Network and time
Test the network in layers
Check name resolution, selected address, route, transport port, TLS, and application response as separate boundaries.
8 minute lesson
“The network is down” combines several systems that can fail independently.
Resolve with dig, inspect ip route get, test the port with nc, and inspect the application with curl -v. Stop at the first failing boundary instead of reconfiguring every layer.
Trace one HTTPS request from the server. Write a single sentence naming the first failure or confirming every tested boundary.
Use one command for each boundary:
ip route get 1.1.1.1
dig +short app.example.com
nc -vz app.example.com 443
curl -v https://app.example.com/healthStop at the first failing layer. A correct DNS answer does not prove the port accepts connections, and an open port does not prove TLS or HTTP works. Save timestamps and exact errors before changing firewall, DNS, certificates, or application configuration.
Lesson completed