Proposal: Support for Dynamic IP obfs4 bridges with unattended proxy information update(aka "Subscription")

Context

Currently, Tor encourages obfs4 bridges with a static address. This reduces the interruption to the users as users won't need to request new bridges frequently, which need to be done manually. However, this also allows censors to block bridges by IP, thus resulting in bridges consistently in an inaccessible state for some users.

Proposal

Support obfs4 bridges with a dynamic IP address by allowing users to perform an unattended update of the bridge IP address. When the user retrieves an obfs4 bridge with a dynamic IP, the user will also receive a ticket. This ticket will then allow the user to receive an updated IP address of that bridge without completing a captcha or any manual interaction. The tor browser will do that on behalf of the user during connecting process. So, the user experience will not be impacted when the bridge operator rotates the IP address.

The operator of the obfs4 bridge can then switch to a new IP when the bridge is blocked. This process can be automated if we have an API to detect blocks.

Additional Backgrounds

The concept of subscription(unattended proxy information update) is a feature supported by almost all Chinese anti-censorship tools. Here are some relevant discussions & screenshots. An example of this will be Shadowsockets SIP008, which is currently supported by Outline and many other clients like SagerNet. (Conflict of Interest Declared)

Here is a screenshot of the configuration screen for a "subscription" in SagerNet.