Introduction

When troubleshooting network services it is essential to be able to see that a service is not just running but also accessible. Not every tool we cover will be available in any given environment; lsof in particular is rarely a part of default installations. It may not be possible to quickly or easily install your favourite utility, particularly on an outdated system. Therefore it is helpful to carry many tools in your belt. A listening service is not necessarily open; we will also cover using a port scanner like nmap to verify accessibility from remote hosts and how to check your netfilter rules with iptables.

lsof
lsof -i -n -P

lsof -i -n -P | grep ":80"

netstat
netstat -plnt

ss
ss -tunlp

nmap
nmap 192.168.0.100 -p 1-65535