This test examines whether websites are reachable and if they are not, it attempts to determine whether access to them is blocked through DNS tampering, TCP connection RST/IP blocking or by a transparent HTTP proxy.
Specifically, this test is designed to perform the following:
HTTP GET request
By default, this test performs the above (excluding the first step, which is performed only over the network of the user) both over a control server and over the network of the user. If the results from both networks match, then there is no clear sign of network interference; but if the results are different, then the websites that the user is testing are likely censored.
Below we provide information about how each step performed under the web connectivity test works.
The domain name system (DNS) is what is responsible for transforming a host name (e.g. torproject.org) into an IP address (e.g. 22.214.171.124). Internet Service Providers, amongst others, run DNS resolvers which map IP addresses to host names. In some circumstances though, ISPs map the requested host names to the wrong IP addresses, which is a form of tampering.
As a first step, the web connectivity test attempts to identify which DNS resolver is being used by the user. It does so by performing a DNS query to special domains (such as whoami.akamai.com) which will disclose the IP address of the resolver.
Once the web connectivity test has identified the DNS resolver of the user, it then attempts to identify which addresses and are mapped to the tested host names by the resolver. It does so by performing a DNS lookup, which asks the resolver to disclose which IP addresses are mapped to the tested host names, as well as which other host names are linked to the tested host names under DNS queries.
The web connectivity test will then try to connect to the tested websites by attempting to establish a TCP session on port 80 (or port 443 for URLs that begin with HTTPS) for the list of IP addresses that were identified in the previous step (DNS lookup).
As the web connectivity test connects to tested websites (through the previous step), it sends requests through the HTTP protocol to the servers which are hosting those websites. A server normally responds to an HTTP GET request with the content of the webpage that is requested.
Once the above steps of the web connectivity test are performed both over a control server and over the network of the user, the collected results are then compared with the aim of identifying whether and how tested websites are tampered with. If the compared results do not match, then there is a sign of network interference.
Below are the conditions under which the following types of blocking are identified:
DNS blocking: If the DNS responses (such as the IP addresses mapped to host names) do not match
TCP/IP blocking: If a TCP session to connect to websites was not established over the network of the user
HTTP blocking: If the HTTP request over the user’s network failed, or the
**HTTP status codes don’t match, or all of the following apply:
The body length of compared websites (over the control server and the network of the user) differs by some percentage
The HTTP headers names do not match
The HTML title tags do not match
The examples below (testing piratebay.se and google.com for censorship in Italy) show what the output of the web connectivity test could look like:
Starting test for http://thepiratebay.se/ * doing DNS query for thepiratebay.se * connecting to 126.96.36.199:443 * doing HTTP(s) request http://thepiratebay.se/ * performing control request with backend Result for http://thepiratebay.se/ ---------------------------------- * BLOCKING DETECTED due to dns * Is NOT accessible Starting test for https://google.com/ * doing DNS query for google.com * connecting to 188.8.131.52:80 * doing HTTP(s) request https://google.com/ * performing control request with backend Result for https://google.com/ ------------------------------ * No blocking detected * Is accessible Summary for web_connectivity ---------------------------- Accessible URLS --------------- * https://google.com/ Not accessible URLS ------------------- * http://thepiratebay.se/ URLS possibly blocked due to dns -------------------------------- * http://thepiratebay.se/
Note: DNS resolvers, such as Google or your local ISP, often provide users with IP addresses that are closest to them geographically. Often this is not done with the intent of network tampering, but merely for the purpose of providing users faster access to websites. As a result, some false positives might arise in OONI measurements. Other false positives might occur when tested websites serve different content depending on the country that the user is connecting from, or in the cases when websites return failures even though they are not tampered with.