Add a torrc option and descriptor line to opt-in as a FallbackDir

This needs:

• a proposal and a design
• a patch to dir-spec.txt
• a patch to the tor man page
• a tor code patch
• an updateFallbackDirs.py code patch
• a wiki update to [[doc/UpdatingFallbackDirectoryMirrors]]

Here's a quick sketch of a design:

1. Relay operators set OfferFallbackDir 1 to offer their relay as a potential FallbackDir.
2. Relays with this option set put offer-fallback-dir in their descriptors
3. updateFallbackDirs.py loads relay fingerprints with offer-fallback-dir, and from the legacy offer list
4. updateFallbackDirs.py does stability checks, and generates the fallback list

changed milestone to %Tor: unspecified

added 034-removed-20180328 034-triage-20180328 component::core tor/tor fallback milestone::Tor: unspecified needs-proposal network-team-roadmap-2020Q1 points::3 priority::medium severity::normal status::new teor-backlog tor-spec type::task labels

We should check if the fallbacks have a DirPort and ContactInfo. A ContactInfo should be required on the relay and by the selection script. No DirPort should result in a warning on the relay. A DirPort should be required by the selection script until #19129 (moved) is implemented.

Trac:
Keywords: N/A deleted, 034-triage-20180328 added

Per our triage process, these tickets are pending removal from 0.3.4.

Trac:
Keywords: N/A deleted, 034-removed-20180328 added

These tickets, tagged with 034-removed-*, are no longer in-scope for 0.3.4. We can reconsider any of them, if time permits.

Trac:
Milestone: Tor: 0.3.4.x-final to Tor: unspecified

This is not part of the minimal fallback rebuild in 0.3.4.

Trac:
Parent: #24786 (moved) to N/A

I also want this on my backlog.

Maintaining the fallback whitelist is very time-consuming, and it adds an extra few weeks to every fallback rebuild.

Trac:
Owner: N/A to teor
Status: new to assigned

Depending on the results of #30952 (moved), we might want to add this feature to get more fallbacks.

Trac:
Parent: N/A to #28793 (moved)

No need for this right now.

Trac:
Parent: #28793 (moved) to N/A

These items are not on our roadmap, and they do not have a sponsor. But I might do them some day.

Trac:
Owner: teor to N/A
Keywords: N/A deleted, teor-backlog added

Trac:
Keywords: N/A deleted, network-team-roadmap-2020Q1 added

Change tickets that are assigned to nobody to "new".

Trac:
Status: assigned to new

Update description, make option shorter.

Trac:
Description: This needs:

• a proposal and a design
• a patch to dir-spec.txt
• a patch to the tor man page
• a tor code patch
• an updateFallbackDirs.py code patch
• a wiki update to [[doc/UpdatingFallbackDirectoryMirrors]]

Here's a quick sketch of a design:

1. Relay operators set OfferFallbackDirServer 1 to offer their relay as a potential FallbackDir.
2. Relays with this option set put offer-fallback-dir-server in their descriptors
3. updateFallbackDirs.py loads relay fingerprints with offer-fallback-dir-server, and from the legacy whitelist (#24838 (moved) will make this easier)
4. updateFallbackDirs.py applies the blacklist, does stability checks, and generates the fallback list

to

This needs:

• a proposal and a design
• a patch to dir-spec.txt
• a patch to the tor man page
• a tor code patch
• an updateFallbackDirs.py code patch
• a wiki update to [[doc/UpdatingFallbackDirectoryMirrors]]

Here's a quick sketch of a design:

1. Relay operators set OfferFallbackDir 1 to offer their relay as a potential FallbackDir.
2. Relays with this option set put offer-fallback-dir in their descriptors
3. updateFallbackDirs.py loads relay fingerprints with offer-fallback-dir, and from the legacy offer list
4. updateFallbackDirs.py does stability checks, and generates the fallback list

changed time estimate to 24h

mentioned in issue #32699 (moved)

moved to tpo/core/tor#24839 (closed)

mentioned in issue tpo/core/fallback-scripts#32699 (closed)