Onebox preview isn't shown for my own blog

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I’m currently setting up Discourse for my community and I’ve encountered a strange issue that isn’t directly related to Discourse itself.

I have a blog, and I want to create a topic for each blog post via the API to enable discussions about the posts. This part works fine. However, the onebox preview for my blog posts isn’t generated:

image

But: The onebox preview doesn’t work only on my Discourse instance. It works perfectly on other Discourse instances, like here (this isn’t an ad for my blog; it just shows that it generally works):

The blog is a Ghost blog running on my internal Docker host. I’ve already tested a ping and nslookup to my blog domain, and even a curl works fine in the console of the Discourse Docker container.

Now, I want to track down the issue. I’ve studied the Ruby code for Onebox, but since I’m not an expert in Ruby, it’s a bit of a “black box” for me.

I suspect the issue might be related to my internal network and DNS structure. I use AdGuard Home as my primary DNS, and I can see in the DNS logs that external links are correctly resolved, but my internal links aren’t.

I understand this isn’t a Discourse problem, but I need some hints to track this issue down. What would you recommend I double-check?

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Update: I already added a test container to the same docker network of discours and checked some stuff.

curl returns the correct content and nslookup resolves to teh correct host.

I also tried to bypass the dns server and routed everything to a public dns. Even that isn’t working.

Any ideas?

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I am not sure. But have noticed lately at times. One of the Discourse forums(Stable branch) and iirc also on my Tests-passed forum. That sometimes onebox is not being generated for links

Even though previous and later posts from the same domain works as expected. Even with links posted from my Tests-passed forum on the the other Stable one.

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Here is link to OpenMR running stable with a link to my Tests-passed site. Notice the link is not one boxed.