Re: PPPoE ip extraction

>----- Oorspronkelijk bericht -----
>Van: Marek Jawurek [mailto:[email protected]]
>Verzonden: donderdag, juli 6, 2006 01:12 PM
>Aan: [email protected]
>CC: [email protected]
>Onderwerp: Re: [Ebtables-devel] PPPoE ip extraction
>
>Thx Bart, i figured it out yesterday the other way around.
>I moved the rest of the packet down in order to overwrite the ppp
>header.
>
>This is what I intended to write:
>
>The skb->data pointer just points to the start of the current protocol
>header. This is the first Layer encapsulated in the Ethernet packet for
>the ebtables code. As changing the data pointer apparantly does not help
>I just overwrote the pppoe+ppp header and copied the IP Proto field into
>the Ethernet header:
>
>memcpy(eth_hdr(*pskb)->h_dest, in->dev_addr, ETH_ALEN);
>eth_hdr(*pskb)->h_proto=htons(ETH_P_IP);
>memcpy((*pskb)->data,(*pskb)->datasizeof(pppoe),(*pskb)->end-(*pskb)->data);

I think the best thing you can do is to skb_pull the data and then copy the
Ethernet header. You must make sure that the packet length is decreased by the 
size of the ppp header.

>Ok this seems to work. In general, I see that I lack a great deal of
>kernel-programming-experience ;-)
>
>One question though, I manage to extract the IP packet but apparently it
>then (after the REDIRECT in ebtables) the INPUT table of the netfilter ?

?

>Would it be possible to direct it to the PREROUTING where I could use
>the iptables REDIRECT to a local port ? 

I don't know where you are doing this modification. But it's very likely you
need to change skb->protocol to htons(ETH_P_IP).

cheers,
Bart





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