ping, dig, curl & ss

Is it reachable?

Lesson 19 of 20 · 12 min · Module 6: Networking from the shell

When "the site is down", a network engineer checks each layer in turn: can I reach the host, does the name resolve, does the service answer, and is it listening?

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Test reachability with ping
Resolve names with dig
Check a service with curl and ss

The idea

ping -c 2 host sends two test packets. dig +short name asks DNS for an address. curl url makes a web request and prints the response. ss -tuln lists ports on which this machine is listening (t = TCP, u = UDP, l = listening, n = numbers).

ping -c 2 192.168.1.1
dig +short codetemper.com
curl http://localhost:8080/health
ss -tuln

01Work from the bottom up

ping tests basic reachability. dig tests DNS. curl tests the application. Each check rules out a layer, so you know where to look next.

02Read ping statistics

0% packet loss with low times means the path is healthy. "Destination Host Unreachable" or 100% loss means the host or the route is the problem, not the website.

ping -c 1 10.9.9.9

Output

From 192.168.1.50 icmp_seq=1 Destination Host Unreachable

03Listening is not the same as reachable

ss shows 127.0.0.1:5432 for the database: it listens, but only for this machine. 0.0.0.0:80 listens on every interface. curl to a port with nothing listening gives "Connection refused".

Your turn

Ping the gateway 192.168.1.1 exactly 2 times. Look up the address of codetemper.com with dig +short. Request http://localhost:8080/health with curl. Finally, list the listening ports with ss -tuln.

What the checks look for

Pings the gateway exactly twice
Resolves codetemper.com to 203.0.113.10
Gets the health response
Shows port 8080 listening

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Take this with you

Check layer by layer: reach the host, resolve the name, ask the service, confirm it listens.

Go a little further

Use curl -I http://codetemper.com to see a redirect and its location header.