or your local metalab mirror. transproxy will not be discussed further in this
document.
</p>
<p>
A cleaner solution is to get a web proxy that is aware of transparent proxying
itself. The one we are going to focus on here is squid. Squid is an Open Source
caching proxy server for Unix systems. It is available from <htmlurl url="http://www.squid-cache.org" name="www.squid-cache.org">
</p>
<p>Alternatively, instead of redirecting the connections to local ports, we could redirect the connections to remote ports. This is discussed in the <ref id="twoboxes" name="Transparent Proxy to a Remote Box"> section. Readers interested in this approach should skip down to that section. Readers interested on doing everything on one box can safely ignore that section.
<sect1>
Scope of this document
<p>
This document will focus on squid version 2.4 and Linux kernel version
2.4, the most current stable releases as of this writing (December 2001). It
should also work with most of the later 2.3 kernels. If you need information
about earlier releases of squid or Linux, you can find some earlier
documents at <htmlurl url="http://www.unxsoft.com/transproxy.html" name="www.unxsoft.com/transproxy.html">.
</p>
<p>
If you are using a development kernel or a development version of squid, you are on your own. This document may help you, but YMMV.
</p>
<p>
Note that this document focuses only on HTTP proxing. I get many emails asking
about transparent FTP proxying. Squid can't do it. Now, allegedly a program
called Frox can. I have not tried this myself, so I cannot say how well it
works. You can find it at <htmlurl url="http://www.hollo32.fsnet.co.uk/frox/" name ="http://www.hollo32.fsnet.co.uk/frox/">.
</p>
<p>
I only focus on squid here, but Apache can also function as a caching proxy
server. (If you are not sure which to use, I recommend squid, since it was
built from the ground up to be
a caching proxy server, Apache's caching proxy features are more of
afterthought additions to an already existing system.)
If you want use Apache instead of squid: follow all the instructions in this
document that pertain to the kernel and iptables rules. Ignore the squid
specific sections, and instead look at
<htmlurl url="http://lupo.campus.uniroma2.it/progetti/mod_tproxy/" name="http://lupo.campus.uniroma2.it/progetti/mod_tproxy/"> for source code and
instructions for a transparent proxy module for Apache (thanks to Cristiano Paris (c.paris@libero.it) for contributing this).
<p>Finally, as far as transparently proxing HTTPS (e.g. secure web pages using
SSL, etc.), you can't do it. Don't even ask. For the explanation, do a
search for 'man-in-the-middle attack'. Note that you probably don't
really need to transparently proxy HTTPS anyway, since squid does not
cache secure pages.
<sect>
Configuring the Kernel
<p>
First, we need to make sure all the proper options are set in your kernel.
If you are using a stock kernel from your distribution, transparent proxying
may or may not be enabled.
If you are unsure, the best way to tell is to simply skip this section, and
if the commands in the next section give you weird errors, it's probably because
the kernel wasn't configured properly.
</p>
<p>
If your kernel is not configured for transparent proxying, you will need
to recompile. Recompiling a kernel is a complex process (at least at first),
and it is beyond the scope of this document. If you need help compiling a kernel,
please see <htmlurl url="http://metalab.unc.edu/pub/Linux/docs/HOWTO/Kernel-HOWTO" name="The Kernel HOWTO">
</p>
<p>
The options you need to set in your configuration are as follows (Note:
none of these can be built as modules)
</p>
<p>
<itemize>
<item> Under General Setup
<itemize>
<item>
Networking support
<item>
Sysctl support
</itemize>
<item> Under Networking Options
<itemize>
<item>Network packet filtering
<item>TCP/IP networking
</itemize>
<item> Under Networking Options -> IP: Netfilter Configuration
<itemize>
<item>Connection tracking
<item>IP tables support
<item>Full NAT
<item>REDIRECT target support
</itemize>
<item>Under File Systems
<itemize>
<item>/proc filesystem support
</itemize>
</itemize>
You must say NO to ``Fast switching'' under Networking Options.
