How blocking works
How does internet filtering work?
How does IP blocking work?
When you enter a URL, the device uses a service called DNS to translate the URL into an IP address. However, an internet service provider can set up a firewall that blocks connections to specific IP addresses.
IP blocking is a more thorough system than DNS blocking (which interferes with the URL to IP translation) as it can’t be bypassed by switching the DNS service. Both methods rely on having access to someone in the loop of communication between the user and website to work. States accomplish this by making ISPs or whoever controls the local internet backbone infrastructure do the blocking.
How does DNS poisoning work?
DNS is like a phonebook that translates URLs (e.g., www.google.com) into IP addresses (e.g., 8.8. 8.8. 8.8). IP addresses are what devices actually use to communicate on the internet - even you have one. But since random numbers are hard to remember, we have a DNS service handle the translation from one to the other. But with DNS filtering in place, the DNS service doesn’t give out an IP address when asked about a blocked URL. In some cases, it may even give an IP that will route users to a website set up by the authorities to warn them of their wrongdoing.
DNS filtering relies on having access to someone in the chain of communication between the user and website. For states, this means instructing local ISPs to modify their DNS databases. However, DNS filtering and poisoning is one of the easiest methods to overcome, as users with the barest minimum of technical knowledge can change the DNS address on their device.
How does deep packet inspection (DPI) work?
This content blocking measure is costly, as the firewall has to check all of the data the user is sending or receiving although the technology is becoming more readily available to operators. State actors employ DPI filtering by forcing ISPs to implement firewalls that can carry out DPI tasks or by enforcing these controls at the local internet backbone infrastructure.
DPI filtering most frequently targets the HTTPS URL address based on the web addresses of the data the user’s device is sending and receiving. If a user tries to connect to a specific domain name that contains sensitive keywords or matches banned addresses, the firewall can drop the request. So if Facebook is a banned website, an HTTPS-based DPI filter would block access to any website address that includes the word “facebook” as it appears in the HTTPS request, also known as the TLS SNI header.