Lesson 3 of 3
Diagnosing connection problems
2 min
When "the site is down", work from the bottom up: name, then reachability, then the port, then the application.
1. Does the name resolve?
dig insidx.com +shortNo answer, or the wrong IP, means a DNS problem: check the records and nameservers.
2. Is the host reachable?
ping 203.0.113.10
traceroute 203.0.113.10ping checks basic reachability (some servers block it, so silence isn't proof of failure). traceroute shows each router on the path and where packets stop.
3. Is the port open?
nc -vz insidx.com 443"Connection refused" means nothing is listening; a timeout usually means a firewall is dropping traffic.
4. Is the application responding?
curl -I https://insidx.comRead the status code: 200 fine, 301/302 redirect, 403 forbidden, 502/504 the web server can't reach the application behind it.
Change one thing at a time and write down what you changed. Fixing two problems at once makes it impossible to know what worked.
Scanning: only what you own
nmap lists open ports on a host. Scanning systems you don't own or have permission to test can be illegal. Practise on your own servers, or try the simulated network in IDX Lab.