Tim Wilde, Software Engineer at Team Cymru, Inc., is a big fan of the Tor Project and everything that they do. In October 2011, a user in China found that their connections to US-based Tor bridge relays were being regularly cut off after a very short period of time. The Great Firewall of China, or GFW, would reach out to the U.S.-based bridge and connect to it shortly thereafter the connection would be blocked by the GFW. This differs slightly from Iran to block Tor in September 2011, using the client-side of the SSL negotiation as its trigger rather than the server-side.”]
Source: https://blog.torproject.org/knock-knock-knockin-bridges-doors