Sherlock Holmes: The Case Of App Not Found

Welcome to our Sherlock Holmes-inspired tech adventure Series! Imagine each technical challenge as a thrilling mystery waiting to be solved. Like Sherlock Holmes with his sharp eye for detail, I'll tackle the problem with wit and precision. Let's dive in and crack these cases together!

Setting Up Nginx as a Proxy

We recently worked on proxying requests to our Heroku app through Nginx. The Nginx configuration was set up as follows:

server {
    listen 80;
    server_name ourdomain;

    location / {
        proxy_pass https://heroku-app.herokuapp.com;

        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
    }
}

After configuring Nginx, we restarted the server and updated the IP address in the domain provider. However, when we tried to access our website using the custom domain, we encountered a "Not Found" page.

Investigating the Issue

At this point, the debugging process began. First, I reviewed the Nginx configuration by inspecting the file to ensure that there were no errors, but everything appeared to be correct.

I started to suspect that the issue might be related to whether the correct configuration was being applied to our domain, especially since we had multiple configurations. To confirm, I changed the proxy_pass directive to point to Google's homepage. After making this change, when I accessed our domain, it successfully redirected to Google, confirming that the correct configuration was being used.

Next, I tested accessing the Heroku app URL directly to verify whether the app itself was working, and it loaded without any issues.

The Breakthrough

With no other options, I decided to go over the Nginx configuration again. Upon reviewing the file, I had an important realization: the issue was with the Host
header configuration. Here’s the problematic line:

proxy_set_header Host $host;

If you've used Heroku before, you may know that when you create a new app, Heroku automatically assigns it a subdomain like appname.herokuapp.com. But how does Heroku route requests to the correct app when you hit a *.herokuapp.com URL?

Heroku uses the Host header to determine which app the request should be routed to. If the Host value does not match any Heroku app or if it's missing altogether, you'll receive an "App Not Found" error.

In our case, the Host header was set to ourdomain, which meant that when the Heroku router tried to forward the request, it looked for an app associated with ourdomain—which obviously doesn’t exist. That's why we were seeing the "App Not Found" error.

The Fix

To resolve the issue, the Host header should be set to the Heroku app's subdomain (appname.herokuapp.com) instead of the custom domain. Here's the corrected line:

proxy_set_header Host heroku-app.herokuapp.com;

By explicitly setting the Host header to the correct value, Heroku can route the request to the appropriate app, and everything works as expected.

Stay tuned for our next adventure, where we continue to unravel the mysteries of the infrastructure world, one case at a time. Until then, keep your magnifying glasses handy and your curiosity alive.

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