1bfc2bc7ad
Recently, transparent proxy support has been added to nf_tables so that this document should be updated with the new information. - Nft commands are added as alternatives to iptables ones. - The link for a patched iptables is removed as it is already part of the mainline iptables implementation (and the link is dead). - tcprdr is added as an example implementation of a transparent proxy Cc: "David S. Miller" <davem@davemloft.net> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Florian Westphal <fw@strlen.de> Cc: KOVACS Krisztian <hidden@sch.bme.hu> Cc: Pablo Neira Ayuso <pablo@netfilter.org> Cc: linux-doc@vger.kernel.org Signed-off-by: Máté Eckl <ecklm94@gmail.com> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
104 lines
3.7 KiB
Text
104 lines
3.7 KiB
Text
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
|