Transparent proxying patches, take six

Hi Dave,

This is the sixth round of transparent proxying patches recently
discussed on the Netfilter Workshop. Since the last incarnation [1]
we've added support for related ICMP packets in the socket
match. Should apply cleanly on top of net-next-2.6. Could you please
apply patches 1-11 (those touching core networking parts) and I'll ask
Patrick McHardy to take care of patches 12-16 (the Netfilter parts).

The aim of the patchset is to make non-locally bound sockets work both
for receiving and sending. The target is IPv4 TCP/UDP at the moment.

Speaking of the patches, there are two big parts:

 * Output path (patches 1-7): these modifications make it possible to
   send IPv4 datagrams with non-local source IP address by:

   - Introducing a new flowi flag (FLOWI_FLAG_ANYSRC) which disables
     source address checking in ip_route_output_slow(). This is
     also necessary for some of the tricks LVS does. [2]

   - Adding the IP_TRANSPARENT socket option (setting this requires
     CAP_NET_ADMIN to prevent source address spoofing).

   - Gluing these together across the TCP/UDP code.

 * Input path (patches 8-15): these changes add redirection support
   for TCP along with an iptables target implementing NAT-less traffic
   interception, and an iptables match to make ahead-of-time socket
   lookups on PREROUTING. These combined with a set of iptables rules
   and policy routing make non-locally bound sockets work.

   - IPv4 TCP and UDP input path is modified to use this stored socket
     reference if it's present.

   - Netfilter IPv4 defragmentation is split into a separate
     module. (This could make sense independently of tproxy and
     conntrack, for example to have a stateless firewall which still
     does fragment reassembly.)

   - The 'socket' iptables match does a socket lookup on the
     destination address and matches if a socket was found.

   - The 'TPROXY' iptables target provides a way to intercept traffic
     without NAT -- it does an ahead-of-time socket lookup on the
     configured address and caches the socket reference in the skb.

The last patch adds a short intro on how to use it. A trivial patch
for netcat demonstrating the necessary modifications for proxies is
available separately at [3]. Squid has support for it in the 3.HEAD
(3.1) branch.


References:
[1] http://lwn.net/Articles/254527/
[2] http://marc.info/?l=linux-netdev&m=118065358510836&;...
[3] http://people.netfilter.org/hidden/tproxy/netcat-ip_trans...

-- 
KOVACS Krisztian


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