ping
NetworkPackage: iputils-ping
Sends ICMP packets and times the reply. It proves a machine is reachable at the IP level; it says nothing about port 443, and a firewall dropping ICMP makes it fail on a perfectly healthy server.
What its options do in the lessons
From the same glossary the lessons render under their commands, so the two cannot disagree.
ping -c- Stops after sending that many packets, instead of running until Ctrl+C.
ping -s- Payload size in bytes, headers excluded:
-s 1472puts a 1500-byte packet on the wire once the 8 ICMP bytes and the 20 IP bytes are added. ping -M- Fragmentation policy.
-M doforbids fragmenting: a packet too large for the MTU is refused locally instead of being split, which is what makes it the way to measure a path's MTU. ping -t- Sets the TTL of the packets sent:
-t 1expires them at the first router, which names itself by returning a Time Exceeded. It is the mechanismtracerouteis built on.
Lessons that teach it
- What actually crosses the wireHow a network works: packets, addresses and neighbours
- Hop by hop: nobody holds the pathHow a network works: packets, addresses and neighbours
- The MTU: when to lower it, and why never by guessConfiguring the network: an address that survives a reboot
- Unreachable: find the broken layer before touching anythingNetworking: how your server is reached
- What the application asks, and why dig answers a different questionDNS: pointing a domain at your server