nc
NetworkPackage: netcat-openbsd
Opens a raw TCP or UDP connection to a port, or listens on one. nc -zv host 5432 says whether the port is open before the database gets blamed.
What its options do in the lessons
From the same glossary the lessons render under their commands, so the two cannot disagree.
nc -v- Announces the verdict of each connection attempt — success or failure — instead of staying silent.
nc -z- Connects without sending a byte, then closes: the test stops at “does the connection open?”.
nc -w- Maximum time, in seconds, before giving up on a connection that does not complete. Without it you wait on the kernel — several minutes.
Lessons that teach it
- Refused or silent: what each failure provesNetworking: how your server is reached
- Unreachable: find the broken layer before touching anythingNetworking: how your server is reached
- Works on the server, refused from outsideNetworking: how your server is reached
- Dropped here or never arrived: one capture decidesNetworking: how your server is reached