connchk
Open Source
Quick Description
connchk checks whether a network path actually works: whether a given source can reach a destination on a specific port. You can feed it hosts and ports directly on the command line or from a file, and it reports success or failure for each combination it tries.
This is invaluable when you're standing up new infrastructure, debugging firewall rules, or validating that a service is reachable before an application depends on it. Rather than juggling telnet, nc, and ping, connchk gives you one scriptable way to confirm connectivity across a whole list of endpoints at once.