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
8From Linux 4.18 transparent proxy support is also available in nf_tables.
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:
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
21Alternatively you can do this in nft with the following commands:
22
23# nft add table filter
24# nft add chain filter divert "{ type filter hook prerouting priority -150; }"
25# nft add rule filter divert meta l4proto tcp socket transparent 1 meta mark set 1 accept
26
27And then match on that value using policy routing to have those packets
28delivered locally:
29
30# ip rule add fwmark 1 lookup 100
31# ip route add local 0.0.0.0/0 dev lo table 100
32
33Because of certain restrictions in the IPv4 routing output code you'll have to
34modify your application to allow it to send datagrams _from_ non-local IP
35addresses. All you have to do is enable the (SOL_IP, IP_TRANSPARENT) socket
36option before calling bind:
37
38fd = socket(AF_INET, SOCK_STREAM, 0);
39/* - 8< -*/
40int value = 1;
41setsockopt(fd, SOL_IP, IP_TRANSPARENT, &value, sizeof(value));
42/* - 8< -*/
43name.sin_family = AF_INET;
44name.sin_port = htons(0xCAFE);
45name.sin_addr.s_addr = htonl(0xDEADBEEF);
46bind(fd, &name, sizeof(name));
47
48A trivial patch for netcat is available here:
49http://people.netfilter.org/hidden/tproxy/netcat-ip_transparent-support.patch
50
51
522. Redirecting traffic
53======================
54
55Transparent proxying often involves "intercepting" traffic on a router. This is
56usually done with the iptables REDIRECT target; however, there are serious
57limitations of that method. One of the major issues is that it actually
58modifies the packets to change the destination address -- which might not be
59acceptable in certain situations. (Think of proxying UDP for example: you won't
60be able to find out the original destination address. Even in case of TCP
61getting the original destination address is racy.)
62
63The 'TPROXY' target provides similar functionality without relying on NAT. Simply
64add rules like this to the iptables ruleset above:
65
66# iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY \
67  --tproxy-mark 0x1/0x1 --on-port 50080
68
69Or the following rule to nft:
70
71# nft add rule filter divert tcp dport 80 tproxy to :50080 meta mark set 1 accept
72
73Note that for this to work you'll have to modify the proxy to enable (SOL_IP,
74IP_TRANSPARENT) for the listening socket.
75
76As an example implementation, tcprdr is available here:
77https://git.breakpoint.cc/cgit/fw/tcprdr.git/
78This tool is written by Florian Westphal and it was used for testing during the
79nf_tables implementation.
80
813. Iptables and nf_tables extensions
82====================================
83
84To use tproxy you'll need to have the following modules compiled for iptables:
85 - NETFILTER_XT_MATCH_SOCKET
86 - NETFILTER_XT_TARGET_TPROXY
87
88Or the floowing modules for nf_tables:
89 - NFT_SOCKET
90 - NFT_TPROXY
91
924. Application support
93======================
94
954.1. Squid
96----------
97
98Squid 3.HEAD has support built-in. To use it, pass
99'--enable-linux-netfilter' to configure and set the 'tproxy' option on
100the HTTP listener you redirect traffic to with the TPROXY iptables
101target.
102
103For more information please consult the following page on the Squid
104wiki: http://wiki.squid-cache.org/Features/Tproxy4
105