If the packet is received via the bridge stack, this cannot reject
packets from the IP stack.
This adds functions to build the reject packet and send it from the
bridge stack. Comments and assumptions on this patch:
1) Validate the IPv4 and IPv6 headers before further processing,
given that the packet comes from the bridge stack, we cannot assume
they are clean. Truncated packets are dropped, we follow similar
approach in the existing iptables match/target extensions that need
to inspect layer 4 headers that is not available. This also includes
packets that are directed to multicast and broadcast ethernet
addresses.
2) br_deliver() is exported to inject the reject packet via
bridge localout -> postrouting. So the approach is similar to what
we already do in the iptables reject target. The reject packet is
sent to the bridge port from which we have received the original
packet.
3) The reject packet is forged based on the original packet. The TTL
is set based on sysctl_ip_default_ttl for IPv4 and per-net
ipv6.devconf_all hoplimit for IPv6.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
523b929d54