Problems with UNIX SCO

Problems with UNIX SCO

[rigoberto perez]

Friends:

I wrote a netfilter module with the follows operations:

OUT
***
1) Modify every packet by paddding 64 bytes
2) Save the original protocol in the 64 bytes trailer (that's enough space!)
3) Modify the protocol header in TCP
4) Calculate the IP checksum


IN
**
1) Restore the original protocol by get the saved in the trailer
2) Trim the 64 bytes in the tail
3) Calculate the IP checksum

I wrote the same funcionality in a SCO OPENSERVER UNIX. The comunication between the boths systems through pings is OK but in the case of TCP report errors in the checksum of the packets in UNIX.

Anybody have some idea about it?

Thanks.


      

Re: Problems with UNIX SCO

[rigoberto perez]

 Friends:
 
 The problems is not only with UNIX. If I put two routers
 Linux with my modules the communications between boths are
 OK but when I try to connect from a Client the TCP reports
 checksum problems. For example:
 
 
 Linux1    ------   Linux2
 
 is OK, but
 
 
 Client (windows or Linux)    ------ Linux1 ------- Linux2
 
 The Client reports checksum TCP errors. The gateway for the
 Client is Linux1 and the TCP is made form Client to the
 Linux2. 
 
 It's like the TCP checksum was modify but the TCP
 checksum depends only of data and the IP directions and this
 not change, beside, the original packet content (protocol,
 length, ..) is restore. I see the TCP checksum header before
 modify the packets and its the same when I restore the
 original packets, so I think that the pseudo header is the
 problem but I don't have clear whats the problem.
 
 
 I glad to receive some comments,
 
 Thanks.
 
 Postdata: I use SUSE Enterprise Server 10.




      

Re: Problems with UNIX SCO

[Oscar N]

Don't know if this has anything to do with your problem but some network
drivers calculate checksum on the network card, so if you to tcpdump
directly on sending machine it can report checksum errors.

/Regards Oscar

>  Friends:
>
>  The problems is not only with UNIX. If I put two routers
>  Linux with my modules the communications between boths are
>  OK but when I try to connect from a Client the TCP reports
>  checksum problems. For example:
>
>
>  Linux1    ------   Linux2
>
>  is OK, but
>
>
>  Client (windows or Linux)    ------ Linux1 ------- Linux2
>
>  The Client reports checksum TCP errors. The gateway for the
>  Client is Linux1 and the TCP is made form Client to the
>  Linux2.
>
>  It's like the TCP checksum was modify but the TCP
>  checksum depends only of data and the IP directions and this
>  not change, beside, the original packet content (protocol,
>  length, ..) is restore. I see the TCP checksum header before
>  modify the packets and its the same when I restore the
>  original packets, so I think that the pseudo header is the
>  problem but I don't have clear whats the problem.
>
>
>  I glad to receive some comments,
>
>  Thanks.
>
>  Postdata: I use SUSE Enterprise Server 10.
>
>
>
>
>
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