Firewalls

Linux filters packets in the kernel (netfilter). You'll meet three front-ends:

Tool	Distro	Example
`firewalld`	RHEL/Fedora	`firewall-cmd --add-service=http --permanent`
`ufw`	Ubuntu/Debian	`ufw allow 22/tcp`
`iptables`/`nftables`	Low-level, all	`iptables -A INPUT -p tcp --dport 22 -j ACCEPT`

firewalld (RHCSA):

$ firewall-cmd --permanent --add-service=https
$ firewall-cmd --reload
$ firewall-cmd --list-all

ufw (Ubuntu):

$ ufw allow 22/tcp
$ ufw enable
$ ufw status verbose

Exam trap: In firewalld, changes need --permanent and a --reload to survive; without --permanent they're lost on reload/reboot.