110 lines
3.8 KiB
ReStructuredText
110 lines
3.8 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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=========================
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Transparent proxy support
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=========================
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This feature adds Linux 2.2-like transparent proxy support to current kernels.
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To use it, enable the socket match and the TPROXY target in your kernel config.
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You will need policy routing too, so be sure to enable that as well.
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From Linux 4.18 transparent proxy support is also available in nf_tables.
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1. Making non-local sockets work
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================================
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The idea is that you identify packets with destination address matching a local
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socket on your box, set the packet mark to a certain value::
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# iptables -t mangle -N DIVERT
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# iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
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# iptables -t mangle -A DIVERT -j MARK --set-mark 1
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# iptables -t mangle -A DIVERT -j ACCEPT
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Alternatively you can do this in nft with the following commands::
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# nft add table filter
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# nft add chain filter divert "{ type filter hook prerouting priority -150; }"
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# nft add rule filter divert meta l4proto tcp socket transparent 1 meta mark set 1 accept
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And then match on that value using policy routing to have those packets
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delivered locally::
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# ip rule add fwmark 1 lookup 100
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# ip route add local 0.0.0.0/0 dev lo table 100
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Because of certain restrictions in the IPv4 routing output code you'll have to
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modify your application to allow it to send datagrams _from_ non-local IP
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addresses. All you have to do is enable the (SOL_IP, IP_TRANSPARENT) socket
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option before calling bind::
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fd = socket(AF_INET, SOCK_STREAM, 0);
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/* - 8< -*/
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int value = 1;
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setsockopt(fd, SOL_IP, IP_TRANSPARENT, &value, sizeof(value));
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/* - 8< -*/
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name.sin_family = AF_INET;
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name.sin_port = htons(0xCAFE);
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name.sin_addr.s_addr = htonl(0xDEADBEEF);
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bind(fd, &name, sizeof(name));
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A trivial patch for netcat is available here:
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http://people.netfilter.org/hidden/tproxy/netcat-ip_transparent-support.patch
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2. Redirecting traffic
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======================
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Transparent proxying often involves "intercepting" traffic on a router. This is
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usually done with the iptables REDIRECT target; however, there are serious
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limitations of that method. One of the major issues is that it actually
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modifies the packets to change the destination address -- which might not be
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acceptable in certain situations. (Think of proxying UDP for example: you won't
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be able to find out the original destination address. Even in case of TCP
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getting the original destination address is racy.)
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The 'TPROXY' target provides similar functionality without relying on NAT. Simply
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add rules like this to the iptables ruleset above::
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# iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY \
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--tproxy-mark 0x1/0x1 --on-port 50080
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Or the following rule to nft:
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# nft add rule filter divert tcp dport 80 tproxy to :50080 meta mark set 1 accept
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Note that for this to work you'll have to modify the proxy to enable (SOL_IP,
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IP_TRANSPARENT) for the listening socket.
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As an example implementation, tcprdr is available here:
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https://git.breakpoint.cc/cgit/fw/tcprdr.git/
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This tool is written by Florian Westphal and it was used for testing during the
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nf_tables implementation.
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3. Iptables and nf_tables extensions
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====================================
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To use tproxy you'll need to have the following modules compiled for iptables:
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- NETFILTER_XT_MATCH_SOCKET
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- NETFILTER_XT_TARGET_TPROXY
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Or the floowing modules for nf_tables:
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- NFT_SOCKET
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- NFT_TPROXY
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4. Application support
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======================
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4.1. Squid
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----------
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Squid 3.HEAD has support built-in. To use it, pass
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'--enable-linux-netfilter' to configure and set the 'tproxy' option on
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the HTTP listener you redirect traffic to with the TPROXY iptables
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target.
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For more information please consult the following page on the Squid
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wiki: http://wiki.squid-cache.org/Features/Tproxy4
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