<!doctype linuxdoc system>

<article>

<title>Transparent Proxy with Linux and Squid mini-HOWTO</title>
<author>Daniel Kiracofe</author>
<date>v1.15, August 2002</date>
<abstract>
This document provides information on how to setup a transparent caching
HTTP proxy server using only Linux and squid.
</abstract>

<toc>

<sect>
Introduction
<sect1>
Comments
<p>
Comments and general feedback on this mini HOWTO are welcome and can be
directed to its author, Daniel Kiracofe, at drk@unxsoft.com. 
</p>
<sect1>
Copyrights and Trademarks 
<p>
Copyright 2000-2002 by Daniel Kiracofe
</p>
<p>
This manual may be reproduced in whole or in part, without fee, subject
 to the following restrictions: 
</p>
<p>
<itemize>
 <item>
The copyright notice above and this permission notice must be preserved
 complete on all complete or partial copies 
 <item>
Translation to another language is permitted, provided that the author
is notified prior to the translation.
 <item>
Any derived work must be approved by the author in writing
 before distribution. 
 <item>
If you distribute this work in part, instructions for obtaining the complete
 version of this manual must be included, and a means for obtaining a complete
 version provided. 
 <item>
Small portions may be reproduced as illustrations for reviews or quotes
 in other works without this permission notice if proper citation is given.
</itemize>
</p><p>
Exceptions to these rules may be granted for academic purposes: Write to
 the author and ask. These restrictions are here to protect us as authors, not
 to restrict you as learners and educators. Any source code (aside from the
 SGML this document was written in) in this document is placed under the GNU
 General Public License, available via anonymous FTP from the GNU archive.
</p>
<sect1>
&num;include &lt;disclaimer.h&gt;
<p>
No warranty, expressed or implied, etc, etc, etc...
</p>
<sect>
Overview of Transparent Proxying
<sect1>
Motivation
<p>
In ``ordinary'' proxying, the client specifies the hostname and port number
 of a proxy in his web browsing software. The browser then makes requests to
 the proxy, and the proxy forwards them to the origin servers. This is all fine
 and good, but sometimes one of several situations arise. Either
</p>
<p>
<itemize>
 <item>
You want to force clients on your network to use the proxy, whether they
 want to or not.
 <item>
You want clients to use a proxy, but don't want them to know they're being
 proxied.
 <item>
You want clients to be proxied, but don't want to go to all the work of
 updating the settings in hundreds or thousands of web browsers.
</itemize>
</p><p>
This is where transparent proxying comes in. A web request can be intercepted
 by the proxy, transparently. That is, as far as the client software knows,
 it is talking to the origin server itself, when it is really talking to
 the proxy server.  (Note that the transparency only applies to the client; 
the server knows that a proxy is involved, and will see the IP address of
the proxy, not the IP address of the user.  Although, squid may pass
an X-Forwarded-For header, so that the server can determine the original
user's IP address if it groks that header).
</p>
<p>
Cisco routers support transparent proxying. So do many switches. But, 
(surprisingly enough)
 Linux can act as a router, and can perform transparent proxying by redirecting
 TCP connections to local ports. However, we also need to make our web proxy
 aware of the affect of the redirection, so that it can make connections to
 the proper origin servers. There are two general ways this works:
</p>
<p>
The first is when your web proxy is not transparent proxy aware. You can
 use a nifty little daemon called transproxy that sits in front of your web
 proxy and takes care of all the messy details for you. transproxy was written
 by John Saunders, and is available from 

<htmlurl url="ftp://ftp.nlc.net.au/pub/linux/www/" name="ftp://ftp.nlc.net.au/pub/linux/www/">
 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 (August 2002). 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://users.gurulink.com/transproxy/" name="http://users.gurulink.com/transproxy/">. Note that this site has moved from it's previous location.
</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).
<sect1>
HTTPS
<p>Finally, as far as transparently proxing HTTPS (e.g. secure web pages using
SSL, TSL, 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 can not
cache secure pages.  
<sect1>
Proxy Authentication
<p>
You cannot use Proxy Authentication transparently.  See the 
<htmlurl url="http://www.squid-cache.org/Doc/FAQ/FAQ.html" name="Squid FAQ">
for (slightly) more details.
<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:
if you prefer modules, some (but not all) of these can be built as modules. Luckily, everything that is not modularizable is probably got in your kernel anyway.)
</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' &gt; /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.STABLE4.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 &lt;filename&gt;'').
 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).
</p>
<p>
Now, the magic words for transparent proxying:
</p>
<p>
<itemize>
 <item>
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3128
</itemize>
</p><p>
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.
</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>
I will present two different approaches here.
<sect1>
First method (simpler, but does not work for some esoteric cases)
<p>

