How to check why a website or server is unreachable

Work through DNS, routing, port and TLS checks with ping, dig, traceroute, curl and ss to find exactly where a connection fails.

Intermediate · about 8 min · Linux, Windows, macOS

"The site is down" can mean DNS, routing, firewall, the service itself or TLS. Testing layer by layer, in order, finds the break in a couple of minutes instead of guessing.

What this guide covers

1. Does the name resolve?
2. Is the host reachable at all?
3. Where does the path break?
4. Is the port open?
5. What does the server actually answer?
6. On the server: is the service listening?

Common pitfalls covered

Testing from one network only: try a phone on mobile data to rule out local DNS or firewall issues.
A cached DNS record can point you at a decommissioned server — flush with … or ….
An expired TLS certificate looks like an outage in browsers but curl will say so explicitly.

Questions answered

Why does ping fail but the website loads?
How do I check DNS from the command line?
How do I know if it is my network or the server?

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