Re: strange kernel message when hacking the NIC driver

[Timothy Covell]

On Thursday 10 January 2002 22:19, Pei Zheng wrote:
> Hi,
>
> I've done some simple hackings to the network device driver. Basically
> i want to connect two NICs as a point-to-point link,ie, packets sending
> out from one NIC will be redirected to another NIC, sort of direct-link
> cable between these two NICs.
> So in the drivers of these two NICs, before hart_start_xmit(), the skb's
> header will be forcibly set to its peer's MAC address, thus each packey
> sending out from one NIC will be directly diverted to its peer NIC. This
> works fine most of the case since tcpdump shows that packets traverse in
> this way. However, i found errors in system log:
>
> Jan 10 18:02:40 em2 kernel: 1130: 09 => cb
> Jan 10 18:02:40 em2 kernel: 388: f4 => 45
> Jan 10 18:02:40 em2 kernel: 26: c0 => 7d
> Jan 10 18:02:40 em2 kernel: 149: 1f => 98
> Jan 10 18:02:40 em2 kernel: After error applied:
> Jan 10 18:02:40 em2 kernel: 00 80 c8 b9 6b b6 00 80 c8 b9 6b b6 08 00
> 45 00
> Jan 10 18:02:40 em2 kernel: 05 dc 73 cd 20 b9 3f 11 44 26 7d a8 0c 0a
> c0 a8
> Jan 10 18:02:40 em2 kernel: 10 0a e3 c1 4c 1f 6f 83 61 dc fc 5e 5c f4
> 66 17
> Jan 10 18:02:40 em2 kernel: 2c 73 19 ce cc 2d 69 69 bd 43 33 6d 82 de
> 4a 87
> Jan 10 18:02:40 em2 kernel: 8d 9d 81 f6 2e b2 8c 6a d5 e4 f2 e6 bc ab
> 5a 01
> Jan 10 18:02:40 em2 kernel: 34 d5 39 f8 d9 5b 16 bc dc 95 bd b4 08 a9
> 5e 11
> Jan 10 18:02:40 em2 kernel: df 38 80 8a ca d8 1c 53 65 97 91 4c 84 5a
> a1 0e
> Jan 10 18:02:40 em2 kernel: c2 5f d4 02 a1 9e 1c 35 d4 95 08 44 81 16
> a3 e0
>
> there are many this kind of messages. Don't understand what it is. the
> beginning part of the data seems to be an icmp packet(00 80 c8 b9 6b
> b6 is the MAC of one NIC). For some reason kernel thinks that this
> packet is not correct. Any idea about this? If i want to modify a
> packet's header before it goes to hard_strat_xmit(), what else should
> i do except setting the skb header to the MAC of the NIC's peer only?
> checksum stuff?
>
> Any helps will be highly appreciated.
>
> -Pei

You have me confused about whether your trying to build some kind of
ethernet bridge or some kind of secret packet sniffer or some other
such thing.   It looks like you're mixing up TCP/IP and Ethernet.

Ethernet is the link level protocol that addresses NICs by the MAC
address.    There can only be one NIC on the network with the same
MAC address.   TCP/IP uses IP addresses, not MAC addresses.  And you
cannot have two IP addresses on the same link if they have the same
network and broadcast address unless you do multipathing.  

I guess that I don't understand what you are trying to achieve.

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