Squid proxy server

48
I
have had a home network for several
years. I started with a router using
Windows XP with ICS (Internet Con-
nection Sharing) and one multi-homed
Ethernet card. The main disadvantages
were instability, low performance, and a
total lack of security. Troubleshooting
was totally impossible. Firewall configu-
ration was at the mercy of inexperienced
users, who clicked randomly at security
settings as if they were playing Russian
roulette.
I finally turned to Linux and set up an
iptables firewall on a Pentium II com-
puter acting as a router. The firewall sys-
tem would keep the attackers off my net-
work and log incoming and outgoing
traffic. Along with the iptables firewall, I
also set up a Squid proxy server to
improve Internet performance, filter out
unwanted popup ads, and block danger-
ous URLs.
A Squid proxy server filters Web traffic
and caches frequently accessed files. A
proxy server limits Internet bandwidth
usage, speeds up Web
access, and lets you
filter URLs. Centrally
blocking advertise-
ments and dangerous
downloads is cost
effective and transpar-
ent for the end user.
Squid is a high per-
formance implementa-
tion of a free Open-
Source, full-featured
proxy caching server.
Squid provides exten-
sive access controls
and integrates easily with an iptables
firewall. In my case, the Squid proxy
server and the iptables firewall worked
together to protect my network from
intruders and dangerous HTML. You’ll
find many useful discussions of firewalls
in books, magazines, and Websites. (See
[1] and [2], for example.) The Squid
proxy server, on the other hand, is not as
A proxy server provides safer and more efficient surfing.
Although commercial proxy solutions are available, all you really
need is Linux and an old PC in the attic.
BY GEERT VAN PAMEL
Necessary Components Specifics
Intel Pentium II CPU, or higher -
Why not a spare Alpha Server? 350 MHz
80 - 100 MB memory minimum more is better
1 or more IDE disks (reuse 2 old disks: 1 GB
system SW + swap & 3 GB for cache + /home disk) 4 GB minimum
2 Ethernet cards, minihub, fast Ethernet modem, 100 Mbit/s if
wireless router or hub possible
CDROM, DVD reader software is
mostly distri-
buted via DVD
Use only normal straight LAN cables [no need for modem and
cross cables] minihub cross
themselves!
Table 1: Recommended Hardware
SAFE HARBOR
Implementing a home proxy server with Squid
SAFE HARBOR
Squid proxy serverKNOW-HOW
48 ISSUE 60 NOVEMBER 2005 WWW.LINUX- MAGAZINE.COM

49
well documented, especially for small
home networks like mine. In this article,
I will show you how to set up Squid.
Getting Started
The first step is to find the necessary
hardware. Figure 1 depicts the network
configuration of the Pentium II computer
I used as a firewall and proxy server.
This firewall system should operate with
minimal human intervention, so after
the system is configured, you’ll want to
disconnect the mouse, keyboard, and
video screen. You may need to adjust the
BIOS settings so that the computer will
boot without a keyboard. The goal is to
be able to put the whole system in the
attic, where you won’t hear it or trip
over it. From the minihub shown in Fig-
ure 1, you can come “downstairs” to the
home network using standard UTP cable
or a wireless connection. Table 1 shows
recommended hardware for the firewall
machine.
Assuming your firewall is working,
the next step is to set up Squid. Squid is
available from the Internet at [3] or one
of its mirrors [4] as tar.gz (compile from
sources). You can easily install it using
one of the following commands:
rpm -i /cdrom/RedHat/RPMS/U
squid-2.4.STABLE7-4.i386.rpmU
# Red Hat 8
rpm -i /cdrom/Fedora/RPMS/U
squid-2.5.STABLE6-3.i386.rpm U
# Fedora Core 3
rpm -i /cdrom/.../U
squid-2.5.STABLE6-6.i586.rpmU
# SuSE 9.2
At this writing, the current stable Squid
version is 2.5.
Configuring Squid
Once Squid is installed, you’ll need
to configure it. Squid has one central
configuration file. Every time this file
changes, the configuration must be
reloaded with the command /sbin/init.
d/squid reload.
