1Transparent proxy support
2=========================
3
4This feature adds Linux 2.2-like transparent proxy support to current kernels.
5To use it, enable the socket match and the TPROXY target in your kernel config.
6You will need policy routing too, so be sure to enable that as well.
7
8
91. Making non-local sockets work
10================================
11
12The idea is that you identify packets with destination address matching a local
13socket on your box, set the packet mark to a certain value, and then match on that
14value using policy routing to have those packets delivered locally:
15
16# iptables -t mangle -N DIVERT
17# iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
18# iptables -t mangle -A DIVERT -j MARK --set-mark 1
19# iptables -t mangle -A DIVERT -j ACCEPT
20
21# ip rule add fwmark 1 lookup 100
22# ip route add local 0.0.0.0/0 dev lo table 100
23
24Because of certain restrictions in the IPv4 routing output code you'll have to
25modify your application to allow it to send datagrams _from_ non-local IP
26addresses. All you have to do is enable the (SOL_IP, IP_TRANSPARENT) socket
27option before calling bind:
28
29fd = socket(AF_INET, SOCK_STREAM, 0);
30/* - 8< -*/
31int value = 1;
32setsockopt(fd, SOL_IP, IP_TRANSPARENT, &value, sizeof(value));
33/* - 8< -*/
34name.sin_family = AF_INET;
35name.sin_port = htons(0xCAFE);
36name.sin_addr.s_addr = htonl(0xDEADBEEF);
37bind(fd, &name, sizeof(name));
38
39A trivial patch for netcat is available here:
40http://people.netfilter.org/hidden/tproxy/netcat-ip_transparent-support.patch
41
42
432. Redirecting traffic
44======================
45
46Transparent proxying often involves "intercepting" traffic on a router. This is
47usually done with the iptables REDIRECT target; however, there are serious
48limitations of that method. One of the major issues is that it actually
49modifies the packets to change the destination address -- which might not be
50acceptable in certain situations. (Think of proxying UDP for example: you won't
51be able to find out the original destination address. Even in case of TCP
52getting the original destination address is racy.)
53
54The 'TPROXY' target provides similar functionality without relying on NAT. Simply
55add rules like this to the iptables ruleset above:
56
57# iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY \
58  --tproxy-mark 0x1/0x1 --on-port 50080
59
60Note that for this to work you'll have to modify the proxy to enable (SOL_IP,
61IP_TRANSPARENT) for the listening socket.
62
63
643. Iptables extensions
65======================
66
67To use tproxy you'll need to have the 'socket' and 'TPROXY' modules
68compiled for iptables. A patched version of iptables is available
69here: http://git.balabit.hu/?p=bazsi/iptables-tproxy.git
70
71
724. Application support
73======================
74
754.1. Squid
76----------
77
78Squid 3.HEAD has support built-in. To use it, pass
79'--enable-linux-netfilter' to configure and set the 'tproxy' option on
80the HTTP listener you redirect traffic to with the TPROXY iptables
81target.
82
83For more information please consult the following page on the Squid
84wiki: http://wiki.squid-cache.org/Features/Tproxy4
85