NF_ARP_OUT skb has NULL link layer header

Hello,

I've been working lately on a netfilter hook extension which rewrites outgoing ARP packets to spoof a host's MAC address.

I've succesfully spoofed ARP replies, but have seen that several OS's tend to update their arp cache based on information in ARP requests. After several panic()'s, I noticed that the link layer header (skb->mac)
is NULL. My intention was to rewrite the ethernet mac address to the spoofed MAC address.

Is there a way to rewrite the link layer header besides modifying skb->mac (skb->mac.ethernet to be precise), or could I possibly just tag on a kmalloc()'ed pointer to the skb->mac?

If it makes any difference, I only noticed this when porting the code to linux 2.6 (2.6.7).

Thanks a lot, in advance

Phillip Whelan
EXIS (http://www.exis.cl)