You can edit the configuration file with
a text editor. You’ll find a detailed
description of the settings inside the
squid.conf file, although the discussion
is sometimes very technical and difficult
to understand. This section summarizes
some of the important settings in the
squid.conf file.
First of all, you can prevent certain
metadata related to your configuration
from reaching the external world when
you surf the Web:
vi /etc/squid/squid.conf
...
anonymize_headers deny U
From Server Via User-Agent
forwarded_for off
strip_query_terms on
Note that you cannot anonymize Referer
and WWW-Authenticate because other-
wise authentication and access control
mechanisms won’t work.
forwarded_for off means that the IP
address of the proxy server will not be
sent externally.
With strip_query_terms on, you do not
log URL parameters after the ?. When
this parameter is set to off, the full URL
is logged in the Squid log files. This fea-
ture can help with debugging the Squid
filters, but it can also violate privacy
rules.
The next settings identify the Squid
host, the (internal) domain where the
machine is operating, and the username
of whoever is responsible for the server.
Note the dot in front of the domain. Fur-
ther on, you find the name of the local
DNS caching server, and the number of
domain names to cache into the Squid
server.
visible_hostname squid
append_domain .mshome.net
cache_mgr sysman
dns_nameservers 192.168.0.1
dns_testnames router.mshome.net
fqdncache_size 1024
http_port 80
icp_port 0
http_port is the port used by the proxy
server. You can choose anything, as long
as the configuration does not conflict
with other ports on your router. A com-
mon choice is 8080 or 80. The Squid
default, 3128, is difficult to remember.
We are not using cp_port, so we set it
to 0. This setting synchronizes proxy
servers.
With log_mime_hdrs on, you can
make mime headers visible in the access.
log file.
Avoid Disk Contention
Squid needs to store its cache some-
where on the hard disk. The cache is a
tree of directories. With the cache_dir
option in the squid.conf file, you can
specify configuration settings such as the
following:
• disk I/O mechanism – aufs
• location of the squid cache on the disk
– /var/cache/squid
• amount of disk space that can be used
by the proxy server – 2.5 GB
• number of main directories – 16
• subdirectories – 256
For instance:
cache_dir aufs U
/var/cache/squid 2500 16 256
Figure 1: Ethernet basic LAN configuration.
Local Network
Internet
KNOW-HOWSquid proxy server
49ISSUE 60 NOVEMBER 2005WWW.LINUX- MAGAZINE.COM

The disk access method options are as
follows:
• ufs – classic disk access (too much I/O
can slow down the Squid server)
• aufs – asynchronous UFS with threads,
less risk of disk contention
• diskd – diskd daemon, avoiding disk
contention but using more memory
UFS is the classic UNIX file system I/O.
We recommend using aufs to avoid I/O
bottlenecks. (When you use aufs, you
have fewer processes.)
# ls -ld /var/cache/squid
lrwxrwxrwx 1 root rootU
19 Nov 22 00:42 U
/var/cache/squid -> U
/volset/cache/squid
I suggest you keep the standard file loca-
tion for the squid cache /var/cache/
squid, then create a symbolic link to the
real cache directory. If you move the
cache to another disk for performance or
capacity reasons, you only have to mod-
ify the symbolic link.
The disk space is distributed among
all directories. You would normally look
for even distribution across all directo-
ries, but in practice, some variation in
the distribution is acceptable. More com-
plex setups using multiple disks are pos-
sible, but for home use, one directory
structure is sufficient.
Cache Replacement
The proxy server uses an LRU (Least
Recently Used) algorithm. Detailed stud-
ies by HP Laboratories [6] have revealed
that an LRU algorithm is not always an
intelligent choice. The GDSF setting
keeps small popular objects in cache,
while removing bigger and lesser used
objects, thus increasing the overall effi-
ciency.
cache_replacement_policyU
heap GDSF
memory_replacement_policyU
heap GDSF
Big objects requested only once can
flush out a lot of smaller objects, there-
fore you’d better limit the maximum
object size for the cache:
cache_mem 20 MB
maximum_object_sizeU
16384 KB
maximum_object_sizeU
_in_memory 2048 KB
Log Format Specification
You can choose between Squid log for-
mat and standard web server log format
using the parameter emulate_httpd_log.
