Transparent Proxying Patches, Take 5

Hi Dave,

This is the fifth round of transparent proxying patches following
recent discussion on netfilter-devel [1,2].

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 1-6): 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. [3]

   - 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 7-13): 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.

   - Netfilter IPv4 defragmentation is split into a separate
     module. It's not particularly pretty but I see no other way of
     making sure the 'socket' match gets no fragmented IPv4 packets.

   - 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.

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

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 [4].


References:
[1] http://marc.info/?l=netfilter-devel&m=119118672703285...
[2] http://marc.info/?l=netfilter-devel&m=119135774918622...
[3] http://marc.info/?l=linux-netdev&m=118065358510836&...
[4] http://people.netfilter.org/hidden/tproxy/netcat-ip_trans...

-- 
KOVACS Krisztian
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