1.. SPDX-License-Identifier: GPL-2.0
2
3=========================
4Transparent proxy support
5=========================
6
7This feature adds Linux 2.2-like transparent proxy support to current kernels.
8To use it, enable the socket match and the TPROXY target in your kernel config.
9You will need policy routing too, so be sure to enable that as well.
10
11From Linux 4.18 transparent proxy support is also available in nf_tables.
12
131. Making non-local sockets work
14================================
15
16The idea is that you identify packets with destination address matching a local
17socket on your box, set the packet mark to a certain value::
18
19    # iptables -t mangle -N DIVERT
20    # iptables -t mangle -A PREROUTING -p tcp -m socket --transparent -j DIVERT
21    # iptables -t mangle -A DIVERT -j MARK --set-mark 1
22    # iptables -t mangle -A DIVERT -j ACCEPT
23
24Alternatively you can do this in nft with the following commands::
25
26    # nft add table filter
27    # nft add chain filter divert "{ type filter hook prerouting priority -150; }"
28    # nft add rule filter divert meta l4proto tcp socket transparent 1 meta mark set 1 accept
29
30And then match on that value using policy routing to have those packets
31delivered locally::
32
33    # ip rule add fwmark 1 lookup 100
34    # ip route add local 0.0.0.0/0 dev lo table 100
35
36Because of certain restrictions in the IPv4 routing output code you'll have to
37modify your application to allow it to send datagrams _from_ non-local IP
38addresses. All you have to do is enable the (SOL_IP, IP_TRANSPARENT) socket
39option before calling bind::
40
41    fd = socket(AF_INET, SOCK_STREAM, 0);
42    /* - 8< -*/
43    int value = 1;
44    setsockopt(fd, SOL_IP, IP_TRANSPARENT, &value, sizeof(value));
45    /* - 8< -*/
46    name.sin_family = AF_INET;
47    name.sin_port = htons(0xCAFE);
48    name.sin_addr.s_addr = htonl(0xDEADBEEF);
49    bind(fd, &name, sizeof(name));
50
51A trivial patch for netcat is available here:
52http://people.netfilter.org/hidden/tproxy/netcat-ip_transparent-support.patch
53
54
552. Redirecting traffic
56======================
57
58Transparent proxying often involves "intercepting" traffic on a router. This is
59usually done with the iptables REDIRECT target; however, there are serious
60limitations of that method. One of the major issues is that it actually
61modifies the packets to change the destination address -- which might not be
62acceptable in certain situations. (Think of proxying UDP for example: you won't
63be able to find out the original destination address. Even in case of TCP
64getting the original destination address is racy.)
65
66The 'TPROXY' target provides similar functionality without relying on NAT. Simply
67add rules like this to the iptables ruleset above::
68
69    # iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY \
70      --tproxy-mark 0x1/0x1 --on-port 50080
71
72Or the following rule to nft:
73
74# nft add rule filter divert tcp dport 80 tproxy to :50080 meta mark set 1 accept
75
76Note that for this to work you'll have to modify the proxy to enable (SOL_IP,
77IP_TRANSPARENT) for the listening socket.
78
79As an example implementation, tcprdr is available here:
80https://git.breakpoint.cc/cgit/fw/tcprdr.git/
81This tool is written by Florian Westphal and it was used for testing during the
82nf_tables implementation.
83
843. Iptables and nf_tables extensions
85====================================
86
87To use tproxy you'll need to have the following modules compiled for iptables:
88
89 - NETFILTER_XT_MATCH_SOCKET
90 - NETFILTER_XT_TARGET_TPROXY
91
92Or the floowing modules for nf_tables:
93
94 - NFT_SOCKET
95 - NFT_TPROXY
96
974. Application support
98======================
99
1004.1. Squid
101----------
102
103Squid 3.HEAD has support built-in. To use it, pass
104'--enable-linux-netfilter' to configure and set the 'tproxy' option on
105the HTTP listener you redirect traffic to with the TPROXY iptables
106target.
107
108For more information please consult the following page on the Squid
109wiki: http://wiki.squid-cache.org/Features/Tproxy4
110