If possible, you should avoid re-using subdomains or domains when installing new Fediverse servers. If you set up a new server from scratch using exactly the same subdomain or domain as a defunct server, this may cause technical problems when you attempt to communicate with the rest of the Fediverse.
Can I set up a new subdomain on a previously used domain?
Yes! If the subdomain is brand new, that will avoid technical problems. With a brand new subdomain, the rest of the Fediverse will immediately see it correctly as a totally new server.
Could you give an example of what to do?
If you have an old server at social.example.com and it closes down, it would be a bad idea to set up a new server with that same address. Instead, you could use a different subdomain such as social1.example.com or fedi.example.com, or whatever you want to replace the word “social”. The important thing is that it isn’t identical to the old server’s address.
Why would re-use of a subdomain or domain cause technical problems?
All Fediverse servers store data about posts and accounts they have federated with. If they already have data about posts and accounts from a defunct server, this may confuse attempts by your new server to connect using the same address. This is especially true if a username on the old and new server is identical, because they will have different keys verifying their identity.
What if I re-install a server and use the database from an old server?
If you’re re-using the database of the old server as well as re-using that old server’s subdomain or domain, then this should avoid problems because it will be technically the same as the old server, using the same keys to verify accounts.
The problems occur if you’re installing on the same subdomain or domain using a brand new database with a brand new set of keys.
What are keys?
On Fediverse servers, keys are special codes used behind the scenes to confirm that a particular account created a particular post or followed a particular other account. Users never see them, but the software that runs servers depends on them to make sure they’re talking to the right account on the right server.
If I’ve already reused a subdomain or domain, will the technical problems ever go away?
The problems may eventually go away when other servers refresh the keys they use to verify identities. However, this can take a long time, and it’s much better to just avoid these problems altogether by using a new subdomain or domain.