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