# Cis Ubuntu2004 V300 4 4 3 4

> Ensure ip6tables firewall rules exist for all open ports

- Skill: `cyberstrikeus/cis-ubuntu2004-v300-4-4-3-4` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu2004-v300-4-4-3-4`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-ubuntu2004-v300-4-4-3-4/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-ubuntu2004-v300-4-4-3-4

---


# CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0 - Control 4.4.3.4

## Profile

- **Level:** Level 1 - Server, Level 1 - Workstation
- **Assessment Status:** Automated

## Description

Any ports that have been opened on non-loopback addresses need firewall rules to govern traffic.

Notes:

- Changing firewall settings while connected over network can result in being locked out of the system
- Remediation will only affect the active system firewall, be sure to configure the default policy in your firewall management to apply on boot as well
- The remediation command opens up the port to traffic from all sources. Consult iptables documentation and set any restrictions in compliance with site policy

## Rationale

Without a firewall rule configured for open ports default firewall policy will drop all packets to these ports.

## Audit Procedure

### Command Line

Run the following command to determine open ports:

```bash
ss -6tuln
```

Run the following command to determine firewall rules:

```bash
ip6tables -L INPUT -v -n
```

Verify all open ports listening on non-localhost addresses have at least one firewall rule.

## Expected Result

All open IPv6 ports on non-localhost addresses have corresponding ip6tables rules.

## Remediation

### Command Line

For each port identified in the audit which does not have a firewall rule establish a proper rule for accepting inbound connections:

```bash
ip6tables -A INPUT -p <protocol> --dport <port> -m state --state NEW -j ACCEPT
```

## Default Value

Not applicable.

## References

1. NIST SP 800-53 Rev. 5: CA-9, SC-7

## CIS Controls

v8 - 4.4 Implement and Manage a Firewall on Servers
v8 - 4.5 Implement and Manage a Firewall on End-User Devices
v7 - 9.4 Apply Host-based Firewalls or Port Filtering

MITRE ATT&CK Mappings: T1562, T1562.004 | TA0011 | M1031, M1037