Chapter 2. introduction to squid

Table of Contents

about proxy servers
open proxy servers
installing squid
port 3128
starting and stopping
client proxy settings
upside down images
access control
testing squid
name resolution

about proxy servers


A proxy server is a server that caches the internet. Clients connect to the proxy server with a request for an internet server. The proxy server will connect to the internet server on behalf of the client. The proxy server will also cache the pages retrieved from the internet server. A proxy server may provide pages from his cache to a client, instead of connecting to the internet server to retrieve the (same) pages.

A proxy server has two main advantages. It improves web surfing speed when returning cached data to clients, and it reduces the required bandwidth (cost) to the internet.

Smaller organizations sometimes put the proxy server on the same physical computer that serves as a NAT to the internet. In larger organizations, the proxy server is one of many servers in the DMZ.

When web traffic passes via a proxy server, it is common practice to configure the proxy with extra settings for access control. Access control in a proxy server can mean user account access, but also website(url), ip-address or dns restrictions.

open proxy servers

You can find lists of open proxy servers on the internet that enable you to surf anonymously. This works when the proxy server connects on your behalf to a website, without logging your ip-address. But be careful, these (listed) open proxy servers could be created in order to eavesdrop upon their users.


This module is an introduction to the squid proxy server ( We will first configure squid as a normal proxy server.

installing squid

This screenshot shows how to install squid on Debian with aptitude. Use yum if you are on Red Hat/CentOS.

root@debian7:~# aptitude install squid
The following NEW packages will be installed:
  squid squid-common{a} squid-langpack{a}
0 packages upgraded, 3 newly installed, 0 to remove and 0 not upgraded.
Need to get 1,513 kB of archives. After unpacking 4,540 kB will be used.
Do you want to continue? [Y/n/?]
...output truncated...
Setting up squid-langpack (20120616-1) ...
Setting up squid-common (2.7.STABLE9-4.1) ...
Setting up squid (2.7.STABLE9-4.1) ...
Creating squid spool directory structure
2014/08/01 15:19:31| Creating Swap Directories
Restarting Squid HTTP proxy: squid.

squid's main configuration file is /etc/squid/squid.conf. The file explains every parameter in great detail.

root@debian7:~# wc -l /etc/squid/squid.conf
4948 /etc/squid/squid.conf

port 3128

By default the squid proxy server will lsiten to port 3128.

root@debian7:~# grep ^http_port /etc/squid/squid.conf
http_port 3128

starting and stopping

You can manage squid with the standard service command as shown in this screenshot.

root@debian7:~# service squid start
Starting Squid HTTP proxy: squid.
root@debian7:~# service squid restart
Restarting Squid HTTP proxy: squid.
root@debian7:~# service squid status
squid is running.
root@debian7:~# service squid stop
Stopping Squid HTTP proxy: squid.

client proxy settings

To enable a proxy server in Firefox or Iceweasel go to Edit Preferences and configure as shown in this screenshot (replace with the ip address of your proxy server).

Test that your internet works with the proxy enabled. Also test that after a service squid stop command on your proxy server that you get a message similar to this schreenshot.

To enable a proxy server with Google Chrome (or Debian Chromium) start the program from the command line like this:

paul@debian7:~$ chromium --proxy-server=''

Disabling the proxy with service squid stop should result in an error message similar to this screenshot.

upside down images

A proxy server sits inbetween your browser and the internet. So besides caching of internet data (the original function of a proxy server) and besides firewall like restrictions based on www content, a proxy server is in the perfect position to alter the webpages that you visit.

You could for instance change the advertising on a webpage (or remove certain advertisers), or like we do in this example; change all images so they are upside down.

