Internet Censorship

Internet Censorship and Circumvention #

  • Working definition: censorship occurs when an entity blocks a user request to a resource
  • Before: only a few governments had access; but now, commercialization of these practices

How do censors restrict access? #

  1. Internet shutdowns
    • e.g. state-sponsored ISP blocks all ingress/egress from country; typically used in times of political turmoil like protests
    • High political and social cost
    • But becomming more common: 2020 saw 155 political internet shutdowns in 29 countries
  2. Throttling
    • Slow down internet content, e.g. Google slows down twitter
  3. Content takedown e.g. DMCA
    • Some legitimate e.g. Bangladesh getting Google Play Store to take down malware app using country’s insignia
  4. Content blocking

Content blocking #

DNS manipulation #

  • ISP usually provides DNS resolver, usually unencrypted
  • When client tries to connect to a server, DNS request made; country ISP provides an ISP like localhost to block it
    • Or redirect to some site showing a blocked message

IP blocking #

  • Client tries to send requested server a TCP handshake
  • But country censor looks for that IP egress, and then drops or sends a RST packet to the client
    • Gives client the impression that the server is unavailable at the moment
  • More common than DNS blocking
  • Now less likely as IP addresses tend to mean services rather than servers (i.e. with virtual hosts)

Application-layer blocking #

  • After TCP handshake, try to send HTTP(s) request to the server
  • Censor could do blocking here, or if there’s a cert decrypt your connection and send you back bad information
  • Could happen with any application-layer protocol

Measuring internet censorship #

  • Motivation:
    • Information controls harm citizens
    • Spreads beyond large countries
    • Frequently opaque in topic and technique
    • Want to know where, how, when, what is being censored
  • Measurements help to:
    • Increase transparency and accountability
    • Improve technical defenses
    • Inform users and policy: users aware of censorship take actions enhancing internet freedom
      • e.g. Turkey spray-painted DNS in 2014

Challenges #

  1. Censorship methods are varied (see above)
  2. Censorship varies around the world: geographical and network variance
  3. Censorship varies across time: cat and mouse game

Methods #

  • Open Observatory of Network Interference (OONI) probe: agent that runs on volunteers’ personal computers
    • Client will send a request to server of interest through censor, but also through control relay
    • Limitations:
      • Scale
      • Coverage
      • Continuity
      • Cost
      • Ethics
        • When is it safe to use volunteers’ devices?
        • What population of users are affected?
        • What risks do volunteers occur, and do benefits balance these risks?
  • Spooky scans (Ensafi et al. PAM 2014): using TCP/IP side channels to detect if clients and servers can communicate
    • Can detect “no blocking”; “server-to-client blocking”, “client-to-server blocking” by using IP spoofing from a measurement to measure behavior of client/server communication through censor
    • This can uncover IP blocking
  • Censored Planet Obeservatory
    • Many different techniques
    • Detect blocking on TCP/IP, HTTP, HTTPS, Echo, Discard
    • Impact: study of 2019 Kazakhstan national TLS interception that led to Firefox and Chrome pushing updates to block usage of Kazakh root CA certificate

Circumventing internet censorship #

  1. Proxying requests through “safe” servers
    • Censor only sees that user is trying to reach benign proxy server, not a blocked service
    • Very easy to detect; becomes a cat-and-mouse game for censors
  2. Imitate non-censored protocols
    • Skypemorph (2012): disguise Tor traffic as skype video traffic using fake skype servers and clients
    • But: also easy for censors to detect using network features (e.g. anomalous-looking UDP packets as a key) - Houmansadr et al. 2013
  3. Refraction networking
    • Bring proxy behavior into the core of the network via cooperation with friendly ISPs
    • When user requests a blocked site, client software requests a reachable site hosted by ISP partner network; censor allows request to pass through; ISP partner refracts request to the blocked site
    • Deployed in the wild: VanderSloot et al. PET 2020