How long do public obfs4 bridges typically last in China?

Public obfs4 bridges (listed in BridgeDB) are typically blocked within hours to a few days of use from China. The Great Firewall's active probing and enumeration is highly sophisticated. Private obfs4 bridges (not in BridgeDB) can last weeks to months depending on how widely they are shared. WebTunnel bridges last significantly longer due to their HTTPS traffic appearance making active probing much harder.

What is 'bridge cycling' and how does it help?

Bridge cycling refers to proactively rotating to new bridge addresses before the current bridge is blocked, rather than waiting for blocking to occur. For operators: change your IP address or redeploy to a new server every 60-90 days. For users: request fresh bridges every 2-4 weeks even if current bridges still work. Regular cycling keeps one step ahead of censor enumeration and provides a larger pool of addresses that have not yet been identified.

Can a bridge operator detect if their bridge has been blocked in a specific country?

Indirectly, yes. OONI (Open Observatory of Network Interference) has probes in many countries that test bridge reachability. A bridge operator can submit their bridge to OONI's bridge testing program and view results. Additionally, a sudden drop in connections from a specific country in bridge metrics suggests blocking. Tor Metrics provides per-country bandwidth statistics for registered bridges.

Does running a bridge on port 443 conflict with web server HTTPS?

Yes, if both services are on the same IP. Solutions: use a different external port for the bridge and NAT/redirect it to 443 on the server, or use separate IP addresses for the web server and the bridge. For WebTunnel bridges, the bridge shares port 443 with the web server by design (the web server reverse-proxies WebSocket connections to WebTunnel) - this is the intended architecture and there is no conflict.

Does a bridge with more users get blocked faster?

A bridge with more users creates more traffic visible to network monitors, potentially attracting attention faster. However, higher usage also means more users successfully using the bridge, which is the point. The balance: do not over-publicize a private bridge (keep it in a small trusted circle), but do not worry about usage volume for public bridges (the Tor Project manages public bridge exposure through BridgeDB's rate limiting). For private bridges, limiting distribution to a trusted circle of 10-50 people provides the best longevity-to-usefulness ratio.

Anti-Blocking Strategies for Tor Bridges: Advanced Guide 2026

Running a Tor bridge is only useful if it works. Censors actively detect and block bridges, often within days of public listing. These strategies extend bridge lifetime and improve resistance to automated and active blocking techniques.

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Why Bridges Get Blocked: Censor Methodology

Understanding how bridges are blocked informs how to resist blocking. Methods used by censors in 2026: passive analysis (monitoring traffic patterns for Tor protocol signatures), BridgeDB enumeration (requesting bridges from BridgeDB and blocking all received addresses - rate limiting slows this), active probing (sending probe connections to suspected bridges to confirm they are running Tor), IP range blocking (blocking all datacenter IP ranges known to host Tor bridges), fingerprinting pluggable transports (identifying obfs4, Snowflake, WebTunnel traffic patterns), and legal pressure (requesting hosting providers to terminate bridge accounts). The Great Firewall specifically combines multiple methods: it identifies Tor-like traffic, then actively probes the source IP to confirm it is a bridge, then blocks it. This typically happens within minutes to hours of a bridge being used from China.

IP Address Selection for Maximum Longevity

IP address type dramatically affects bridge longevity. Residential IPs (home internet, ISP consumer ranges): not in datacenter IP blacklists, indistinguishable from normal traffic sources. Longest lifespan against unsophisticated censors. Downsides: requires running bridge on home hardware (Pi, spare computer) or residential VPS services. Cloud/datacenter IPs (AWS, Google, Azure, common VPS providers): quickly identified as potential hosting sources, blocked en masse in advanced censors. Avoid for bridges targeting China or Iran. Privacy-focused datacenter IPs (smaller providers in Iceland, Romania, Netherlands): slower identification than AWS/Google, but still datacenter range. Moderate lifespan. Best strategy: residential IP (home bridge) for highest longevity, privacy-focused VPS for operational convenience with moderate longevity. Rotate IP addresses proactively: before a block occurs rather than after. If your hosting provider allows IP rotation or re-allocation, change IP every 60-90 days.

Port Selection and Protocol Mixing

Port selection affects block resistance. Port 443 (HTTPS): the most valuable port to use for ORPort because blocking port 443 breaks all HTTPS traffic (unacceptable collateral damage). Using port 443 for the ORPort makes IP-based blocking without port distinction much more costly. Port 80 (HTTP): similarly blocking-resistant due to collateral damage. Ports 8080, 8443: commonly used alternatives, moderate resistance. High-number ports (10000-65535): easier for censors to block without collateral damage. Prefer low-number or privileged ports. Transport port: the obfs4 or WebTunnel listener port can be on any port. Use 443 or 80 if your ORPort is on a different port. Running both ORPort and transport on well-known ports maximizes cost of blocking.

Active Probing Resistance

Active probing is when a censor sends a probe request to a suspected bridge IP to confirm it runs Tor before blocking. obfs4's active probing resistance: the obfs4 protocol requires a shared secret (the bridge fingerprint) in the handshake. A probe without the correct secret receives a random response indistinguishable from random data - the censor cannot confirm the bridge without the bridge address. WebTunnel's active probing resistance: the server hosts real web content. A probe hits a real website. The censor cannot distinguish a WebTunnel bridge from a legitimate web server without the bridge URL. ScrambleSuit (deprecated, but the principle carries to obfs4 and WebTunnel): the initial handshake is indistinguishable from random data to anyone without the shared secret. Ensure your pluggable transport is properly configured for active probing resistance - check the Tor Project's bridge operator documentation for current best practices.

Multi-Transport Bridges and Dynamic Adaptation

Running multiple transports on a single bridge gives users fallback options if one transport is blocked. Configuration: a single server can run obfs4, WebTunnel, and (with additional setup) Snowflake server simultaneously. Each transport listens on a different port. The bridge line includes multiple transport entries. Users try each transport in sequence until one works. Distribution: provide users the full multi-transport bridge line so their Tor client can try alternatives automatically. Dynamic transport switching: Tor clients with multiple bridge options will switch transports if one fails. Bridge operators can help by publishing updated transport options when one is blocked. Some bridge management tools provide automated alerts when a transport stops accepting connections (indicating potential blocking). Regular testing: test your own bridge from a censored environment (or use OONI probes) to verify it is accessible. Proactive testing catches blocking events before users experience them.

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