1Transparent proxy support
2=========================
3
4This feature adds Linux 2.2-like transparent proxy support to current kernels.
5To use it, enable NETFILTER_TPROXY, the socket match and the TPROXY target in
6your kernel config. You will need policy routing too, so be sure to enable that
7as well.
8
9
101. Making non-local sockets work
11================================
12
13The idea is that you identify packets with destination address matching a local
14socket on your box, set the packet mark to a certain value, and then match on that
15value using policy routing to have those packets delivered locally:
16
17# iptables -t mangle -N DIVERT
18# iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
19# iptables -t mangle -A DIVERT -j MARK --set-mark 1
20# iptables -t mangle -A DIVERT -j ACCEPT
21
22# ip rule add fwmark 1 lookup 100
23# ip route add local 0.0.0.0/0 dev lo table 100
24
25Because of certain restrictions in the IPv4 routing output code you'll have to
26modify your application to allow it to send datagrams _from_ non-local IP
27addresses. All you have to do is enable the (SOL_IP, IP_TRANSPARENT) socket
28option before calling bind:
29
30fd = socket(AF_INET, SOCK_STREAM, 0);
31/* - 8< -*/
32int value = 1;
33setsockopt(fd, SOL_IP, IP_TRANSPARENT, &value, sizeof(value));
34/* - 8< -*/
35name.sin_family = AF_INET;
36name.sin_port = htons(0xCAFE);
37name.sin_addr.s_addr = htonl(0xDEADBEEF);
38bind(fd, &name, sizeof(name));
39
40A trivial patch for netcat is available here:
41http://people.netfilter.org/hidden/tproxy/netcat-ip_transparent-support.patch
42
43
442. Redirecting traffic
45======================
46
47Transparent proxying often involves "intercepting" traffic on a router. This is
48usually done with the iptables REDIRECT target; however, there are serious
49limitations of that method. One of the major issues is that it actually
50modifies the packets to change the destination address -- which might not be
51acceptable in certain situations. (Think of proxying UDP for example: you won't
52be able to find out the original destination address. Even in case of TCP
53getting the original destination address is racy.)
54
55The 'TPROXY' target provides similar functionality without relying on NAT. Simply
56add rules like this to the iptables ruleset above:
57
58# iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY \
59  --tproxy-mark 0x1/0x1 --on-port 50080
60
61Note that for this to work you'll have to modify the proxy to enable (SOL_IP,
62IP_TRANSPARENT) for the listening socket.
63
64
653. Iptables extensions
66======================
67
68To use tproxy you'll need to have the 'socket' and 'TPROXY' modules
69compiled for iptables. A patched version of iptables is available
70here: http://git.balabit.hu/?p=bazsi/iptables-tproxy.git
71
72
734. Application support
74======================
75
764.1. Squid
77----------
78
79Squid 3.HEAD has support built-in. To use it, pass
80'--enable-linux-netfilter' to configure and set the 'tproxy' option on
81the HTTP listener you redirect traffic to with the TPROXY iptables
82target.
83
84For more information please consult the following page on the Squid
85wiki: http://wiki.squid-cache.org/Features/Tproxy4
86