Entirely ignoring TCP and UDP checksum in kernel level

Well, the device fails only in one type of operation, that is when it has to receive packets from an internal port, it sends packets via its external IP address.

I have managed to disable IP header checksumming by hacking the kernel (in file ip_input.c). Now I have to do the same for tcp_input.c and udp.c . Packet mangling will not be required because when I turn the rp_filter off, the kernel does not care whether it is coming from the intended source or not.

Now after disabling IP protocol checksumming, I passed to udp checksumming. However; now the code is more complicated. And for the TCP there is 100K of code. Well of course, with correct patch it would work, but as far as I can see, there are many locations that needs to be changed in order to get this work.

I also thought about packet mangling, but again something needs to be done for checksums. By the way, it is not necessary to do as such. Perhaps, I would later need it in case I see such errors in future but my current problem has a rather easy solution.

I have received many replies about what to change in these codes. However; none of them were correct at all. Either they disabled some part of checksumming or cast the TCP/IP totally unavailable.

Any reference to code would be appreciated.
Regards...

-----Original Message-----
From: Chris Siebenmann [mailto:[email protected]]
Sent: Sunday, August 22, 2004 2:59 AM
To: Josan Kadett
Subject: Re: Entirely ignoring TCP and UDP checksum in kernel level

At least with TCP, I believe that if TCP is establishing a circuit
between (IP1, port1) and (IP2, port2), SYN+ACKs from (IP3, port2) will generally be ignored. So it seems likely that you will not only have to ignore the checksum failure but also mangle the IP packets that you receive that claim to be from 'IP3' to claim to be from 'IP2' instead, so that they match up with what the kernel expects.

(The alternative approach is to mangle on the outgoing path, such
that the Linux box at the high level thinks it's talking to IP3, but IP3 is replaced by IP2 as each packet goes out.)

Has this particular chunk of hardware ever successfully talked to
anything via TCP/IP?

[And just to check: you are entirely 100% certain that the IP checksum
on incoming packets is incorrect?]
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Just a completely random thought. What about re-calculating the checksum and correcting it when the packet hits the IP layer prior to it being passed out to the relevant protocol handlers?

Regards,
Brad

That is certainly what I require but I need some guidelines; I investigated into the issue and fount out that;

- By only changing the source address coded in the IP header
- And by just applying checksum to IP header [Not TCP or UDP]

The problem would be gone since when the source IP in the IP address field is validated, the checksum in the underlying protocol is automatically validated (because the sender system had already computed this CRC using the IP address which we want to write onto the packet)

I do not have much time to build a code just for this purpose and thus I am looking for a simple app. or some other way...

-----Original Message-----
From: Brad Campbell [mailto:[email protected]]
Sent: Sunday, August 22, 2004 8:01 AM
To: Josan Kadett
Cc: 'Chris Siebenmann'; [email protected]
Subject: Re: Entirely ignoring TCP and UDP checksum in kernel level

Just a completely random thought. What about re-calculating the checksum and correcting it when the
packet hits the IP layer prior to it being passed out to the relevant protocol handlers?

Regards,
Brad