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this post was submitted on 19 Dec 2024
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So I take a peek at the Lemmy's source code.
When you see a thumbnail and title of a URL (including YouTube video links) on Lemmy, what happens is that Lemmy server fetched the website HTML file and get the
og:titleandog:image(this convention is called OpenGraph protocol ^1) located inside<head>HTML tag.For example, for this video https://www.youtube.com/watch?v=MbYuEEU5e50, the OpenGraph metadata inside the HTML looks like this:
Try opening the "Create Post" page, then enter URL of a YouTube video. You might notice that a loading spinner will appear. During this, the frontend of Lemmy is actually creating a request to
/api/v3/post/site_metadata?url=[insert your URL]. When the backend side of Lemmy receives the request from front end, it will run this code ^2 to fetch the HTML and parse it to find the OpenGraph metadata. If the metadata is found, the backend will return it to front end, and you willl see recommended title and the "Thumbnail URL" field should be filled.In theory, this should work fine, regardless of any website. I personally tried to view the HTML code of a YouTube video and the HTML file indeed contains OpenGraph metadata.
But it doesn't for YouTube videos in Lemmy somehow. And I'm not sure.
I'm suspecting YouTube thought that Lemmy servers are bots and because of that instead of receiving the HTML file of a YouTube video, instead it received a HTML file of a captcha page. I think I'm gonna try contacting admins of my instance to see whether YouTube URL works or not.
Also, this is a good tip in general:
You can't fetch title and thumbnail for YouTube links that starts with
youtu.be. This applies not only in Lemmy, but other websites too, for example Twitter (even before Elon's era).This is, IMO due to the link doesn't really contain HTML and OpenGraph metadata. It only redirects you. If you try cURL-ing the youtu.be link, you will see that it doesn't have a response body, only some header including
Locationheader that will tell you where to redirect. The response code itself is 301.There are people on shitjustworks posting youtu.be links on !videos@lemmy.world that do have thumbnails, so whatever they're doing doesn't seem to have that problem
My .ee account has a field to put in a thumbnail URL, so other comments in this thread are kinda leading me to believe it may be a combination of instance version, and how the instance is configured
My Lemmy instance actually can't fetch YouTube thumbnail, so I actually have to put it manually lol.
See http://www.get-youtube-thumbnail.com/
Thats a super helpful site, thank you! If nothing else now I can remake my post from my .ee account and add the thumbnail manually