# Cis Ubuntu1804 V220 3 4 2 3

> Ensure iptables are flushed with nftables

- Skill: `cyberstrikeus/cis-ubuntu1804-v220-3-4-2-3` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu1804-v220-3-4-2-3`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-ubuntu1804-v220-3-4-2-3/raw
- Safety review: PASS (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: cyberstrikeus (https://skillmd.com/u/cyberstrikeus)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/cyberstrikeus/cis-ubuntu1804-v220-3-4-2-3

---


# CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0 - Control 3.4.2.3

## Description

nftables is a replacement for iptables, ip6tables, ebtables and arptables.

## Rationale

It is possible to mix iptables and nftables. However, this increases complexity and is not recommended. Flushing all iptables rules clears the way for a clean nftables configuration.

## Impact

Flushing all iptables rules will remove all firewall rules. Ensure that nftables rules are configured before flushing iptables.

## Audit Procedure

### Command Line

Run the following commands to verify iptables are flushed:

```bash
iptables -L
```

No rules should be returned.

```bash
ip6tables -L
```

No rules should be returned.

## Expected Result

Both commands should return empty chain outputs with no rules:

```
Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
```

## Remediation

### Command Line

Run the following commands to flush iptables:

```bash
iptables -F
ip6tables -F
```

## Default Value

iptables rules are empty by default.

## References

1. NIST SP 800-53 Rev. 5: CA-9, SC-7
2. CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0

## CIS Controls

Version 8

4.4 Implement and Manage a Firewall on Servers - Implement and manage a firewall on servers, where supported.

4.5 Implement and Manage a Firewall on End-User Devices - Implement and manage a host-based firewall or port-filtering tool on end-user devices.

Version 7

9.4 Apply Host-based Firewalls or Port Filtering - Apply host-based firewalls or port filtering tools on end systems, with a default-deny rule that drops all traffic except those services and ports that are explicitly allowed.

## Profile Applicability

- Level 1 - Server
- Level 1 - Workstation

## Assessment Status

Manual