2004.06_Web Filtering-Blocking Advertising with Your Browser or Proxy.pdf

KNOW HOW

Web Filtering

Banner-Free Surfing

Advertising on websites makes it

difficult to find your way around, and

consumes bandwidth. In this article

we will be looking at tools that allow

you to block advertising banners on

your own machine or the local

network. BY OLIVER FROMMEL

does not use banner advertising. In

addition to banners that cover the

full width of the screen, there is an

increasing tendency toward large images

that replace headings, making it difficult

for users to find their way around.

Although this may be understandable

from the content provider’s point of

view, most users are anything but

amused when confronted with ban-

ner ads, especially as they impact

surfing speed. In fact, it is quite com-

mon for garish ads in image or flash

formats to take up more memory

than the actual content.

These issues have led to the devel-

opment of a number of programs

that prevent unwanted banners from

appearing on Websites, and even

prevent the ads from loading in the

first place. Blocking tools filter the

datastream allowing interesting con-

tent to pass, and rejecting unsolicited

advertising. This works because the

browser retrieves the requested Web

page from the server first.

Intelligent software checks the

page for advertising images, and

removes the offending sections from the

HTML file. Only then will the browser

retrieve the elements to render the page.

There are several approaches. Some

browsers have an integrated feature, or

can use a plug-in. Filter programs that

run as proxy servers independently of

the browser, but on the same machine

are an alternative. They can also run on

a separate machine if required, providing

a proxy service to the computers on your

home or office network.

Browser-Integrated

Mozilla users have a simple option. The

Mozilla browser has a plug-in that blocks

advertising. The appropriately named

AdBlock tool filters on the basis of the

URL . It is easy to install the plug-in –

just click the link on the project home-

page [1]. You do not need

administrative privileges to do so, as

the AdBlock plug-in installs below

the current user’s home directory in

~/.mozilla .

You need to re-launch your

browser to enable the plug-in. The

plug-in menu is available in the

adBlock section below Tools . If you

add the patterns shown in Figure 2,

the plug-in will block an impressive

number of ads. The same menu

allows you to view the elements on

the current page. If you discover an

unsolicited ad, you can use its URL

to create a new filtering rule.

You can also right-click an ad, and

select Adblock Image in the drop-

down menu to access the same

Figure 1: A Web page full of advertising makes it hard to see

the actual content.

June 2004

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Blocking advertising with your browser or proxy

T here is hardly a website today that

Web Filtering

KNOW HOW

to enable the Load Images | for the

originating site only option below

Options | Web Features . This works

quite well, because most banner

advertising is not actually served up

by the Web server itself, but sup-

plied by third parties that handle ad

click invoicing for the content

provider. To use this feature while

surfing, right-click an ad, and select

Block images from server .

Note that this method might back-

fire on you, as it will prevent you from

downloading any other images which

are not stored directly on the original

server. Also, this approach will not block

ads from the original site. The adBlock

plug-in definitely provides more granular

settings. If you are still not satisfied, you

might prefer to use a more flexible and

powerful proxy that will run with other

browsers such as Konqueror or Opera.

the Jar file, and then select Save link to

disk . Otherwise, your browser may

decide to launch the Jar file directly.

If the java program is not in your path,

add the directory where the program is

stored, for example:

export PATH=$PATH:/usr/java U

/j2sdk1.4.2_02/bin

Figure 2: Using the Mozilla menu to configure AdBlock.

You can then launch Muffin by typing

java -jar muffin-0.9.3a.jar (see Figure 4).

Muffin uses the NoThanks filter by

default. The filter applies simple rules to

block banner advertising. The proxy

includes a few extra filters, which are not

enabled by default, such as Anima-

tionkiller for removing gif animations,

and Cookiemonster , which takes care of

the ubiquitous browser Cookies . The

NoThanks filter will not run out-of-the-

box. Instead, it needs to load a small

configuration file, the so-called killfile,

first. A sample killfile is available from

the Muffin website below Samples .

To load the killfile, select the entry for

the NoThanks filter below Enabled Fil-

ters Click Preferences . In the dialog box

that appears, select Browse below Kill

File and locate the killfile in your Muffin

directory. Click Apply , and then Load , to

tell Muffin to use the killfile. The Save

button stores this setting permanently,

features. Doing so pops up a small dialog

showing the address of this image. You

can use a wildcard (*) to modify the

URL to match similar addresses. Assum-

ing that the following URL is displayed:

http://img-cdn.mediaplex.com/ U

ads/2399/9556/DE_DE_mofg_dim U

4600_dhs_q1w0304_300x200_15k_ U

FL_gif.gif

You can remove the section following the

server name and the ads directory, and

type an asterisk as a wildcard instead:

Tasty Java

Muffin is one such proxy. It resides

between the Web server and your

browser. As a Jar package, Muffin can be

run directly with Java, and does not

need installing or compiling. You do

need the Java Runtime Environment

(JRE), but most distributions install JRE

by default. When downloading from [2],

make sure that you right-click the link to

http://img-cdn.mediaplex.com U

/ads/*

This line tells the plug-in to block any

files from the ads directory on the Medi-

aplex server. Now click the Reload

button, and, hey presto, the ad disap-

pears. Likely candidates are easy to find

using the menu mentioned previously, or

by inspecting the HTML file source code

(see Figure 3). Users with the Mozilla

Firefox browser (previously known as

Firebird) will be pleased to hear that the

plug-in will work with their browser too

(we tested this with Firefox 0.8).

