Sending HTTP requests to private EC2 DNS from other EC2 host works, but not from container on EC2 host

I want to make a container hosted on one EC2 host communicate with another container hosted on another EC2 host.

Currently, if I send an HTTP request from host A to host B using host B’s private DNS and the exposed container port, I receive a response correctly.
However, if I try the same thing within the container on host B, I get a “Host is unreachable” error.
To me, this would indicate it is not a host networking issue, which is configured in AWS, so I won’t enter into detail on it.

What puzzles me is that I have done the exact same thing in our pre-production environment and it works perfectly.

What could be the issue / where do you think I should look to troubleshoot this? Please keep in mind I’m a noob 🙂

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Author: Konrad