First, we need to machine that squid will be running on, squid-box.
You do not need iptables 
or any special kernel options on this machine, just squid.  You *will*, 
however, need the 'http_accel' options as described above.  (Previous version 
of this HOWTO suggested that you did not need those options.  That was a 
mistake. Sorry to have confused people...) 
</p><p>
Now, the machine that iptables will be running on, iptables-box You will need to configure the kernel as described in section 3 above, except that you don't need the REDIRECT target support).  Now, for the iptables commands. You need three:
</p><p>
<itemize>
<item>iptables -t nat -A PREROUTING -i eth0 -s ! <bf>squid-box</bf> -p tcp --dport 80 -j DNAT --to <bf>squid-box</bf>:3128
<item>iptables -t nat -A POSTROUTING -o eth0 -s <bf>local-network</bf> -d <bf>squid-box</bf> -j SNAT --to <bf>iptables-box</bf>
<item>iptables -A FORWARD -s <bf>local-network</bf> -d <bf>squid-box</bf> -i eth0 -o eth0 -p tcp --dport 3128 -j ACCEPT
</itemize>
</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>
(Thanks to Giles Coochey for help writing this section).
<sect1>
Second method (more complicated, but more general)
<p>
Our first shot at this works good, but there is a minor drawback in that
HTTP/1.0 connections without the Host header do not get handled properly.
Connections that are fully or partially HTTP/1.1 compliant work fine.  As
most modern web browsers send the Host header, this is not a problem for
most people.  However, some small programs or embedded devices may send
only very simple HTTP/1.0 requests.  If you want to support these, we'll
 need to do a little more work.
Namely, on iptables-box we'll need the following options enabled in the kernel
in addition to what was specified above:

<itemize>
<item>IP: advanced router                        
<item>IP: policy routing                              
<item>IP: use netfilter MARK value as routing key   
<item>IP: Netfilter Configuration ->  Packet mangling
<item>IP: Netfilter Configuration ->  MARK target support 
</itemize>

You'll also need the iproute2 tools.  Your distribution probably already
has them installed, but if not, look at ftp://ftp.inr.ac.ru/ip-routing/
<p>
You'll want to use the following set of commands on iptables-box:
<itemize>
<item>iptables -t mangle -A PREROUTING -j ACCEPT -p tcp --dport 80 -s <bf>squid-box</bf>
<item>iptables -t mangle -A PREROUTING -j MARK --set-mark 3 -p tcp --dport 80
<item>ip rule add fwmark 3 table 2
<item>ip route add default via <bf>squid-box</bf> dev eth1 table 2
</itemize>
Note that the choice of firewall mark (3) and routing table (2) was fairly
arbitrary.  If you are already using policy routing or firewall marking for
some other purpose, make sure you choose unique numbers here. Otherwise,
don't worry about it. 
<p>

Next, squid-box.  Use this command, which should look remarkably similar to a
command we've seen previously.
<itemize>
<item>iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3128
</itemize>
As before, add all of these commands to the appropriate startup scripts.
<p>
Here is a brief explanation of how this works: in method one, we used Network
Address Translation to get the packets to the other box.  The result of this
is that the packet gets altered.  This alteration is what causes some kinds
of clients mentioned above to fail. In method two,
we use a magic thing called policy routing.  The first thing we do is to
select the packets we want.  Thus, all packets on port 80, except those
coming from squid-box itself, are MARKed.  Then, when the kernel goes to
make a routing decision, the MARKed packets aren't routing using the normal
routing table that you access with the ``route'' command but with a special
table.  This special table has only one entry, a default gateway to squid-box.
Thus, the packet is sent merrily on it's way without every having been altered.
So, even HTTP/1.0 connections can be handled perfectly. 
(Thanks to Michal Svoboda for suggesting and helping to write this section)
<p>

<sect1>Method One: 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:
</p><p>
<itemize>
<item>iptables -t nat -A POSTROUTING -o eth0 -s <bf>local-network</bf> -d <bf>squid-box</bf> -j MASQUERADE
</itemize>
<p>
This change avoids having to specify the IP address of iptables-box in the command.  Since it will change often, you'd have to change your commands to 
reflect it.  This will save you a lot of hassle.  
</p>
</sect1>
</sect>
<sect>
Transparent Proxy With Bridging
<p>
Warning, this is really esoteric stuff.  If you need it, you'll know.  If
not, skip this section.  Thanks to Lewis Shobbrook (lshobbrook@fasttrack.net.au) for contributing to this section.
<p>
If you are trying to setup a transparent proxy on a Linux machine that has
been configured as a bridge, you will need to add one additional iptables 
command to what we had in section 5.
Specifically, you need to explicitly allow connections to the machine
on port 3128 (or any other port squid is listening on), otherwise the 
machine will just forward them over to the other interface like a good 
little bridge.  Here's the magic words:
<itemize><item>
iptables -A INPUT -i <bf>interface</bf> -p tcp -d <bf>your_bridge_ip</bf> -s <bf>local-network</bf> --dport 3128 -m state --state NEW,ESTABLISHED -j ACCEPT
</item></itemize>
Replacing <bf>interface</bf> with the interface that corresponds to
<bf>your_bridge_ip</bf> (typically eth0 or eth1).  First time bridge users 
should also note that you'll probably want to repeat the same command with
``3128'' replaced by ``telnet'' if you want to administer your bridge
remotely.
</sect>

<sect>
Put it all together
<p>
If everything has gone well so far, go to another machine, change it's
 gateway to the IP of the box with iptables running on it, and surf away. To make sure that 
requests are really being forwarded through your proxy instead of straight 
to the origin server, check the log file /usr/local/squid/logs/access.log
</p>
<sect>
Troubleshooting
<p>
There is one problem that occurs often enough to mention here.  If you 
get the following error:
<tscreen>
/lib/modules/2.4.2-2/kernel/net/ipv4/netfilter/ip_tables.o
init_modules: Device or resource busy
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.x.  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.  To permanently
remove these commands from your startup scripts, the following command
should work:  ``/sbin/chkconfig --level 2345 ipchains off''.
(Thanks to Rasmus Glud for pointing this command out to me).

<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.  And please
make sure you've read the whole HOWTO before asking a question.
Regrettably, even though this document has been translated to many
different languages, I can only answer questions asked in English.
</p>

</article>