</p><p>
Once you have your new kernel up and running, you may need to enable IP
forwarding. IP forwarding allows your computer to act as a router. Since this
is not what the average user wants to do, it is off by default and must be
explicitly enabled at run-time. However, your distribution might do this for
you already. To check, do ``cat /proc/sys/net/ipv4/ip_forward''. If you see
``1'' you're good. Otherwise, do ``echo '1' > /proc/sys/net/ipv4/ip_forward''.
You will then want to add that command to your appropriate bootup scripts (depending on your distribution, these may live in /etc/rc.d, /etc/init.d, or maybe somewhere else entirely).
</p>
<sect>
Setting up squid
<p>
Now, we need to get squid up and running. Download the latest source tarball
from <htmlurl url="http://www.squid-cache.org" name="www.squid-cache.org">.
Make sure you get a STABLE version, not a DEVEL version.
The latest as of this writing was squid-2.4.STABLE3.tar.gz. Note that AFAIK, you must have squid-2.4 for linux kernel 2.4. The reason is that the
mechanism by which the process determines the original destination address
has changed from linux 2.2, and only squid-2.4 has this new code in it. (For those of you who are interested, previously the getsockname() call was hacked to provide the original destination address, but now the call is getsockopt() with a level of SOL_IP and an option of SO_ORIGINAL_DST).
</p>
<p>
Now, untar and gunzip the archive (use ``tar -xzf <filename>'').
Run the autoconfiguration script and tell it to include netfilter code
(``./configure --enable-linux-netfilter''), compile (``make'') and
then install (``make install'').
</p>
<p>
Now, we need to edit the default squid.conf file (installed to /usr/local/squid/etc/squid.conf, unless you changed the defaults). The squid.conf file is heavily
commented. In fact, some of the best documentation available for squid is in
the squid.conf file. After you get it all up and running, you should go back
and reread the whole thing. But for now, let's just get the minimum required.
Find the following directives, uncomment them, and change them to the
appropriate values:
</p>
<p>
<itemize>
<item>
httpd_accel_host virtual
<item>
httpd_accel_port 80
<item>
httpd_accel_with_proxy on
<item>
httpd_accel_uses_host_header on
</itemize>
</p><p>
Next, look at the cache_effective_user and cache_effective_group directives.
Unless the default nobody/nogroup has been created on your system (AFAIK, it
is not created out of the box on many popular distributions, including RH7.1),
you'll either need to create those, or create another username/group for
squid to run under. I strongly recommend that you create a username/group of
squid/squid and run under that, but you could use any existing user/group
if you want.
</p><p>
Finally, look at the http_access directive. The default is usually ``http_access
deny all''. This will prevent anyone from accessing squid. For now, you can
change this to ``http_access allow all'', but once it is working, you will
probably want to read the directions on ACLs (Access Control Lists), and setup
the cache such that only people on your local network (or whatever) can access
the cache. This may seem silly, but you should put some kind of restrictions
on access to your cache. People behind filtering firewalls (such as porn
filters, or filters in nations where speech is not very free) often ``hijack'' onto
wide open proxies and eat up your bandwidth.
</p>
<p>
Initialize the cache directories with ``squid -z'' (if this is a not a
new installation of squid, you should skip this step).
</p>
<p>
Now, run squid using the RunCache script in the /usr/local/squid/bin/ directory.
If it works, you should be able to set your web browser's proxy settings to
the IP of the box and port 3128 (unless you changed the default port number)
and access squid as a normal proxy.
</p>
<p>
For additional help configuring squid, see the squid FAQ at <htmlurl url="http://www.squid-cache.org" name="www.squid-cache.org">
</p>
<sect>
Setting up iptables (Netfilter)
<p>
iptables is a new thing for Linux kernel 2.4 that replaces ipchains.
