  Transparent Proxy with Linux and Squid mini-HOWTO
  Daniel Kiracofe
  v1.5, May 2001

  This document provides information on how to setup a transparent
  caching HTTP proxy server using only Linux and squid.
  ______________________________________________________________________

  Table of Contents


  1. Introduction

     1.1 Comments
     1.2 Copyrights and Trademarks
     1.3 #include <disclaimer.h>

  2. Overview of Transparent Proxying

     2.1 Motivation
     2.2 Scope of this document

  3. Configuring the Kernel

  4. Setting up squid

  5. Setting up iptables (Netfilter)

  6. Put it all together

  7. Further Resources



  ______________________________________________________________________

  1.  Introduction

  1.1.  Comments

  Comments and general feedback on this mini HOWTO are welcome and can
  be directed to its author, Daniel Kiracofe, at drk@unxsoft.com.

  1.2.  Copyrights and Trademarks

  Copyright 2000-2001 by UnxSoft Ltd (www.unxsoft.com)

  This manual may be reproduced in whole or in part, without fee,
  subject to the following restrictions:


  ·  The copyright notice above and this permission notice must be
     preserved complete on all complete or partial copies

  ·  Translation to another language is permitted, provided that the
     author is notified prior to the translation.

  ·  Any derived work must be approved by the author in writing before
     distribution.

  ·  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.

  ·  Small portions may be reproduced as illustrations for reviews or
     quotes in other works without this permission notice if proper
     citation is given.

  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.

  1.3.  #include <disclaimer.h>

  No warranty, expressed or implied, etc, etc, etc...

  2.  Overview of Transparent Proxying

  2.1.  Motivation

  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


  ·  You want to force clients on your network to use the proxy, whether
     they want to or not.

  ·  You want clients to use a proxy, but don't want them to know
     they're being proxied.

  ·  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.

  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).

  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:

  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

  ftp://ftp.nlc.net.au/pub/linux/www/ or your local metalab mirror.
  transproxy will not be discussed further in this document.

  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 www.squid-cache.org



  2.2.  Scope of this document

  This document will focus on squid version 2.3 and linux kernel version
  2.4, the most current stable releases as of this writing (Jan 2001).
  It should also work with squids as early as 2.0, and most of the later
  2.3 kernels.  If you need information about earlier releases, you may
  find some earlier documents at www.unxsoft.com.

  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.

  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
  http://www.hollo32.fsnet.co.uk/frox/.

  3.  Configuring the Kernel

  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.

  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 The Kernel HOWTO

  The options you need to set in your configuration are as follows
  (Note: none of these can be built as modules)


  ·  Under General Setup

  ·  Networking support

  ·  Sysctl support

  ·  Under Networking Options

  ·  Network packet filtering

  ·  TCP/IP networking

  ·  Under Networking Options -> IP: Netfilter Configuration

  ·  Connection tracking

  ·  IP tables support

  ·  Full NAT

  ·  REDIRECT target support

  ·  Under File Systems

  ·  /proc filesystem support

     You must say NO to ``Fast switching'' under Networking Options.

  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).

  4.  Setting up squid

  Now, we need to get squid up and running. Download the latest source
  tarball from www.squid-cache.org.  Make sure you get a STABLE version,
  not a DEVEL version.  The latest as of this writing was
  squid-2.3.STABLE4.tar.gz.

  Now, untar and gunzip the archive (use ``tar -xzf <filename>'').  Run
  the autoconfiguration script (``./configure''), compile (``make'') and
  then install (``make install'').

  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:


  ·  httpd_accel_host virtual

  ·  httpd_accel_port 80

  ·  httpd_accel_with_proxy on httpd_accel_uses_host_header on

  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.

  Initialize the cache directories with ``squid -z'' (if this is a not a
  new installation of squid, you should skip this step).

  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.

  For additional help configuring squid, see the squid FAQ at www.squid-
  cache.org

  5.  Setting up iptables (Netfilter)

  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 netfilter.kernelnotes.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.

  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).

  Now, the magic words for transparent proxying:


  ·  iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT
     --to-port 3128

  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, as far as the author can tell, this is the only command
  needed.  2.2 kernels required two extra commands in order to prevent
  forwarding loops.  The author was unable to generate any loops.  If
  anyone can generate a forwarding loop using this rule, please send an
  e-mail to drk@unxsoft.com.

  6.  Put it all together

  If everything has gone well so far, go to another machine, change it's
  gateway to the IP of your new squid box, 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

  7.  Further Resources

  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...