When the parameter is set to on, stan-
dard web log format is used; if the
parameter is set to off, you get more
details with the Squid format. See [7] for
more on analyzing Squid log files.
Proxy Hierarchy
The Squid proxy can work in a hierarchi-
cal way. If you want to avoid the parent
proxy for some destinations, you can
allow a direct lookup. The browser will
still use your local proxy!
acl direct-domain U
dstdomain .turboline.be
always_direct allow U
direct-domain
acl direct-path urlpath_regexU
-i "/etc/squid/direct-path.reg"
always_direct allow direct-path
Some ISPs allow you to use their proxy
server to visit their own pages even if
you are not a customer. This can help
you speed up your visits to their pages.
The closer the proxy to the original
pages, the more likely the page is to be
cached. Because your own ISP is more
remote, the ISP is less likely to be cach-
ing its competitor’s contents…
cache_peer proxy.tiscali.beU
parent 3128 3130 U
no-query default
cache_peer_domain U
proxy.tiscali.be .tiscali.be
no-query means that you do not use, or
cannot use, ICP (the Internet Caching
Protocol), see [8]. You can obtain the
same functionality using regular expres-
sions, but this gives you more freedom.
cache_peer proxy.tiscali.beU
parent 3128 3130 U
no-query default
acl tiscali-proxy U
dstdom_regex -i U
.tiscali.be$
cache_peer_access U
proxy.tiscali.be allow U
tiscali-proxy
• the order of the rules is important
• first list all the deny rules
• the first matching rule is executed
• the rest of the rules are ignored
• the last rule should be an allow all
Table 2: ACL Guidelines
01 acl block-ip dst "/etc/squid/block-ip.reg"
02 deny_info filter_spam block-ip
03 http_access deny block-ip
04
05 acl block-hosts dstdom_regex -i "/etc/squid/block-hosts.reg"
06 deny_info filter_spam block-hosts
07 http_access deny block-hosts
08
09 acl noblock-url url_regex -i "/etc/squid/noblock-url.reg"
10 http_access allow noblock-url Safe_ports
11
12 acl block-path urlpath_regex -i "/etc/squid/block-path.reg"
13 deny_info filter_spam block-path
14 http_access deny block-path
15
16 acl block-url url_regex -i "/etc/squid/block-url.reg"
17 deny_info filter_spam block-url
18 http_access deny block-url
Listing 1: Blocking Unwanted Pages

The ACL could also include a regular
expression (regex for short) with the
URL using an url_regex construct.
For Squid, regular expressions can be
specified immediately, or they can be in
a file name between double quotes, in
which case the file should contain one
regex expression per line – no empty
lines. The -i (ignore case) means that
case-insensitive comparisons are used.
If you are configuring a system with
multiple proxies, you can specify a
round-robin to speed up page lookups
and minimize the delay when one of the
servers is not available. Remember that
most browsers issue parallel connections
when obtaining all the elements from a
single page. If you use multiple proxy
servers to obtain these elements, your
response time might be better.
cache_peer 80.200.248.199 U
parent 8080 7 U
no-query round-robin
cache_peer 80.200.248.200 U
parent 8080 7 U
...
cache_peer 80.200.248.207U
parent 8080 7 U
FTP files are normally downloaded just
once, so will not normally want to cache
them, except when downloading repeat-
edly. Also, local pages are not normally
cached, since they already reside on
your network :
acl FTP proto FTP
always_direct allow FTP
acl local-domain dstdomain U
.mshome.net
local-domain
acl localnet-dst dst U
192.168.0.0/24
localnet-dst
Filtering with Squid
The preceding sections introduced
some important Squid configura-
tion settings. You have already
learned earlier in this article that
ACLs (Access Control Lists) can be
used for allowing direct access to
pages without using the parent proxy. In
this section, I’ll show you how to use
ACLs for more fine-grained access con-
trol.
