What is the difference between firewall-cmd and iptables and can they co-exist on the same system and if so, is it advisable or not?

Difference Between `firewall-cmd` and `iptables`

1. Nature of the Tools

iptables:
A low-level command-line tool used to directly manage the Linux kernel’s netfilter firewall.
Operates at the packet filtering level.
Rules are defined manually for packet inspection, NAT, and filtering.
firewall-cmd:
A high-level management tool provided by the firewalld service to configure firewall rules dynamically.
Abstracts the complexities of iptables by providing an easier interface.
Rules are organized into zones to simplify management based on trust levels.

2. Ease of Use

iptables:
Offers granular control but requires deep technical knowledge of netfilter syntax.
Static configuration—rules must be reloaded after changes.
firewall-cmd:
User-friendly, dynamic, and allows rule changes without restarting the service.
Automatically persists changes if desired.

3. Zones and Concepts

iptables:
No concept of zones; all rules are global.
Users must define and chain rules explicitly.
firewall-cmd:
Uses pre-defined zones (e.g., public, internal, trusted) for different trust levels.
Simplifies applying rules to specific network interfaces or traffic types.

Can `firewall-cmd` and `iptables` Co-Exist?

1. Co-Existence

Yes, they can technically coexist on the same system because firewall-cmd is a front-end to netfilter (which iptables directly manipulates). Both tools interact with the same underlying kernel structures for packet filtering. However:

Conflicts:
firewalld overwrites rules defined by iptables whenever it is restarted or a rule is added using firewall-cmd.
Manual modifications using iptables can cause unexpected behavior when firewalld is active.

2. Is It Advisable?

It is not advisable to use both simultaneously unless you are confident about what you are doing because:

It can lead to conflicts and unexpected results.
Rules managed by iptables might get wiped out by firewalld or vice versa.

Recommended Approach

Choose One Tool:

Use firewall-cmd (firewalld) if you prefer a simpler, zone-based approach.
Use iptables for more fine-grained, low-level control.

Disable the Other:

If you decide to use iptables, stop and disable firewalld:
bash systemctl stop firewalld systemctl disable firewalld
If you prefer firewalld, avoid direct interaction with iptables.

Summary

iptables is powerful but complex and static.
firewall-cmd is user-friendly and dynamic but abstracts away some control.
Co-existence is possible but not recommended due to potential conflicts. It’s better to commit to one based on your system’s needs and your familiarity with the tools.