A hybrid two-stage blocking system (IP-redirect first stage, URL-proxy second stage) can be exploited as an oracle to enumerate blocked IP addresses by sending TCP packets with a TTL sufficient to reach the first-stage redirector but insufficient to reach the destination. Non-redirected IPs return ICMP TTL-expired from an intermediate router, while redirected IPs return a SYN/ACK from the web proxy impersonating the destination. A live scan of a /24 subnet confirmed 17 redirected IP addresses, yielding 91 associated hostnames across 9 of those IPs.

Using a simple dialup connection, the CleanFeed oracle scan enumerated IP addresses at up to 98 addresses/second. At this rate, the ~8.3 million Russian IP addresses (the IWF reported 25% of known illegal sites were hosted in Russia) could be scanned in under 24 hours, and the full routable IPv4 space (32% of 2^32 addresses) in approximately 160 days. A suitable filtered dialup account was available for free, with phone costs under £15.

§5.2 evaluation

The CleanFeed first stage populates its IP blocklist by automatically resolving hostnames from the IWF database via DNS. Content providers can serve false DNS results pointing to high-traffic third-party IP addresses (e.g., Google cache servers at 66.102.9.104), causing the first stage to redirect legitimate traffic through the proxy. Automated IP-update processes cannot reliably distinguish a genuine IP migration from a spoofed DNS result, and this can cause legitimate sites to be blocked collaterally.

§4.3 detection

The hybrid two-stage design's architectural vulnerability is that circumventing either stage independently defeats the system: end-users can tunnel via Tor or JAP to bypass both stages entirely, while content providers can serve different content to IWF crawlers versus real users, exploiting the fact that only 33% of IWF hotline reports were substantiated as potentially illegal. The system's precision is entirely contingent on content-provider cooperation, which cannot be assumed.

§4, §6 defense

Brightview's countermeasure requiring a minimum probe TTL of 24 (to prevent low-TTL scans from stopping at the proxy) was bypassed by sending probes with TTL=128 and examining the TTL of returned SYN/ACK packets. The UK web proxy consistently returned TTL=49 (64−15 hops), while Russian destination servers returned TTL=45–49 or TTL=113–238 depending on initial OS TTL settings. The two populations were cleanly distinguishable, defeating the fix with no change to scan logic beyond raising the probe TTL.

Postscript defense

Dominant failure modes differ systematically by country: China (AS45090) shows connect timeouts in 75% of DoT failures (IP-level blocking); Kazakhstan (AS48716) shows post-TLS-handshake timeouts in 72% of DoT failures (likely ACK or segment discard after handshake); Iran (AS197207) shows TLS handshake timeouts in 80% of DoT failures. Packet capture analysis confirmed that timeouts during and after the TLS handshake correspond to unacknowledged TCP segments, not connection resets.

2021-basso-measuring §V-F, §V-G, Table VIII ip-blockingtls-fingerprintmiddlebox-interference

Residual censorship—where a censor continues blocking all traffic on a 3- or 4-tuple after an initial censorship event—is active in China (HTTP: 90s 3-tuple RST injection; ESNI: 120–180s 3+4-tuple null routing), Iran (HTTP+SNI: 180s 4-tuple null routing, occasionally up to 5 minutes; protocol filter: 60s), and Kazakhstan (HTTP+SNI: 120s 4-tuple null routing). A December 2020 Quack scan found 3-tuple stateful disruption in 33 countries and null-routing censorship in 18, suggesting much broader applicability.

2021-bock-your §IV, Table I rst-injectionip-blockingmiddlebox-interference

All tested censors (China, Iran, Kazakhstan) can be triggered statelessly—without completing a TCP 3-way handshake—using a SYN with decremented sequence number followed by a PSH+ACK containing the forbidden payload. This stateless triggering enables fully off-path, source-spoofed attacks: an adversary with packet-spoofing capability can residually censor a victim pair they have no on-path access to.

2021-bock-your §IV, §V.A rst-injectionip-blockingmiddlebox-interference

Iran and Kazakhstan reset the residual censorship timer whenever the censor observes any matching packet from the victim, so TCP retransmissions from the victim's own stack inadvertently extend the blocking window far beyond the nominal 120–180s. China's HTTP residual censorship has only ~50% per-request reliability from some vantage points due to heterogeneous GFW middlebox load-balancing, but reliability plateaus near 100% after 7 repeated censorship triggers sent ahead of time.

2021-bock-your §IV (timer reset), §IV (reliability), Fig. 2 ip-blockingmiddlebox-interferencerst-injection

All three prior end-to-middle (E2M) schemes — Telex, Cirripede, and Decoy Routing — require an inline flow-blocking component at the participating ISP, which adds latency, introduces a single point of failure, and may violate carrier SLAs. In private discussions with ISPs, the authors found that despite willingness to assist Internet freedom technically and financially, none were willing to deploy existing E2M technologies due to these operational impacts. TapDance removes the inline blocking requirement entirely, requiring only a passive tap and packet-injection capability.

2014-wustrow-tapdance §1, §6 middlebox-interferenceip-blocking

An adversary aware of a decoy router's location can force decoy-routed flows to be unprocessable by fragmenting all packets below the size of a complete TCP header in the first fragment, preventing flow assignment and forcing the router into expensive reassembly. Alternatively, the adversary can use small-fragment attacks to grow the router's state table, analogous to NAT resource exhaustion. The paper identifies fragmentation-based denial as a harder-to-mitigate attack class than sentinel replay.

2011-karlin-decoy §4.2 ip-blockingmiddlebox-interference