Table 2 provides some guidelines for
creating ACL lists. It is a very good idea
to only allow what-you-see-is-what-you-
get (WYSIWYG) surfing. If you do not
want to see certain pages or frames, then
you can automatically block the corre-
sponding URLs for those pages on the
proxy server.
You can filter on:
• domains of client or server
• IP subnets of client or server
• URL path
• Full URL including parameters
• keywords
• ports
• protocols: HTTP, FTP
• methods: GET, POST, HEAD, CON-
NECT
• day & hour
• browser type
• username
Listing 1 shows examples of commands
that block unwanted pages.
The script in Listing 2 will make
unwanted pages invisible:
Whenever Squid executes the deny_
info tag, it sends the file /etc/squid/
errors/filter_spam to the browser instead
of the real Web page… effectively filter-
ing away the unwanted object. The trail-
ing 
07 </script>
08 <noscript><plaintext><!--
Listing 2: Making a Page
Invisible

times executable zip files that install
software. Squid lets you block files by
path, filename, or file extension, as
shown in Listing 4.
Squid also lets you filter for regular
expressions used in the URL.
Of course, your filter may occasionally
turn up a false positive. You can add reg-
ular expressions for URLs you specifi-
cally don’t want to block to /etc/squid/
noblock-url.reg.
vi /etc/squid/noblock-url.reg
...
^http://ads.com.com/
You can find an up-to-date version of
those configuration files at [11]
Protect your Ports
For security reasons, you should disable
all ports and only allow well known web
ports using the syntax shown in Listing 5.
The same can be done for connected
ports. You can allow SSL ports when
connected, and deny them otherwise.
Remember that the normal HTTP proto-
col is not connected. The client and the
browser always establish a new connec-
tion for every page visit.
acl SSL_ports port 443 563
acl SSL_ports port 1863 U
# Microsoft Messenger
acl SSL_ports port 6346-6353 U
# Limewire
http_access allow U
CONNECT SSL_ports
http_access deny U
CONNECT
Do not allow others to misuse your
cache! You only want your cache to be
used by your own intranet. Users on the
external Internet should not be able
to access your cache:
acl localhost src U
127.0.0.1/255.255.255.255
acl localnet-src src U
192.168.0.0/24
http_access deny !localnet-src
Allowing All the Rest
To allow only the protocols and the
methods that you want:
acl allow-proto proto HTTP
http_access deny !allow-proto
01 vi /etc/squid/block-path.reg
02 ...
03 .ad[ep](?.*)?$
04 .ba[st](?.*)?$
05 .chm(?.*)?$
06 .cmd(?.*)?$
07 .com(?.*)?$
08 .cpl(?.*)?$
09 .crt(?.*)?$
10 .dbx(?.*)?$
11 .hlp(?.*)?$
12 .hta(?.*)?$
13 .in[fs](?.*)?$
14 .isp(?.*)?$
15 .lnk(?.*)?$
16 .md[abetwz](?.*)?
17 .ms[cpt](?.*)?$
18 .nch(?.*)?$
19 .ops(?.*)?$
20 .pcd(?.*)?$
21 .p[ir]f(?.*)?$
22 .reg(?.*)?$
23 .sc[frt](?.*)?$
24 .sh[bs](?.*)?$
25 .url(?.*)?$
26 .vb([e])?(?.*)?$
27 .vir(?.*)?$
28 .wm[sz](?.*)?$
29 .ws[cfh](?.*)?$
Listing 4: Blocking by Path or Extension
01 acl Safe_ports port 80 # http
02 acl Safe_ports port 21 # ftp
03 acl Safe_ports port 2020 # BeOne Radio
04 acl Safe_ports port 2002 # Local server
05 acl Safe_ports port 8044 # Tiscali
06 acl Safe_ports port 8080 # Turboline port scan
07 acl Safe_ports port 8081 # Prentice Hall
08
09 # Deny requests to unknown ports
10 http_access deny !Safe_ports
Listing 5: Protecting Ports
[1] Presentation for the HP-Interex user
group in Belgium on 17/03/2005 about
“Implementing a home Router, Fire-
wall, Proxy server, and DNS Caching
Server using Linux” http://users.
