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[Russia] Investigate obsf4 blocking by protocol in mobile 4G in some ISPs

Since August 2024, we've seen a rise in user support tickets from Russia. Users reported issues with bridges provided by GetBridgesBot, noting that these bridges didn't connect/work for them.

Initially, we suspected that some bridges distributed via Telegram had been enumerated and blocked. However, more recent findings from @nina (full report: #40046 (comment 3125365)) suggest that obfs4 is blocked in mobile/4G network in some ISPs in Russia.

  • The same obfs4 bridge connects successfully on residential connections, but fails on mobile networks (4G).
  • Downgrading to 3G allows these bridges to connect to Tor without issues.
  • WebTunnel bridges remain unaffected.

Just yesterday, a user reported that obfs4 bridges were not working on MTS/Moscow (AS8359). I checked the IPs of these bridges using MTS Looking Glass, and they were reachable, which could be an evidence of a new type of block (by protocol and not by IP).

Providers mentioned by users: Tele2, Beeline, and MTS.

edit: If you're reading this ticket and have access to MTS, Tele2, or Beeline, we would greatly appreciate it if you could share Tor logs with obfs4 bridges, a pcap, and any additional relevant information. Please note that in this context, OONI Tor tests aren't particularly useful. 🖤

cc @meskio.

Edited by Gus