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Add README from balabit.com - it's nice doc about TPROXY.

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Leonid Evdokimov 2012-12-04 20:57:31 +04:00
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These are the Transparent Proxying patches for Linux kernel 2.6.
The latest version can always be found at
http://www.balabit.com/download/files/tproxy/
What does the term 'proxy' mean?
--------------------------------
A proxy is a server-like program, receiving requests from clients,
forwarding those requests to the real server on behalf of users,
and returning the response as it arrives.
Proxies read and parse the application protocol, and reject invalid
traffic. As most attacks violate the application protocol, disallowing
protocol violations usually protects against attacks.
What is transparent proxying?
-----------------------------
To simplify management tasks of clients sitting behind proxy
firewalls, the technique 'transparent proxying' was invented.
Transparent proxying means that the presence of the proxy is invisible
to the user. Transparent proxying however requires kernel support.
We have a 'REDIRECT' target, isn't that enough?
----------------------------------------------
Real transparent proxying requires the following three features from
the IP stack of the computer it is running on:
1. Redirect sessions destined to the outer network to a local process
using a packet filter rule.
2. Make it possible for a process to listen to connections on a
foreign address.
3. Make it possible for a process to initiate a connection with a
foreign address as a source.
Item #1 is usually provided by packet filtering packages like
Netfilter/IPTables, IPFilter. (yes, this is the REDIRECT target)
All three were provided in Linux kernels 2.2.x, but support for this
was removed.
How to install it?
------------------
Download the latest tproxy-kernel-<kernelversion>*.tar.bz2 tarball
for your kernel (from v2.6.24), with the tproxy-iptables-*.patch file.
Patch your kernel using:
cd /usr/src/linux
cat <path_to_tproxy>/00*.patch | patch -p1
then enable tproxy support, `socket' and `TPROXY' modules
(with optional conntrack support if you need SNAT), compile your kernel
and modules.
The required modules are automatically loaded if the iptables commands
are used.
The IPtables patches:
cd /usr/src/iptables-1.4.X
cat <path_to_tproxy>/tproxy-iptables*.patch | patch -p1
then compile it on the usual way:
./autogen.sh
./configure && make && make install
Squid-3 has official support of TProxy v4.1:
checkout the source code of squid-3 as in
http://wiki.squid-cache.org/Squid3VCS
then compile it:
cd ~/source/squid
./bootstrap.sh
./configure --enable-linux-netfilter && make && make install
Of course you might need to change the path in the examples above.
How to start using it?
----------------------
This implementation of transparent proxying works by marking packets and
changing the route based on packet mark. The foreign address bind and tproxy
redirection is enabled via a new socket option, IP_TRANSPARENT, without it
neither the bind nor the tproxy target works.
Now let's see what happens when a proxy tries to use the required tproxy
features I outlined earlier.
1. Redirection
This is easy, as this was already supported by iptables. Redirection is
equivalent with the following nat rule:
iptables -t nat -A PREROUTING -j DNAT --to-dest <localip> --to-port <proxyport>
<localip> is one the IP address of the interface where the packet
entered the IP stack
<proxyport> is the port where the proxy was bound to
To indicate that this is not simple NAT rule, a separate target, 'TPROXY'
was created:
iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY --on-port <proxyport> \
--tproxy-mark 0x1/0x1
The local IP address is determined automatically, but can be overridden
by the --on-ip parameter.
The marked sockets has to be routed locally:
ip rule add fwmark 1 lookup 100
ip route add local 0.0.0.0/0 dev lo table 100
2. Listening for connections on a foreign address
There are protocols which use more than a single TCP channel for
communication. The best example is FTP which uses a command channel for
sending commands, and a data channel to transfer the body of files. The
secondary channel can be established in both active and passive mode,
active meaning the server connects back to the client, passive meaning
the client connects to the server on another port.
Let's see the passive case, when the client establishes a connection to
the address returned in the response of the PASV FTP command.
As the presence of the proxy is transparent to the client, the target
IP address of the secondary channel (e.g. the address in the PASV
response) is the server (and not the firewall) and this connection must
also be handled by the proxy.
The first solution that comes to mind is to add a a TPROXY rule
automatically (e.g. to redirect a connection destined to a given server
on a given port to a local process), however it is not feasible, adding
rules on the fly should not be required as it would mess the
administrator's own rules, the NAT translation should be done
implicitly without touching the user rulebase.
To do this on a Linux 2.2 kernel it was enough to call bind() on a
socket with a foreign IP address, and if a new connection to the given
foreign IP was routed through the firewall the connection was
intercepted. This solution however distracted the core network kernel
hackers and removed this feature. This implementation is similar to
the old behaviour although it works a bit differently:
* the proxy sets the IP_TRANSPARENT socket option on the listening
socket
* the proxy then binds to the foreign address
* the proxy accepts incoming connections
It requires additional iptables rules with the socket module of the
tproxy patches:
iptables -t mangle -N DIVERT
iptables -t mangle -A PREROUTING -p tcpo -m socket -j DIVERT
iptables -t mangle -A DIVERT -j MARK --set-xmark 0x1/0xffffffff
iptables -t mangle -A DIVERT -j ACCEPT
the best if the second rule is before using the TPROXY target.
3. Initiating connections with a foreign address as a source
Similarly to the case outlined above, it is sometimes necessary to be
able to initiate a connection with a foreign IP address as a source.
Imagine the active FTP case when the FTP client listens for connections
with source address equal to the server. Another example: a webserver
in your DMZ which does access control based on client IP address. If
the proxy could not initiate connections with foreign IP address, the
webserver would see the inner IP address of the firewall itself.
In Linux 2.2 this was accomplished by bind()-ing to a foreign address
prior calling connect(), and it worked. In this tproxy patch it is done
somewhat similar to the case 2 outlined above.
* the proxy calls setsockopt with IP_TRANSPARENT
* the proxy bind to a foreign address
* the tproxy calls connect()
The iptables rules with the socket match are also required here.
How to use it?
--------------
The following use-case assumes a transparent proxy listening on port
50080 and any ip address (0.0.0.0).
First, set up the routing rules with iproute2:
ip rule add fwmark 1 lookup 100
ip route add local 0.0.0.0/0 dev lo table 100
Or, if you want to use packet marking for anything else, the least
significant bit is enough for transparent proxying.
ip rule add fwmark 0x1/0x1 lookup 100
ip route add local 0.0.0.0/0 dev lo table 100
Note that this latter example is only working with newer versions of
iproute2.
For supporting foreign address bind, the socket match is required with
packet marking:
iptables -t mangle -N DIVERT
iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
# DIVERT chain: mark packets and accept
iptables -t mangle -A DIVERT -j MARK --set-mark 1
iptables -t mangle -A DIVERT -j ACCEPT
The last rule is for diverting traffic to the proxy:
iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY \
--tproxy-mark 0x1/0x1 --on-port 50080
If it is a Squid-3 proxy, in /etc/squid/squid.conf the following
rule is necessary for transparent proxying:
http_port 50080 tproxy transparent
Then set up the ACL rules according to your local policy.