I maintain multiple tools at work that all run in Docker containers on the same machine. The overall setup looks like the following diagram:

The router container on top (nginx) routes traffic to the various application containers based on the hostname seen in each request (each tool has its own internal domain name). Each application has an nginx container for serving static assets, and a gunicorn container to serve the dynamic parts of the application (using the Django framework).

Earlier this week, I was trying to add a redirect rule to one of my application containers (at the application nginx layer), because a URL was changing. As a convenience for users, I wanted to redirect them to the new location so they don't get the annoying "404: Not Found" error. I set up the redirect as a permanent redirect using a rewrite rule in nginx. For some strange reason, the port of the application's nginx layer, which should never be exposed to the outside world, was being appended to the redirect!

Adding the port_in_redirect off; directive to my nginx rules made no difference (or so I thought), and I struggled for an entire day on why this redirect wasn't working properly. At the end of the day, I learned that permanent redirects are aggressively cached by the browser! This annoyance means you need to clear your browser's cache to remove bogus redirects. I wasted an entire day because my stupid browser was using a bogus cached reference. Ugh!