What techniques does Tor use like bridges and pluggable transports to evade ISP blocking?

1. Bridges: hidden first hops that hide Tor’s addresses

When a censor or ISP simply blocks known Tor relay IPs, Tor's primary countermeasure is bridges: Tor relays that do not appear in the public directory so censors cannot straightforwardly block them by address. Users obtain bridge addresses via BridgeDB or built-in Tor Browser options; if built-in bridges are blocked, users are advised to request custom bridges ^{[1] [2] [3]}.

2. Pluggable transports: change the shape of the traffic

Bridges alone do not stop a censor that inspects traffic patterns. Pluggable transports (PTs) sit between the Tor client and a bridge and transform Tor flows into other formats so DPI and protocol fingerprinting see "innocent-looking" traffic instead of raw Tor protocol. The Tor Project documents this transformation as the core role of PTs: they “transform the Tor traffic flow between the client and the bridge” ^{[2] [1]}.

3. obfs4 and randomizing transports: hide everything as randomness

obfs4 is the go-to recommendation for first-time users: it randomizes the wire image of Tor so traffic looks like random bytes rather than a recognizable protocol, and it is designed to resist active probing by censors. Tor Project guidance recommends trying obfs4 first because it “works for most people” ^{[1] [5]}.

4. meek and domain fronting: tunnel via large web services

Meek sends Tor traffic inside an HTTPS tunnel and uses a domain‑fronting technique so to a censor it appears the client is talking to major services (e.g., Google, Amazon). Meek’s trade-off is latency and reliance on third‑party infrastructure; Tor developers have noted meek can feel slower and its backend options have changed over time as providers changed support for domain fronting ^{[6] [2]}.

5. Snowflake, FTE and other technique-specific transports

Snowflake leverages ephemeral proxies (often run by volunteers) to provide resistance to address‑based blocking through rapid, transient endpoints; FTE (Format‑Transforming Encryption) makes Tor traffic conform to a specified protocol fingerprint so it can mimic allowed protocols ^{[7] [2]}. Different PTs are effective in different censorship environments, and Tor Browser recommends trying obfs4, snowflake, or meek‑azure if you encounter blocks ^[3].

6. Newer developments: WebTunnel and continuing evolution

The Tor Anti‑censorship Team and community continue to develop transports — for example, WebTunnel disguises Tor traffic as ordinary web traffic using HTTPT and integrates with standard webserver setups (Nginx, TLS) to make bridges harder to detect and block. Tor has actively called for more WebTunnel bridge operators where censors are escalating blocking ^{[4] [8]}.

7. Operational trade-offs and detection risks

No single PT is a silver bullet. Randomizing transports like obfs4 aim to resist DPI and active probing but can be discovered through traffic analysis or by blocking ranges once enough bridge IPs are known ^{[1] [5]}. Meek depends on third‑party infrastructure and can be slower; domain‑fronting options have been restricted when providers change their policies ^{[6] [5]}. Tor documentation explicitly says effectiveness depends on “your individual circumstances” and recommends trying multiple transports ^[3].

8. Practical user guidance and what reporting shows

Tor Project and related documentation tell users to declare censorship in the connection setup to reach the PT configuration screen, to try obfs4 first, and to fall back to FTE or meek (or request custom bridges) if needed ^{[6] [1] [3]}. In places with strong, evolving censorship — such as recent reports from Russia — Tor has urged volunteers to run specific bridge types (WebTunnel, obfs4) because censors are increasingly blocking popular transports and even removing circumvention apps from stores ^[8].

Limitations: available sources do not provide a full technical fingerprint-by-fingerprint comparison of every pluggable transport, nor exhaustive testing results across all national firewalls; the claims above are drawn from Tor Project documentation, technical papers, and recent reporting cited in this summary ^{[1] [2] [6] [4] [7] [3] [8]}.