# Cis Ubuntu1804 V220 3 4 3 3 1

> Ensure ip6tables default deny firewall policy

- Skill: `cyberstrikeus/cis-ubuntu1804-v220-3-4-3-3-1` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu1804-v220-3-4-3-3-1`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-ubuntu1804-v220-3-4-3-3-1/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-v220-3-4-3-3-1

---


# CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0 - Control 3.4.3.3.1

## Description

A default deny all policy on connections ensures that any unconfigured network usage will be rejected.

## Rationale

With a default accept policy the firewall will accept any packet that is not configured to be denied. It is easier to allow list acceptable usage than to deny list unacceptable usage.

## Impact

- 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

## Audit Procedure

### Command Line

Run the following command and verify that the policy for the INPUT, OUTPUT, and FORWARD chains is DROP or REJECT:

```bash
ip6tables -L -n
```

-OR-

Verify IPv6 is disabled:
Run the following script. Output will confirm if IPv6 is enabled on the system.

```bash
#!/usr/bin/bash
{
  if grep -Pqs '^\h*0\b' /sys/module/ipv6/parameters/disable; then
    echo -e " - IPv6 is enabled on the system"
  else
    echo -e " - IPv6 is not enabled on the system"
  fi
}
```

## Expected Result

```
Chain INPUT (policy DROP)
Chain FORWARD (policy DROP)
Chain OUTPUT (policy DROP)
```

Or IPv6 is not enabled on the system.

## Remediation

### Command Line

IF IPv6 is enabled on your system:
Run the following commands to implement a default DROP policy:

```bash
ip6tables -P INPUT DROP
ip6tables -P OUTPUT DROP
ip6tables -P FORWARD DROP
```

## Default Value

accept

## 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

Automated