If your distribution came with a 2.4 kernel, it probably has iptables
already installed. If not, you'll have to download it (and possibly
compile it). The homepage is <htmlurl url="http://netfilter.samba.org/" name="netfilter.samba.org">.
You make be able to find binary RPMs elsewhere, I haven't looked. For the
curious, there is plenty of documentation on the netfilter site.
</p>
<p>
To set up the rules, you will need to know two things, the interface that
the to-be-proxied requests are coming in on (I'll use eth0 as an example)
and the port squid is running on (I'll use the default of 3128 as an example).
You will want to add the above commands to your appropriate bootup script
under /etc/rc.d/. Readers upgrading from 2.2 kernels should note that
this is the only command needed. 2.2 kernels required two extra commands
in order to prevent forwarding loops. The infastructure of netfilter is
much nicer, and only this command is needed.
</p>
<sect>
Transparent Proxy to a Remote Box <label id="twoboxes">
<p>
Now, the question naturally arises, if we can do all this nifty stuff
redirecting HTTP connections to local ports, could we do the same thing
but to a remote box (e.g., the machine with squid running is not the same
machine as iptables is running on). The answer is yes, but it takes a little different magic words. If you only want to redirect to the local box
(the normal case), skip this section.
(Thanks to Giles Coochey for help writing this section).
</p><p>
For the purposes of example commands, let's assume we have two boxes called squid-box and iptables-box, and that they are on the network local-network. In the commands below, replace these strings with the actual IP addresses or
name of your machines and network.
</p><p>
First, the machine that squid will be running on, squid-box.
Now, the machine that iptables will be running on, iptables-box You will need to configure the kernel as described above, except that you don't need the REDIRECT target support). Now, for the iptables commands. You need three:
</p><p> The first one sends the packets to squid-box from iptables-box. The second
makes sure that the reply gets sent back through iptables-box, instead of
directly to the client (this is very important!). The last one makes sure
the iptables-box will forward the appropriate packets to squid-box. It may
not be needed. YMMV. Note that we specified '-i eth0' and then '-o eth0',
which stands for input interface eth0 and output interface eth0. If your
packets are
entering and leaving on different interfaces, you will need to adjust the
commands accordingly.
</p><p>
Add these commands to your appropriate startup scripts under /etc/rc.d/
</p><p>
<sect1>What if iptables-box is on a dynamic IP?
<p>
If the iptables-box is on a dynamic IP address (e.g. a dialup PPP connection, or a DHCP assigned IP address from a cable modem, etc.), then you will want to
make a slight change to the above commands. Replace the second command with this one:
Hints: insmod errors can be caused by incorrect module parameters;
including invalid IO or IRQ parameters.<p>
perhaps iptables or your kernel needs to be upgraded...<p>
</tscreen>
then you are probably running Red Hat 7.1. The folks at Red Hat, in all
their wisdom, decided to load the ipchains module by default on startup.
I guess this was for backwards compatibility for those who haven't learned
iptables yet. However, the problem is that ipchains and iptables are
mutually incompatible. Since ipchains has been secretly loaded by RH, you
cannot use iptables commands. To see if this is your problem, do the
command ``lsmod'' and look for the module named ``ipchains''. If you
see it, that is your problem. The quick fix is to execute the command
``rmmod ipchains'' before you issue any iptables commands. Long term,
you'll want to search through RH's setup scripts and remove the reference
to ipchains in the first place (I don't really know where this is, as
I've never used RH 7.1 myself).
<sect>
Further Resources
<p>
Should you still need assistance, you may wish to check the squid FAQ or
the squid mailing list at www.squid-cache.org. You may also e-mail me at drk@unxsoft.com, and I'll try to answer your questions if time permits (sometimes
it does, but sometimes it doesn't). Please, please, please, send the output of
``iptables -t nat -L'' and relavent portions of any configuration files in your e-mail, or else I will probably not be able to help you out much.
Regrettably, even though this document has been translated to many
different languages, I can only answer questions asked in English.