Re: Wrong MAC in redirected packet

[Kristian Evensen <kristian.evensen@gmail.com>]

Jan Engelhardt skrev:
> On Wednesday 2009-03-25 20:56, Kristian Evensen wrote:
>
>   
>> Jan Engelhardt skrev:
>>     
>>> On Wednesday 2009-03-25 20:47, Kristian Evensen wrote:
>>>  
>>>       
>>>> However, I have noticed a similar problem when using my module on incoming
>>>> packets in PREROUTING (on the multihomed receiver), the IP adress is changed
>>>> (accoring to my dmesg-output) but then they are not heard from again.
>>>>    
>>>>         
>>> Probably dropped due to L3 or L4 checksum mismatches. While we try to
>>> change the checksum, I have not verified it for all cases.
>>>
>>>  
>>>       
>> Hm, I am noe sure. I recalculate the checksum (using csum_replace4 for l3 and
>> inet....replace4 for l4) after modifying the destination IP, and when used with
>> OUTPUT the checksum is correct. Since tcpdump/Wireshark outputs the packets
>> before they are modified I haven't been able to verify this though.
>>     
>
> You can use hexdump in the xtables module, or any other form, to
> dump the checksum.
>   
I had completely forgot that, thank you. However, tcpdump tells me that 
the checksum is ok, but I will look into it and simply compare a packet 
that has not been redirected with one that has. Thank you.

-Kristian