Documentation/networking/tproxy.rst

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Documentation/networking/tproxy.rst
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.. 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 --transparent -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

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