.. SPDX-License-Identifier: GPL-2.0
  
  =========================

  Transparent proxy support
  =========================
  
  This feature adds Linux 2.2-like transparent proxy support to current kernels.

  To use it, enable the socket match and the TPROXY target in your kernel config.
  You will need policy routing too, so be sure to enable that as well.

  From Linux 4.18 transparent proxy support is also available in nf_tables.

  
  1. Making non-local sockets work
  ================================
  
  The idea is that you identify packets with destination address matching a local

  socket on your box, set the packet mark to a certain value::

      # iptables -t mangle -N DIVERT
      # iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
      # iptables -t mangle -A DIVERT -j MARK --set-mark 1
      # iptables -t mangle -A DIVERT -j ACCEPT

  Alternatively you can do this in nft with the following commands::

      # nft add table filter
      # nft add chain filter divert "{ type filter hook prerouting priority -150; }"
      # nft add rule filter divert meta l4proto tcp socket transparent 1 meta mark set 1 accept

  
  And then match on that value using policy routing to have those packets

  delivered locally::

      # ip rule add fwmark 1 lookup 100
      # ip route add local 0.0.0.0/0 dev lo table 100

  
  Because of certain restrictions in the IPv4 routing output code you'll have to
  modify your application to allow it to send datagrams _from_ non-local IP
  addresses. All you have to do is enable the (SOL_IP, IP_TRANSPARENT) socket

  option before calling bind::
  
      fd = socket(AF_INET, SOCK_STREAM, 0);
      /* - 8< -*/
      int value = 1;
      setsockopt(fd, SOL_IP, IP_TRANSPARENT, &value, sizeof(value));
      /* - 8< -*/
      name.sin_family = AF_INET;
      name.sin_port = htons(0xCAFE);
      name.sin_addr.s_addr = htonl(0xDEADBEEF);
      bind(fd, &name, sizeof(name));

  
  A trivial patch for netcat is available here:
  http://people.netfilter.org/hidden/tproxy/netcat-ip_transparent-support.patch
  
  
  2. Redirecting traffic
  ======================
  
  Transparent proxying often involves "intercepting" traffic on a router. This is
  usually done with the iptables REDIRECT target; however, there are serious
  limitations of that method. One of the major issues is that it actually
  modifies the packets to change the destination address -- which might not be
  acceptable in certain situations. (Think of proxying UDP for example: you won't
  be able to find out the original destination address. Even in case of TCP
  getting the original destination address is racy.)
  
  The 'TPROXY' target provides similar functionality without relying on NAT. Simply

  add rules like this to the iptables ruleset above::

      # iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY \
        --tproxy-mark 0x1/0x1 --on-port 50080

  Or the following rule to nft:
  
  # nft add rule filter divert tcp dport 80 tproxy to :50080 meta mark set 1 accept

  Note that for this to work you'll have to modify the proxy to enable (SOL_IP,
  IP_TRANSPARENT) for the listening socket.

  As an example implementation, tcprdr is available here:
  https://git.breakpoint.cc/cgit/fw/tcprdr.git/
  This tool is written by Florian Westphal and it was used for testing during the
  nf_tables implementation.

  3. Iptables and nf_tables extensions
  ====================================

  To use tproxy you'll need to have the following modules compiled for iptables:

   - NETFILTER_XT_MATCH_SOCKET
   - NETFILTER_XT_TARGET_TPROXY

  Or the floowing modules for nf_tables:

   - NFT_SOCKET
   - NFT_TPROXY

  
  4. Application support
  ======================
  
  4.1. Squid
  ----------
  
  Squid 3.HEAD has support built-in. To use it, pass
  '--enable-linux-netfilter' to configure and set the 'tproxy' option on
  the HTTP listener you redirect traffic to with the TPROXY iptables
  target.
  
  For more information please consult the following page on the Squid
  wiki: http://wiki.squid-cache.org/Features/Tproxy4