How India Censors the Web

A proposed HTTP censorship detection algorithm combining status-code comparison, response-length Z-score, HTML TF-vector cosine similarity, and redirect-hostname matching achieves F1 scores of 0.83 (censored) and 0.77 (uncensored), outperforming OONI (0.80 / 0.70), length-difference methods (0.70 / 0.66), and HTML-similarity methods (0.52 / 0.34) on a manually annotated set of 3,000 responses across six Indian ISPs.

§4.3, Table 1 evaluation measurement-platformkeyword-filteringrst-injectionpacket-injection in

All detected HTTP censorship events in BSNL and MTNL are attributable to infrastructure shared with or operated by Airtel and ACT, demonstrating that upstream ISP filtering creates collateral censorship visible to downstream networks. Isolated cross-ISP leakage was also observed: Vodafone's censorship notice appeared in 2 Jio tests, and Airtel's appeared in 2 Vodafone tests.

§5.1.3 evaluation keyword-filteringrst-injectionpacket-injection in

Indian ISPs use heterogeneous and overlapping censorship mechanisms with no single technique common across all providers: DNS tampering (ACT, Airtel, BSNL, MTNL), HTTP header filtering (all six ISPs), and SNI inspection (Jio only). Individual ISPs such as ACT simultaneously apply DNS-only blocking to 233 sites, HTTP-only to 1,873 sites, and both to 1,615 sites.

§5.1, Table 2 evaluation dns-poisoningkeyword-filteringsni-blocking in

Across six ISPs covering 98.82% of Indian internet subscribers, only 1,115 out of 4,033 tested blocked websites (27.64%) are blocked by all six ISPs simultaneously. ACT blocks 3,721 websites while Airtel blocks only 1,892, and 215 websites are blocked by exactly one ISP with no apparent legal basis.

§5.2, Table 3 evaluation measurement-platform in

Jio, India's largest ISP serving 49.7% of internet users, employs SNI inspection to block 2,951 out of 3,340 websites it censors — the first documented use of SNI-based blocking in India. No other of the six tested ISPs uses this technique.

§5.1.4 evaluation sni-blockingdpi in