Deploying an application with HTTPS feels straightforward.
Point your domain to the server, configure a reverse proxy like Caddy, and let Let's Encrypt automatically issue a certificate.
That was exactly what I expected.
Instead, certificate issuance kept failing, even though everything looked correct.
At first, I assumed something was wrong with Caddy.
It wasn't.
The real lesson wasn't about Caddy at all-it was about understanding how ACME actually validates a domain.
The Problem
I had a server running behind Caddy, with DNS records pointing to the correct IP address.
The application was reachable, the firewall allowed HTTP and HTTPS traffic, and Caddy was listening on the expected ports.
Yet every attempt to obtain a certificate failed.
Initially, I started checking the obvious things:
- Was DNS configured correctly?
- Were ports 80 and 443 open?
- Was Caddy configured correctly?
- Was the firewall blocking traffic?
Everything appeared to be fine.
So why wasn't Let's Encrypt issuing a certificate?
My Wrong Assumption
At first, I thought HTTPS certificate issuance was simply a conversation between my server and Let's Encrypt.
If my server could reach Let's Encrypt, surely the certificate should be issued.
That assumption was completely wrong.
The important connection isn't just your server reaching Let's Encrypt.
It's Let's Encrypt successfully reaching your server.
That single realization changed how I approached the entire problem.
Understanding ACME
Let's Encrypt uses the ACME (Automatic Certificate Management Environment) protocol to verify that you actually own the domain you're requesting a certificate for.
The process is roughly:
- Your ACME client requests a certificate.
- Let's Encrypt sends a validation challenge.
- Your server proves ownership by responding correctly.
- Only after successful validation is the certificate issued.
In other words, certificate issuance isn't automatic trust.
It's a verification process.
Looking at the Logs
Once I stopped guessing and started reading the logs, things became much clearer.
Instead of generic "certificate failed" messages, the ACME logs explained which validation step was failing.
That immediately narrowed the investigation.
Rather than assuming TLS was broken, I started asking more useful questions:
- Can Let's Encrypt reach my server?
- Is the expected challenge actually being served?
- Is DNS resolving to the correct address?
- Is traffic arriving where I think it is?
Those questions were far more valuable than repeatedly changing configuration files.
The Bigger Lesson
One thing I learned during this debugging session is that HTTPS failures are often not TLS failures at all.
They can be:
- DNS issues
- Routing problems
- Network interface problems
- Reverse proxy configuration mistakes
- Port forwarding issues
The certificate request simply exposes those underlying networking problems.
In my case, the root cause wasn't ACME itself-it was a networking issue preventing successful validation.
Understanding how ACME worked helped me stop treating the symptoms and start investigating the actual cause.
A Better Way to Debug ACME Failures
Instead of randomly changing configuration files, I found it much more effective to work through a simple checklist.
First, verify that DNS resolves to the expected IP address.
Next, confirm that ports 80 and 443 are reachable from outside the network.
Then inspect the ACME client logs to determine which validation step is failing.
Finally, validate that your server is actually serving the expected challenge response.
Each step eliminates an entire class of possible problems.
What Changed My Thinking
Before this experience, I viewed HTTPS certificate issuance as a feature provided by the reverse proxy.
Now I see it differently.
ACME is fundamentally a validation protocol.
TLS certificates are simply the end result of successfully proving domain ownership.
Once I understood that, the debugging process became much more systematic.
Instead of asking:
"Why won't Let's Encrypt issue my certificate?"
I started asking:
"What is the ACME server trying to verify, and why is that verification failing?"
That shift in thinking made all the difference.
Final Thoughts
Debugging production systems often comes down to replacing assumptions with understanding.
At first, I thought the problem was Caddy.
Then I suspected DNS.
Then I questioned my firewall.
In reality, none of those were the underlying issue.
The real lesson was understanding how ACME validates a deployment before a certificate is ever issued.
Once that mental model clicked, the logs became easier to interpret, the investigation became more focused, and the actual root cause was much easier to identify.
Sometimes the hardest part of debugging isn't fixing the system.
It's understanding how the system is supposed to work in the first place.
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