The server needs command line tools to manipulate images and a perl script that uses these tools (and wget to download the images locally and serve them with apache2). In this example we use imagemagick (which provides tools like convert and mogrify).

root@debian7:~# aptitude install imagemagick wget perl apache2
...output truncated...
root@debian7:~# dpkg -S $(readlink -f $(which mogrify))
imagemagick: /usr/bin/mogrify.im6

The perl script that is shown in the screenshot below can be found on several websites, yet I have not found the original author. It is however a very simple script that uses wget and mogrify to download images (.jpg .gif and .png), flip them and store them in /var/www/images.

root@debian7:~# cat /usr/local/bin/
$count = 0;
$pid = $$;
while (<>) {
 chomp $_;
 if ($_ =~ /(.*\.jpg)/i) {
  $url = $1;
  system("/usr/bin/wget", "-q", "-O","/var/www/images/$pid-$count.jpg", "$url");
  system("/usr/bin/mogrify", "-flip","/var/www/images/$pid-$count.jpg");
  print "$pid-$count.jpg\n";
 elsif ($_ =~ /(.*\.gif)/i) {
  $url = $1;
  system("/usr/bin/wget", "-q", "-O","/var/www/images/$pid-$count.gif", "$url");
  system("/usr/bin/mogrify", "-flip","/var/www/images/$pid-$count.gif");
  print "$pid-$count.gif\n";
 elsif ($_ =~ /(.*\.png)/i) {
  $url = $1;
  system("/usr/bin/wget", "-q", "-O","/var/www/images/$pid-$count.png", "$url");
  system("/usr/bin/mogrify", "-flip","/var/www/images/$pid-$count.png");
  print "$pid-$count.png\n";
 else {
         print "$_\n";;

Change (or enable) also the following line in /etc/squid/suiqd.conf.

http_access allow localnet
http_port 3128 transparent
url_rwwrite_program /usr/local/bin/

The directory this script uses is /var/www/images and should be accessible by both the squid server (which uses the user named proxy and by the apache2 webserver (which uses the user www-data. The screenshot below shows how to create this directory, set the permissions and make the users a member of the other groups.

root@debian7:~# mkdir /var/www/images
root@debian7:~# chown www-data:www-data /var/www/images
root@debian7:~# chmod 755 /var/www/images
root@debian7:~# usermod -aG www-data proxy
root@debian7:~# usermod -aG proxy www-data

Test that it works after restarting squid and apache2.


The standard log file location for squid is /var/log/squid.

[root@RHEL4 ~]# grep "/var/log" /etc/squid/squid.conf
# cache_access_log /var/log/squid/access.log
# cache_log /var/log/squid/cache.log
# cache_store_log /var/log/squid/store.log

access control

The default squid setup only allows localhost access. To enable access for a private network range, look for the "INSERT YOUR OWN RULE(S) HERE..." sentence in squid.conf and add two lines similar to the screenshot below.


acl company_network src
http_access allow company_network

testing squid

First, make sure that the server running squid has access to the internet.

[root@RHEL4 ~]# wget -q
[root@RHEL4 ~]# ls -l index.html 
-rw-r--r--  1 root root 2269 Sep 18 13:18 index.html
[root@RHEL4 ~]#

Then configure a browser on a client to use the proxy server, or you could set the HTTP_PROXY (sometimes http_proxy) variable to point command line programs to the proxy.

[root@fedora ~]# export HTTP_PROXY=
[root@ubuntu ~]# export http_proxy=

Testing a client machine can then be done with wget (wget -q is used to simplify the screenshot).

[root@RHEL5 ~]# > /etc/resolv.conf
[root@RHEL5 ~]# wget -q
[root@RHEL5 ~]# ls -l index.html 
-rw-r--r-- 1 root root 2269 Sep 18  2008 index.html
[root@RHEL5 ~]#

name resolution

You need name resolution working on the squid server, but you don't need name resolution on the clients.

[paul@RHEL5 ~]$ wget
Resolving failed: Temporary failure in name resolution.
[paul@RHEL5 ~]$ export http_proxy=
[paul@RHEL5 ~]$ wget
Connecting to connected.
Proxy request sent, awaiting response... 200 OK
Length: 5390 (5.3K) [text/html]
Saving to: `index.html.1'

100%[================================>] 5,390       --.-K/s   in 0.1s

14:38:29 (54.8 KB/s) - `index.html' saved [5390/5390]

[paul@RHEL5 ~]$