Domain fronting is a technique that circumvents internet censorship by obfuscating the domain of a HTTPS connection. Working in the application layer, domain fronting allows a user to connect to a service that may otherwise be blocked by DNS, IP or deep packet inspection.
In one configuration, a large hosting service such as Amazon S3 or Google App Engine uses a shared SNI certificate containing both a common and target HTTPS domains. The domain name of the actual, blocked endpoint is only communicated after the establishment of an encrypted HTTPS connection, in the HTTP Host header, making it invisible to censors. This can be done if the blocked and the innocuous sites are both hosted by the same large provider, such as Google App Engine.
These techniques work by using different domain names at different layers of communication. The domain name of an innocuous site is used to initialize the connection. This domain name is exposed to the censor in clear-text as part of the DNS request and the TLS Server Name Indication. For any given domain name, censors are typically unable to differentiate circumvention traffic from legitimate traffic. As such, they are forced to either allow all traffic to the domain name, including circumvention traffic, or block the domain name entirely, which may result in expensive collateral damage.
Google disabled domain fronting in April 2018, saying that it had "never been a supported feature at Google." Amazon also decided to disable domain fronting for CloudFront in April 2018, claiming it was "already handled as a breach of AWS Terms of Service". This effort by both Google and Amazon was in part due to pressure from the Russian government over Telegram domain fronting activity using both of the cloud providers' services.
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