1Transparent proxy support 2========================= 3 4This feature adds Linux 2.2-like transparent proxy support to current kernels. 5To use it, enable NETFILTER_TPROXY, the socket match and the TPROXY target in 6your kernel config. You will need policy routing too, so be sure to enable that 7as well. 8 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, and then match on that 15value using policy routing to have those packets delivered locally: 16 17# iptables -t mangle -N DIVERT 18# iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT 19# iptables -t mangle -A DIVERT -j MARK --set-mark 1 20# iptables -t mangle -A DIVERT -j ACCEPT 21 22# ip rule add fwmark 1 lookup 100 23# ip route add local 0.0.0.0/0 dev lo table 100 24 25Because of certain restrictions in the IPv4 routing output code you'll have to 26modify your application to allow it to send datagrams _from_ non-local IP 27addresses. All you have to do is enable the (SOL_IP, IP_TRANSPARENT) socket 28option before calling bind: 29 30fd = socket(AF_INET, SOCK_STREAM, 0); 31/* - 8< -*/ 32int value = 1; 33setsockopt(fd, SOL_IP, IP_TRANSPARENT, &value, sizeof(value)); 34/* - 8< -*/ 35name.sin_family = AF_INET; 36name.sin_port = htons(0xCAFE); 37name.sin_addr.s_addr = htonl(0xDEADBEEF); 38bind(fd, &name, sizeof(name)); 39 40A trivial patch for netcat is available here: 41http://people.netfilter.org/hidden/tproxy/netcat-ip_transparent-support.patch 42 43 442. Redirecting traffic 45====================== 46 47Transparent proxying often involves "intercepting" traffic on a router. This is 48usually done with the iptables REDIRECT target; however, there are serious 49limitations of that method. One of the major issues is that it actually 50modifies the packets to change the destination address -- which might not be 51acceptable in certain situations. (Think of proxying UDP for example: you won't 52be able to find out the original destination address. Even in case of TCP 53getting the original destination address is racy.) 54 55The 'TPROXY' target provides similar functionality without relying on NAT. Simply 56add rules like this to the iptables ruleset above: 57 58# iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY \ 59 --tproxy-mark 0x1/0x1 --on-port 50080 60 61Note that for this to work you'll have to modify the proxy to enable (SOL_IP, 62IP_TRANSPARENT) for the listening socket. 63 64 653. Iptables extensions 66====================== 67 68To use tproxy you'll need to have the 'socket' and 'TPROXY' modules 69compiled for iptables. A patched version of iptables is available 70here: http://git.balabit.hu/?p=bazsi/iptables-tproxy.git 71 72 734. Application support 74====================== 75 764.1. Squid 77---------- 78 79Squid 3.HEAD has support built-in. To use it, pass 80'--enable-linux-netfilter' to configure and set the 'tproxy' option on 81the HTTP listener you redirect traffic to with the TPROXY iptables 82target. 83 84For more information please consult the following page on the Squid 85wiki: http://wiki.squid-cache.org/Features/Tproxy4 86