Blocking-resistant communication through domain fronting

TitleBlocking-resistant communication through domain fronting
Publication TypeConference Paper
Year of Publication2015
AuthorsFifield D, Lan C, Hynes R, Wegmann P, Paxson V
Published inProceedings of the Privacy Enhancing Technologies Symposium (PETS)
Date Published06/2015
Keywordscensorship, circumvention

We describe “domain fronting,” a versatile censorship circumvention technique that hides the remote endpoint of a communication. Domain fronting works at the application layer, using HTTPS, to communicate with a forbidden host while appearing to communicate with some other host, permitted by the censor. The key idea is the use of different domain names at different layers of communication. One domain appears on the “outside” of an HTTPS request—in the DNS request and TLS Server Name Indication—while another domain appears on the “inside”—in the HTTP Host header, invisible to the censor under HTTPS encryption. A censor, unable to distinguish fronted and non-fronted traffic to a domain, must choose between allowing circumvention traffic and blocking the domain entirely, which results in expensive collateral damage. Domain fronting is easy to deploy and use and does not require special cooperation by network intermediaries. We identify a number of hard-to-block web services, such as content delivery networks, that support domain-fronted connections and are useful for censorship circumvention.Domain fronting, in various forms, is now a circumvention workhorse. We describe several months of deployment experience in the Tor, Lantern, and Psiphon circumvention systems, whose domain-fronting transports now connect thousands of users daily and transfer many
terabytes per month.


We would like to thank Yawning Angel, George Ka-dianakis, Georg Koppen, Lunar, and the members of the tor-dev, tor-qa, and traffic-obf mailing lists who responded to our design ideas, reviewed source code, and tested our prototypes. Arlo Breault wrote the flashproxy-reg-appspot program mentioned in Section 3, an early application of domain fronting. Leif Ryge and Jacob Appelbaum tipped us off that domain fronting was possible. Sadia Afroz, Michael Tschantz, and Doug Tygar were sources of inspiring conversation. Johanna Amann provided us with an estimate of the fraction of SNI-bearing TLS handshakes. This work was supported in part by the National Science Foundation under grant 1223717. The opinions,findings, and conclusions expressed herein are those of the authors and do not necessarily reflect the views of the sponsors.

ICSI Research Group

Networking and Security