pering. In India, for example, the Internet Service Providers Association of India reportedly has
sent instructions to ISPs showing how to block by DNS instead of by IP.
11
In proxy-based filtering strategies, Internet traffic passing through the filtering system is
reassembled and the specific HTTP address being accessed is checked against a list of
blocked Web sites. These can be individual domains, subdomains, specific long URL paths,
or keywords in the domain or URL path. When users attempt to access blocked content they
are subsequently blocked. An option in this method of filtering is to return a blockpage that
informs the user that the content requested has been blocked.
Saudi Arabia uses SmartFilter as a filtering proxy and displays a blockpage to users when
they try to access a site on the country’s block list. The blockpage also contains information
on how to request that a block be lifted. Saudi Arabia blocks access to specific long URLs.
For example, www.humum.net/ is accessible, while www.humum.net/country/saudi.shtml is
blocked. United Arab Emirates, Oman, Sudan, and Tunisia also use SmartFilter. Tunisia uses
SmartFilter as a proxy to filter the Internet. But instead of showing users a blockpage indicating that the site has been blocked, they have created a blockpage that looks like the Internet
Explorer browser’s default error page (in French), presumably to disguise the fact that they are
blocking Web sites.
A proxy-based filtering system can also be programmed such that Internet traffic passing
through the filtering system is reassembled and the specific HTTP address requested is
checked against a list of blocked keywords. No country that ONI tested blocked access to a
Web site as a result of a keyword appearing in the body content of the page, however, there
are a number of countries that block by keyword in the domain or URL path, including China,
Iran, and Yemen.
China filters by keywords that appear in the host header (domain name) or URL path. For
example, while the site http://archives.cnn.com/ is accessible, the URL http://archives.cnn
.com/2001/ASIANOW/east/01/11/falun.gong.factbox/ is not. When this URL is requested,
reset (RST) packets are sent that disrupt the connection, presumably because of the keyword
falun.gong. Iran uses a filtering proxy that displays a blockpage when a blocked Web site is
requested. On some ISPs in Iran, such as Shatel and Datak, keywords in URL paths are
blocked. This most often affects search queries in search engines. For example, here is a
query run on Google for naked in Arabic (www.google.com/search?hl=fa&q=%D9%84%
D8%AE%D8%AA&btnG=%D8%A8%D9%8A%D8%A7%D8%A8) that was blocked. Ynet in
Yemen blocks any URL containing the word sex. The domain www.arabtimes.com is blocked
in Oman and the UAE but the URL for the Google cached version (http://72.14.235.104/
search?q=cache:8utpDVLa1yYJ:www.arabtimes.com/+arabtimes&hl=en&ct=clnk&cd=1) is
also blocked because www.arabtimes.com appears in the URL path.
Filtering systems can also be configured to redirect users to another Web site. In most
cases, redirection is identical to blockpage filtering, the only difference being the route used
Measuring Global Internet Filtering
15
sent instructions to ISPs showing how to block by DNS instead of by IP.
11
In proxy-based filtering strategies, Internet traffic passing through the filtering system is
reassembled and the specific HTTP address being accessed is checked against a list of
blocked Web sites. These can be individual domains, subdomains, specific long URL paths,
or keywords in the domain or URL path. When users attempt to access blocked content they
are subsequently blocked. An option in this method of filtering is to return a blockpage that
informs the user that the content requested has been blocked.
Saudi Arabia uses SmartFilter as a filtering proxy and displays a blockpage to users when
they try to access a site on the country’s block list. The blockpage also contains information
on how to request that a block be lifted. Saudi Arabia blocks access to specific long URLs.
For example, www.humum.net/ is accessible, while www.humum.net/country/saudi.shtml is
blocked. United Arab Emirates, Oman, Sudan, and Tunisia also use SmartFilter. Tunisia uses
SmartFilter as a proxy to filter the Internet. But instead of showing users a blockpage indicating that the site has been blocked, they have created a blockpage that looks like the Internet
Explorer browser’s default error page (in French), presumably to disguise the fact that they are
blocking Web sites.
A proxy-based filtering system can also be programmed such that Internet traffic passing
through the filtering system is reassembled and the specific HTTP address requested is
checked against a list of blocked keywords. No country that ONI tested blocked access to a
Web site as a result of a keyword appearing in the body content of the page, however, there
are a number of countries that block by keyword in the domain or URL path, including China,
Iran, and Yemen.
China filters by keywords that appear in the host header (domain name) or URL path. For
example, while the site http://archives.cnn.com/ is accessible, the URL http://archives.cnn
.com/2001/ASIANOW/east/01/11/falun.gong.factbox/ is not. When this URL is requested,
reset (RST) packets are sent that disrupt the connection, presumably because of the keyword
falun.gong. Iran uses a filtering proxy that displays a blockpage when a blocked Web site is
requested. On some ISPs in Iran, such as Shatel and Datak, keywords in URL paths are
blocked. This most often affects search queries in search engines. For example, here is a
query run on Google for naked in Arabic (www.google.com/search?hl=fa&q=%D9%84%
D8%AE%D8%AA&btnG=%D8%A8%D9%8A%D8%A7%D8%A8) that was blocked. Ynet in
Yemen blocks any URL containing the word sex. The domain www.arabtimes.com is blocked
in Oman and the UAE but the URL for the Google cached version (http://72.14.235.104/
search?q=cache:8utpDVLa1yYJ:www.arabtimes.com/+arabtimes&hl=en&ct=clnk&cd=1) is
also blocked because www.arabtimes.com appears in the URL path.
Filtering systems can also be configured to redirect users to another Web site. In most
cases, redirection is identical to blockpage filtering, the only difference being the route used
Measuring Global Internet Filtering
15