As of July 2019, approximately 10.93% of the Alexa top 1 million websites support ESNI (all via Cloudflare CDN, which enabled ESNI across all its platforms in September 2018), with 92.56% of Cloudflare-hosted sites using encrypted SNI over TLS 1.3. However, fewer than 0.01% of observed TLS ClientHello messages in the wild contained an ESNI extension, revealing a severe gap between server-side readiness and client-side adoption.

The paper identifies 47 Cloudflare IP addresses that are already blocked by the GFW despite being shared by at least 85 websites, contradicting the prior assumption that censors avoid blocking shared CDN IPs due to collateral damage. This suggests censors will accept significant collateral damage to block CDN-hosted content when the set of co-hosted non-forbidden pages is deemed manageable.

§4.3 detection

Of the Alexa top 1 million websites censored in China, 84.5% are blocked by IP address, meaning that even if both DNS hijacking and SNI filtering are fully circumvented, the vast majority of blocked sites remain inaccessible. Only 66 currently censored sites can be unblocked by ESNI alone (combined with an encrypted DNS channel), while 101,049 ESNI-supported sites remain blocked by IP.

§4.1, §4.3 evaluation

Monitoring ESNI-related censorship across 14 geographic regions — including Mainland China, Iran, UAE, South Korea, and 10 others — found no blocking of ESNI traffic or interference with ESNIKey retrieval via DNS TXT records as of mid-2019, contradicting a widely circulated report claiming South Korea had already blocked ESNI. Additionally, the GFW's residual censorship window after a triggered RST was measured at 60 seconds (down from the previously reported 90 seconds).

§4.4, §6 evaluation

In China's Great Firewall, SNI filtering is almost never the sole blocking mechanism: only 70 of the 21,446 SNI-filtered sites are exclusively censored via SNI. The GFW uses SNI filtering as a 'third gatekeeper' — applied after DNS hijacking and IP blocking — and maintains separate blacklists for SNI filtering and DNS hijacking, evidenced by 2,764 sites under DNS injection but not SNI filtering.

§4.1 detection

In AS197207 (Iran, MCI), approximately 50% of DoT endpoints failed consistently — the only case across all tested ASN/protocol combinations where failure exceeded 20%. In Kazakhstan (AS48716) and China (AS45090), more than 80% of DoT and DoH endpoints were always reachable.

2021-basso-measuring §V-F, Table VII ip-blockingsni-blockingtls-fingerprint

Real-world CDN HTTPS deployments leak the identity of visited websites through three distinct channels — TLS certificate contents (A2, B1, B2 deployments), the plaintext SNI field (B1), and dedicated IP address mappings (B2) — enabling censors to block CDNBrowsing connections via standard DPI or IP filtering without collateral damage to non-forbidden CDN content. Each leakage channel requires inspecting only a single packet from an HTTPS connection, making the attack low-cost and deployable on off-the-shelf censorship boxes.

2016-zolfaghari-practical §3.1 dpisni-blockingtls-fingerprintip-blocking

As of 2015, TLS tampering detection was implemented by only a small minority of surveyed censorship measurement tools: explicitly by Holz et al.'s Crossbear (2012) and OONI (2012), and partially by Soghoian and Stamm (2011) and UBICA (2013). The majority of the 13+ surveyed platforms detected DNS tampering and HTTP manipulation but lacked TLS coverage, creating a systematic blind spot in published censorship measurement.

2015-aceto-internet Table 2 tls-fingerprintsni-blocking

Tor's TLS handshake exhibited multiple distinguishing fingerprints — including the client cipher list, server certificates, and randomly generated SNIs — that were used for TLS-based filtering in Ethiopia, China, and Iran. Inferring the exact byte-level pattern matched by DPI boxes required manual analysis and remains a difficult open problem as of 2013.

2013-winter-towards §3.1.5, §5 tls-fingerprintdpisni-blocking

The paper identifies a fundamental architectural vulnerability in single-IP circumvention designs: a relay must generate new observable flows (via DNS or TLS SNI) to reach end services after receiving client connections, creating a detectable server-and-client behavioral contrast. A relay accessing user-facing domains (news, social media) scores high on a Relay Suspicion Score (w=0.9) versus infrastructure domains (w=0.1). The paper argues this host-level signal is censorship-invariant and cannot be concealed by link obfuscation.

2026-almutairi-server §2.4, §4 traffic-shapedpisni-blocking

AnyTLS implements a persistent idle-session pool with configurable parameters: idle_session_check_interval (default 30s), idle_session_timeout (default 30s), and min_idle_session (default 5). The client maintains at least 5 pre-established TLS sessions at all times to enable fast connection reuse without a new TLS handshake per request.

2026-anon-anytls-anytls-sing-box-2026 §2.4, §5.3 tls-fingerprinttraffic-shape