Network and DNS in Linux: ping and dig

ping checks whether packets reach a host, and dig looks up an IP address from a domain name.

Common cases

ping example.com            # do packets get through, and how fast
ping -c 4 example.com       # exactly 4 packets, then stop
dig example.com             # the IP for a name
dig +short example.com      # just the address, nothing else
dig MX example.com          # the domain's mail servers
cat /etc/resolv.conf        # which DNS server is used

How it works: name, address and reachability

These are two different questions. dig answers 'what address does this name have': it asks a DNS server (its address is in /etc/resolv.conf) and returns an IP. ping answers 'do packets reach this address': it sends short ICMP packets and waits for a reply. So if ping by name fails, check dig first. Maybe the name does not resolve, and the link is fine. For debugging you can hard-code names in /etc/hosts, bypassing DNS.

In the book

This topic is covered in full in the chapter Networking basics.

See also

ip addr and ip link, Sockets: ss, curl and wget.