For those wondering what "domain fronting" is,
https://en.wikipedia.org/wiki/Domain_fronting circumvents Internet censorship by hiding the true endpoint of a connection
https://en.wikipedia.org/wiki/Domain_fronting circumvents Internet censorship by hiding the true endpoint of a connection
Host: signal.org
This will only work with compatible servers. In Signal's case they use AppEngine which is behind Google's network and the servers for google.com are able to connect to their app servers when given the appropriate Host header.