Future of CJDNS support #36041

CJDNS support was added in v23.0 (#23077, released in early 2022).

Unfortunately, it never got much traction among users, in spite of a meaningful amount of work that was put into it:

• I recently tried -onlynet=cjdns out of curiosity on mainnnet, never got more than 4 peers. Today, I got 3 peers. There are 11 fixed seeds, so many of them appear to be offline.
• In the past, usually the complicated setup (one had to manually find an initial peer) was seen as the reason why there are so few nodes. However, this was improved some time ago (documented in #34811), but it doesn't seem to have made much of an effect so far.
• Maintenance cost is not zero: CJDNS is not cleanly separable from IPv6, leading to the necessity of the -cjdnsreachable and MaybeFlipIPv6toCJDNS workarounds. Especially the latter has been the source of multiple bugs and fixes.

Maybe we could try to advertise cjdns more, so that more people know that it is an option, and that it is no longer hard to set up.

I think that for a network to justify continued support, it should reach more interest at some point, especially because cjdns-only nodes (which is discouraged after #35963) would be in danger of being eclipsed. As a rough number, I feel that the number of nodes should be at least in the low triple digits.

What do other people think about this? Should a network meet certain criteria to keep being supported, and should we consider removing cjdns support at some point if nothing changes?

fyi @vasild @jonatack

Not sure if this has been considered, but could the threshold be grounded in eclipse cost, roughly how many nodes an attacker would need to fill a cjdns-only node's 8 full-relay and 2 block-relay-only outbounds? That might give a criterion that generalizes to future networks.

Looking at my seeder's database, it currently has only 25 CJDNS nodes in its database, of which 22 have been reached, and of those, only 7 are marked "good".

Concept ACK on deprecating CJDNS support.

In addition to low raw node count numbers there, I question the value of a network where connections need to be manually established in practice anyway. Something quite close can be achieved using -addnode with less configuration overhead.

I'm running a couple of nodes reachable via CJDNS (primarily for monitoring), and the maximum number of connections to CJDNS nodes I had on one of these nodes was 5 at the same time. I don't know if these connections were to my nodes, but seeing the numbers here makes it sound like they were.

Unless there's going to be a lot more usage of CJDNS, I don't think it's worth maintaining it for much longer.