Open and close ports with firewalld

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$ als gewone gebruiker# als root (of met sudo)

Examendoelen in dit hoofdstuk

Netwerktoegang beperken met firewalld en firewall-cmd
Firewallregels instellen met firewall-cmd en firewalld

Understand Linux firewalling

The kernel part is netfilter. Tools write rules into it: first iptables, now nftables. On RHEL you normally do not write nftables rules yourself — firewalld does it for you with zones and services.

Tool	Role on RHEL 10
netfilter	Kernel framework that filters packets
nftables (`nft`)	Rule engine and low-level tool (firewalld backend)
iptables	Old tool; replaced by nftables
firewalld (`firewall-cmd`)	The service and tool you use

$systemctl status firewalld
#nft list ruleset | head -20

Let op

Do not mix your own nft or iptables rules with firewalld; firewalld may overwrite them at reload.

23.2

Understand zones

A zone is a trust level with its own rules. Every network interface (or source address) belongs to one zone. Traffic is checked by the rules of that zone.

Zone	Default behaviour
`public`	Default on RHEL: only selected services (ssh, cockpit, dhcpv6-client)
`drop`	Drop everything incoming, no answer
`block`	Reject incoming with an ICMP message
`external`, `dmz`, `work`, `home`, `internal`	Different preset services
`trusted`	Allow everything

$firewall-cmd --get-default-zone
$firewall-cmd --get-active-zones
$firewall-cmd --get-zones
#firewall-cmd --list-all
#firewall-cmd --list-all --zone=internal

23.3

Understand services

A firewalld service is a name for one or more ports (and helpers). http = 80/tcp, ssh = 22/tcp, nfs = 2049/tcp … Using names keeps rules readable.

$firewall-cmd --get-services | tr ' ' '\n' | grep -E '^(http|https|ssh|nfs|samba)$'
$firewall-cmd --info-service=http
$ls /usr/lib/firewalld/services/ | head

You can define your own service in /etc/firewalld/services/NAME.xml.

23.4

Open services and ports

Add a service or port to the zone. Without --permanent it is lost at reload or reboot; with --permanent it is not active until a reload.

Open a service and a port permanently, then reload.

#firewall-cmd --add-service=http --permanent
#firewall-cmd --add-port=8443/tcp --permanent
#firewall-cmd --reload

Check.

#firewall-cmd --list-services
#firewall-cmd --list-ports

Close them again.

#firewall-cmd --remove-service=http --permanent
#firewall-cmd --remove-port=8443/tcp --permanent
#firewall-cmd --reload

Way	Effect
without `--permanent`	Active now, gone after reload/reboot
`--permanent` + `--reload`	Saved and active
`--runtime-to-permanent`	Save all current runtime rules

Examentip

Use --permanent and --reload for every firewall task, and check with firewall-cmd --list-all. Test from another machine.

23.5

Work with zones and sources

Change the default zone, move an interface to another zone, or give a source network its own zone.

#firewall-cmd --set-default-zone=public
#firewall-cmd --zone=internal --change-interface=enp2s0 --permanent
#firewall-cmd --zone=trusted --add-source=192.168.122.0/24 --permanent
#firewall-cmd --reload
$firewall-cmd --get-active-zones

Notitie

NetworkManager can store the zone of a connection: nmcli con mod enp2s0 connection.zone internal.

23.6

Use rich rules and forwarding

Rich rules allow more detail: a service only for one network, logging, rejecting one IP. Port forwarding sends traffic to another port or host.

#firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="192.168.122.0/24" service name="mysql" accept'
#firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="203.0.113.50" drop'
#firewall-cmd --permanent --add-forward-port=port=80:proto=tcp:toport=8080
#firewall-cmd --reload
#firewall-cmd --list-rich-rules

Op het werk

Open only what you need, and limit admin services (ssh, cockpit, databases) to admin networks with rich rules or a separate zone.

23.7

Test the firewall

Check the rules on the server, then test from outside.

#firewall-cmd --list-all
#firewall-cmd --query-service=http
yes
$curl -I http://server1
$nc -zv server1 8443
#firewall-cmd --set-log-denied=all
#journalctl -k | grep -i 'filter_IN_public_REJECT'

23.8

Remember the main points

The most important facts of this chapter, in short.

firewalld manages the firewall (nftables underneath); the tool is firewall-cmd.
Zones set trust levels; every interface or source belongs to one zone; the default zone is public.
Open a service (--add-service=http) or a port (--add-port=8080/tcp).
Changes without --permanent are lost at reload or reboot; with --permanent they wait until --reload. Do both, or use --runtime-to-permanent.
firewall-cmd --list-all shows the active zone; --get-services lists known services.
Rich rules allow more detail, for example one source network for one service.
Test from another host with curl, nc or nmap.

23.9

Learn the key terms

The words of this chapter in simple English.

Term	Meaning
firewalld	The firewall service of RHEL.
Zone	A set of rules with one trust level.
Default zone	The zone for interfaces that have no other zone.
Service	A name for one or more ports, like `ssh` or `http`.
Runtime config	The rules that are active now.
Permanent config	The rules saved on disk for the next start.
Rich rule	A detailed rule with source, service, port and action.
nftables	The kernel firewall that firewalld uses.