Documentation/networking/tproxy.txt

   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 the socket match and the TPROXY target in your kernel config.
   6 You will need policy routing too, so be sure to enable that as well.
   7
   8
   9 1. Making non-local sockets work
  10 ================================
  11
  12 The idea is that you identify packets with destination address matching a local
  13 socket on your box, set the packet mark to a certain value, and then match on that
  14 value using policy routing to have those packets delivered locally:
  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
  21 # ip rule add fwmark 1 lookup 100
  22 # ip route add local 0.0.0.0/0 dev lo table 100
  23
  24 Because of certain restrictions in the IPv4 routing output code you'll have to
  25 modify your application to allow it to send datagrams _from_ non-local IP
  26 addresses. All you have to do is enable the (SOL_IP, IP_TRANSPARENT) socket
  27 option before calling bind:
  28
  29 fd = socket(AF_INET, SOCK_STREAM, 0);
  30 /* - 8< -*/
  31 int value = 1;
  32 setsockopt(fd, SOL_IP, IP_TRANSPARENT, &value, sizeof(value));
  33 /* - 8< -*/
  34 name.sin_family = AF_INET;
  35 name.sin_port = htons(0xCAFE);
  36 name.sin_addr.s_addr = htonl(0xDEADBEEF);
  37 bind(fd, &name, sizeof(name));
  38
  39 A trivial patch for netcat is available here:
  40 http://people.netfilter.org/hidden/tproxy/netcat-ip_transparent-support.patch
  41
  42
  43 2. Redirecting traffic
  44 ======================
  45
  46 Transparent proxying often involves "intercepting" traffic on a router. This is
  47 usually done with the iptables REDIRECT target; however, there are serious
  48 limitations of that method. One of the major issues is that it actually
  49 modifies the packets to change the destination address -- which might not be
  50 acceptable in certain situations. (Think of proxying UDP for example: you won't
  51 be able to find out the original destination address. Even in case of TCP
  52 getting the original destination address is racy.)
  53
  54 The 'TPROXY' target provides similar functionality without relying on NAT. Simply
  55 add rules like this to the iptables ruleset above:
  56
  57 # iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY \
  58   --tproxy-mark 0x1/0x1 --on-port 50080
  59
  60 Note that for this to work you'll have to modify the proxy to enable (SOL_IP,
  61 IP_TRANSPARENT) for the listening socket.
  62
  63
  64 3. Iptables extensions
  65 ======================
  66
  67 To use tproxy you'll need to have the 'socket' and 'TPROXY' modules
  68 compiled for iptables. A patched version of iptables is available
  69 here: http://git.balabit.hu/?p=bazsi/iptables-tproxy.git
  70
  71
  72 4. Application support
  73 ======================
  74
  75 4.1. Squid
  76 ----------
  77
  78 Squid 3.HEAD has support built-in. To use it, pass
  79 '--enable-linux-netfilter' to configure and set the 'tproxy' option on
  80 the HTTP listener you redirect traffic to with the TPROXY iptables
  81 target.
  82
  83 For more information please consult the following page on the Squid
  84 wiki: http://wiki.squid-cache.org/Features/Tproxy4