Re: Wrong MAC in redirected packet

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

>>>>> 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.
>>>

Turned out that it was the checksums. They were calculated correctly,
but since I processed packets on input, tcp_hdr and udp_hdr of course
returned invalid pointers. My code is now more or less RAWNAT, so if
it is of interest to anybody else I will merge them and make it
available.

-Kristian