belgacombusiness.net/linuxug/pub/
router/linux-router-firewall-proxy.zip
[2] Firewalls: http://www.linux-magazine.
com/issue/40/Checkpoint_FW1_Fire-
wall_Builder.pdf http://www.
linux-magazine.com/issue/34/
IPtables_Firewalling.pdf
[3] About Squid in general: http://www.
squid-cache.org http://squid-docs.
sourceforge.net/latest/book-full.
html#AEN1685 http://www.
squid-cache.org/FAQ/FAQ-10.html
[4] Squid mirror sites: http://www1.de.
squid-cache.org http://www1.fr.
squid-cache.org http://www1.nl.
squid-cache.org http://www1.uk.
squid-cache.org
[5] Suse 9.2 Professional – DVD software
distribution http://www.linux-maga-
zine.com/issue/54/Linux_Magazine_
DVD.pdf
[6] For more information about the GDSF
and LFUDA cache replacement poli-
cies see: http://www.hpl.hp.com/
techreports/1999/HPL-1999-69.html
http://fog.hpl.external.hp.com/
techreports/98/HPL-98-173.html
[7] Reporting and analysing Squid log
files: http://www.linux-magazine.com/
issue/36/Charly_Column.pdf
[8] ICP – Internet Caching Protocol: http://
en.wikipedia.org/wiki/Internet_Cache_
Protocol
[9] The whois database: http://www.ripe.
net/db/other-whois.html
[10] About Regular Expressions: http://
www.python.org/doc/current/lib/
module-re.html
[11] Example configuration files for
Squid: http://members.lycos.nl/
geertivp/pub/squid
INFO

acl allow-method U
method GET POST
http_access deny U
!allow-method
The last rule should be an allow-all,
since the previous rule was a deny…
http_access allow all
Remember after changing parameters to
always restart the Squid server with the
following command:
/sbin/init.d/squid reload
For SuSE the /sbin/init.d folder is stan-
dard. For Fedora, create a symbolic link:
cd /sbin
ln -s /etc/init.d
When you have finished the configura-
tion, use setup (Fedora), yast2 (SuSE), or
an equivalent tool to activate the Squid
service. Remember to reload the server
when you change a file.
/sbin/init.d/squid reload
If anything does not work as expected,
you can look for a reason in the cache
log file /var/log/squid/cache.log.
Conclusions
This article is the result of a presentation
for the Belgium HP-Interex organization.
Slides are available from [1] giving more
details about the setup of the iptables
firewall, the router, the DNS caching
server, the DHCP server, and the NTP
server.
If you are looking for better perfor-
mance, safer surfing, and a way to block
access to dangerous Web content, try
putting a Squid proxy server in your
attic. For the cost conscious among you:
a Pentium II router consumes about 11
kWh/week. You should balance this
expense against the increased security
and reduced headaches of operating
your own firewall with Squid proxy
caching. ■
You should configure your iptables fire-
wall so that it blocks all outgoing HTTP
traffic unless the proxy server is used.
Since the proxy server is on the local net-
work, it allows all incoming requests from
local browser clients.
Any attempt to bypass the Squid filters is
blocked by the FORWARD firewall rule
blocking HTTP outgoing traffic.
Spyware programs mostly use the HTTP
protocol (remember: port 80) for outgo-
ing connections, but it seems that
Spyware hardly ever uses the proxy
(because Spyware authors are too lazy to
inspect the rconfiguration). Spyware is
thus blocked by the firewall rules.
Enforce Parental Control and Block Spyware
Geert Van Pamel has worked as a
project manager at Belgacom in Bel-
gium since 1997. He has been a
member of DECUS since 1985 and a
board member of HP-Interex since
2002. He learned UNIX on a PDP
system in 1982, and he currently
works with Linux in a mixed envi-
ronment with other servers such as
Tru64 UNIX, HP-UX, OpenVMS,
NonStop Tandem, SUN, and NCR
Teradata.THEAUTHOR
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