Some ads can be blocked without the

plug-in. The easiest way of doing this is

Figure 3: You can inspect the HTML source code to identify the advertising addresses. Many sites actu-

ally use “ad” as part of their names.

GLOSSARY

Proxy servers (or proxies for short) reside

between a client (e.g. a Web browser) and a

server. Viewed from the client’s perspective,

the proxy is a server, whereas the proxy is a

client from the server’s viewpoint. Proxies

cache websites thus improving access speeds.

In some cases, a proxy is needed to allow

clients without a direct connection to access

the Internet.

URL: A Uniform Resource Locator comprises a

service acronym (http, ftp, ..), the address of an

Internet server, and optional directory and file

names. This allows documents on the Internet

to be uniquely identified, for example,

http://www.linux-magazine.com/issue/34/

KDETricks.pdf.

Cookies: Small snippets of text-based infor-

mation which a browser associates with a

website. Content providers use cookies to

store user-specific information on a user’s

Internet surfing behavior between two visits

to a website (“When did the user last visit the

website?”).

Wildcard: Many Linux programs (such as the

shell) use specific characters to represent one

or more letters. For example, the asterisk (*)

typically represents an arbitrary string, al-

though it can mean any number of repetitions

of a certain character (in regular expressions).

When you type ls *.jpg, the shell will display all

filenames with the .jpg suffix, no matter what

letters the filenames start with.

Port: As multiple server programs can run on

a single machine, a combination of the port

number and the IP address is used to uniquely

identify a connection. The Internet standards

assign well-known ports to specific services,

for example port 80 for the World Wide Web

(HTTP), and port 25 for email (SMTP).

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June 2004

KNOW HOW

Web Filtering

tings below Network | Proxy Servers ;

Firefox uses Tools | Options , and

finally Connection Settings .

maximum of 20 simultaneous connec-

tions. The commercial version has

special filters for Javascript which are

not available in the free version. Web-

washer needs administrative privileges

to install, and there is no easy

workaround. Webwasher provides two

package formats, a RPM and a gzipped

tar archive, which contains an installa-

tion script.

The software immediately launches,

without being asked to do so, and listens

for requests on port 9090. The configura-

tion files are located in /etc/wwasher , the

logfiles in /var/log/wwasher . Logging is

Figure 4: The Java-based Muffin program has its own

GUI, and tells you what files it is handling.

Filter with a History

This software is based on a classic

tool, Junkbuster [3]. Privoxy [4] is

still under active development, in

contrast to other programs discussed

in this article. The Privoxy website

has packages for several distributions.

By default, the package will install glob-

ally using the administrative account.

On Red Hat, the operating system

launches the filter on booting, along

with other server programs. You can

do this manually using /etc/rc.d/

init.d/privoxy start . The configuration

files are located in /etc/privoxy , the

main configuration file being

/etc/privoxy/config . Use this file for

more granular settings, but Privoxy

will perform quite well using the

defaults.

After you set the proxy port to 8118

in your browser, as described previ-

ously, Privoxy will start filtering

advertising out of the Web pages you

visit. The software has a useful feature for

tagging locations where it has removed

ads, and can display both the

filtered image, and the filter

rule that it matched. This

allows you to check if the tool

is just removing advertising, or

also preventing access to

images you would prefer to

view. You can also set up Pri-

voxy directly from your brow-

ser using a special address,

http://p.p (see Figure 6).

and is a good idea, unless you want to

repeat this procedure each time you

launch Muffin. The filter program cre-

ates a Muffin directory below your home

directory, and uses this directory to store

its configuration and logfile.

You need to modify your browser set-

tings, by entering the new tool as a

proxy, for this to work. This applies to

any other proxy tool you might use. If

you have the Mozilla browser, open the

settings in ( Edit | Preferences ) and click

on the small triangle labeled Advanced .

Look for the Proxies item; this is where

you can enable the manual configuration

and enter the correct values for HTTP

Proxy and Port (see Figure 5). The first

field is typically localhost . Enter the port

number for your proxy in the second

field (see Table 1). Opera has similar set-

Figure 6: The Privoxy configuration page in a browser,

where you can click to enable or disable the filter.

disabled by default, but can be enabled

using the Web-based front-end, which

you can access via a special address,

http://-web.washer-/ . The default user

name is admin , and the password is

webwasher .

Spoilt for Choice

Your choice of filtering tool is basically a

question of taste. If you don’t like Java,

you won’t like Muffin. If you insist on

entirely free software, you can rule out

Webwasher. Mozilla and Firefox users

can load the adBlock plug-in, although it

is not as powerful as other filters.

Privoxy is a mature tool which is under

active development, and available as a

package for many distributions. It is not

difficult to install any of the programs we

have looked at. The DVD with this issue

provides a compact installation guide.

Figure 5: Proxy configuration in Mozilla showing the settings

for the Muffin Web filter running on port 51966 of the same

machine (localhost).

Whitewashed

Home users might like to

check out Webwasher [5], a

free tool by the Webwasher

company. This version is

restricted to two users, or a

■

Table 1: Web Filter Overview

INFO

Adblock

Muffin

Privoxy

Webwasher

[1] AdBlock: http://adblock.mozdev.org

[2] Muffin: http://muffin.doit.org

[3] Junkbuster:

http://internet.junkbuster.com/

[4] Privoxy: http://www.privoxy.org

[5] Webwasher: http://www.webwasher.

com/client/download/private_use/linux

Non-administrative install possible

yes

Proxy

yes

Standard port

–

51966

8118

9090

Under development

yes

Pre-configured

yes

License

free

Restricted, free for

private use

June 2004

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