Working through connectivity faults.
WHAT TO ESTABLISH FIRST
Whether the problem is local, on the path, or at the destination.
HOW
Test from more than one place.
WHAT TO TEST IN ORDER
The interface has an address The default route exists An address outside responds DNS resolves The destination port accepts a connection The service responds correctly
WHY THAT SEQUENCE
It isolates the layer at fault instead of guessing.
WHAT TO USE FOR PORT TESTING
A connection attempt with curl, telnet or nc.
WHAT A REFUSED CONNECTION MEANS
Something answered and declined, so the host is reachable and nothing is listening.
WHAT A TIMEOUT MEANS
Nothing answered, usually a firewall dropping traffic.
WHY THAT DISTINCTION IS THE MOST USEFUL IN NETWORKING
It separates a service problem from a filtering problem immediately.
WHAT TO CHECK WHEN DNS SEEMS WRONG
Which resolver is configured What an authoritative server returns Whether a local cache is stale
WHAT TO CHECK WHEN ONLY SOME USERS ARE AFFECTED
Routing, and whether an address range is blocked.
WHAT TO LOOK AT LOCALLY
Whether the firewall dropped it Whether the connection limit was reached
WHAT TO RECORD
What you tested and what it returned.
