A community to talk about the Fediverse and all it's related services using ActivityPub (Mastodon, Lemmy, KBin, etc).
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When viewing an instance's main page in a web browser, scroll all the way to the bottom and click on "Instances". That will show all instances federated with, or blocked by, the given instance.
I'm not sure I can answer your other questions, though you might be interested in https://fba.ryona.agency. It will let you enter an instance and see who is blocking it. For example, here are the instances blocking my home instance, sh.itjust.works: https://fba.ryona.agency/?domain=sh.itjust.works
This is very helpful. Unfortunately the source code is not available... How did you find this service? Maybe you can point me to some other resource
Someone linked it in a comment a few months ago, and I happened to bookmark it.
Here are a few other sites that might be interesting:
You can see the list of federated instances of each instance at https://example.com/instances
You can also get that data in json format by using https://example.com/api/v3/federated_instances
https://lemmy.readme.io/reference/getfederatedinstances
Mastodon and Lemmy are both using the ActivityPub protocol (Lemmy uses some extensions to federate downvotes and some other actions) to communicate between instances, but the API to interrogate a Mastodon, Lemmy, PixelFed or kbin instance will be different.
This is exactly what I am looking for. I am interested also in how other federated applications do it. I had a look at Pixelfed's documentation, but they don't seem to have an API for that... Do you know anything about other services?