Or is the checksum algorithm indifferent to which is the IP src address
>> and
>> IP dest address in the ip header ?
>
> What is the I here? Is this in a driver? In a user-space program?
> Normally, the program that sends the actual packet will compute the
> checksum, so you shouldn't need to change it no matter what it is and
> whether it's right or not. Unless you're writing part of a network
> stack, in which case you should definitely already know this stuff.
>
> DS

Or trying to hide the source of the packet?

--
David M (dmacchiarolo)
http://home.triad.rr.com/redsled
T/S 53
sled351 Linux 2.4.18-14 has been up 1 day 18:35

David,
thnks!
> Is this in a driver? In a user-space program?

no, it is not a user-space program but a driver (kernel module). But I
am a newbie in
this and I am not sure wheter the checksum of the IP header is changed
when
src and dsr addresses are switched when we are talking about kernel
space.

Regards,
Mark





David Schwartz wrote:
> (E-Mail Removed) wrote:
>
> > Suppose that I switch source and dest addresses in an IP header of an
> > IP packet - should the checksum of the ip header be changed ?
> > Or is the checksum algorithm indifferent to which is the IP src address
> > and
> > IP dest address in the ip header ?
>
> What is the I here? Is this in a driver? In a user-space program?
> Normally, the program that sends the actual packet will compute the
> checksum, so you shouldn't need to change it no matter what it is and
> whether it's right or not. Unless you're writing part of a network
> stack, in which case you should definitely already know this stuff.
>
> DS

(E-Mail Removed) wrote:
> no, it is not a user-space program but a driver (kernel module). But I
> am a newbie in
> this and I am not sure wheter the checksum of the IP header is changed
> when
> src and dsr addresses are switched when we are talking about kernel
> space.

Bothered to look at any of the pre-existing kernel-space code out there, for
instance, ipt? Working within kernel land is not to be taken lightly. Sounds
like some form of rudimentary proxying or naive NAT-ing of some sort.

Boy that was hard, first hit in google for "tcp checksum":

http://www.netfor2.com/tcpsum.htm