How does Elk's AS2 content negotiation/discovery work?

[snarfed]

Hi! I build and run another ActivityPub service, https://fed.brid.gy/ , and I'm trying to get it to interoperate with Elk, but I haven't figured out how to get Elk to serve AS2 data. When I fetch a profile or post URL with AS2 content negotiation, eg Accept: application/activity+json or Accept: application/ld+json, I always get Content-Type: text/html returned. I've tried both signed (with HTTP Signatures) and unsigned fetches; no luck. I also don't see any rel=alternate links to AS2 URLs in the HTML or headers.

However, when I enter Elk profile/post URLs into Mastodon search, it finds them ok. Am I missing something obvious? Example curl command lines below. cc snarfed/bridgy-fed#709. Thanks in advance!

$ curl -vL -H 'Accept: application/activity+json' https://elk.zone/m.webtoo.ls/@antfu
...
< HTTP/2 200 
< content-type: text/html;charset=utf-8
...

$ curl -vL -H 'Accept: application/ld+json' https://mozilla.social/@mconley/111348170686611324
...
< HTTP/2 200 
< content-type: text/html;charset=utf-8
...
``

[trwnh]

the web frontend has to forward requests based on conneg, and elk seems to only respond to Accept: application/ld+json; profile="https://www.w3.org/ns/activitystreams"

[snarfed]

Interesting, thanks! You're right, mozilla.social at least does serve AS2 to that Accept header, but not any variations, eg my more complicated Accept: application/activity+json; q=0.9, application/ld+json; profile="https://www.w3.org/ns/activitystreams"; q=0.8, text/html; charset=utf-8; q=0.7 gets text/html back, even though it includes that exact stanza.

I guess I'll file an issue to improve their conneg handling.

Other interesting minor points:

• Elk returns Content-Type: application/activity+json; charset=utf-8, ie not ld+json, but it requires ld+json in the Accept header; it returns text/html to Accept: application/activity+json; profile="https://www.w3.org/ns/activitystreams". Conneg, whee!
• elk.zone doesn't forward conneg requests at all, even with exactly Accept: application/ld+json; profile="https://www.w3.org/ns/activitystreams". I guess it doesn't have to, but that definitely hurts interop and AP/AS2 discovery. 🤷

[snarfed]

...and confirmed re elk.zone's AP discovery specifically. If you search for an elk.zone post URL on a Mastodon instance (logged in) that hasn't seen it before, eg https://elk.zone/m.webtoo.ls/@antfu/110045168476286703 on https://indieweb.social/ , you get no results. No AP/AS2 discovery happening there. Sigh.

[snarfed]

Filed #2463