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Based on kernel version 3.9. Page generated on 2013-05-02 23:11 EST.

1	Transparent proxy support
2	=========================
3	
4	This feature adds Linux 2.2-like transparent proxy support to current kernels.
5	To use it, enable NETFILTER_TPROXY, the socket match and the TPROXY target in
6	your kernel config. You will need policy routing too, so be sure to enable that
7	as well.
8	
9	
10	1. Making non-local sockets work
11	================================
12	
13	The idea is that you identify packets with destination address matching a local
14	socket on your box, set the packet mark to a certain value, and then match on that
15	value 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	
25	Because of certain restrictions in the IPv4 routing output code you'll have to
26	modify your application to allow it to send datagrams _from_ non-local IP
27	addresses. All you have to do is enable the (SOL_IP, IP_TRANSPARENT) socket
28	option before calling bind:
29	
30	fd = socket(AF_INET, SOCK_STREAM, 0);
31	/* - 8< -*/
32	int value = 1;
33	setsockopt(fd, SOL_IP, IP_TRANSPARENT, &value, sizeof(value));
34	/* - 8< -*/
35	name.sin_family = AF_INET;
36	name.sin_port = htons(0xCAFE);
37	name.sin_addr.s_addr = htonl(0xDEADBEEF);
38	bind(fd, &name, sizeof(name));
39	
40	A trivial patch for netcat is available here:
41	http://people.netfilter.org/hidden/tproxy/netcat-ip_transparent-support.patch
42	
43	
44	2. Redirecting traffic
45	======================
46	
47	Transparent proxying often involves "intercepting" traffic on a router. This is
48	usually done with the iptables REDIRECT target; however, there are serious
49	limitations of that method. One of the major issues is that it actually
50	modifies the packets to change the destination address -- which might not be
51	acceptable in certain situations. (Think of proxying UDP for example: you won't
52	be able to find out the original destination address. Even in case of TCP
53	getting the original destination address is racy.)
54	
55	The 'TPROXY' target provides similar functionality without relying on NAT. Simply
56	add 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	
61	Note that for this to work you'll have to modify the proxy to enable (SOL_IP,
62	IP_TRANSPARENT) for the listening socket.
63	
64	
65	3. Iptables extensions
66	======================
67	
68	To use tproxy you'll need to have the 'socket' and 'TPROXY' modules
69	compiled for iptables. A patched version of iptables is available
70	here: http://git.balabit.hu/?p=bazsi/iptables-tproxy.git
71	
72	
73	4. Application support
74	======================
75	
76	4.1. Squid
77	----------
78	
79	Squid 3.HEAD has support built-in. To use it, pass
80	'--enable-linux-netfilter' to configure and set the 'tproxy' option on
81	the HTTP listener you redirect traffic to with the TPROXY iptables
82	target.
83	
84	For more information please consult the following page on the Squid
85	wiki: http://wiki.squid-cache.org/Features/Tproxy4
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