What basic sanity checking on packet headers is done

[Patrick McHardy]

Wayne Schroeder wrote:
> I've written a new target module that uses the ip header fields of ihl
> and tot_len for offsets into the packets.  How safe is the data in the
> ip headers?  Is there sanity checking when the packet is received off
> the wire... for instance -- is it safe to assume in the prerouting chain
> of the mangle table that tot_len will not put me past the memory
> allocated for the packet?  Same goes for ihl?

Check out ip_rcv() in ip_input.c. ihl and tot_len are _usually_ valid
within netfilter. The only exception is raw sockets, on the outgoing
hooks the only guarantee is that ihl * 4 >= sizeof(struct iphdr) (
only for ip_tables, see iptable_filter.c). We could consider providing
stricter guarantees, I'm pretty sure some modules only perform
insufficient checks.