# Cis Ubuntu2004 V300 4 4 3 3

> Ensure ip6tables outbound and established connections are configured

- Skill: `cyberstrikeus/cis-ubuntu2004-v300-4-4-3-3` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu2004-v300-4-4-3-3`
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- Safety review: PASS (external: skill-scanner PASS, skillspector CAUTION)
- 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-v300-4-4-3-3

---


# CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0 - Control 4.4.3.3

## Profile

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

## Description

Configure the firewall rules for new outbound, and established IPv6 connections.

Note:

- 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

## Rationale

If rules are not in place for new outbound, and established connections all packets will be dropped by the default policy preventing network usage.

## Audit Procedure

### Command Line

Run the following command and verify all rules for new outbound, and established connections match site policy:

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

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

```bash
#!/usr/bin/env bash

{
  l_ipv6_enabled="is"
  ! grep -Pqs -- '^\h*0\b' /sys/module/ipv6/parameters/disable && l_ipv6_enabled="is not"
  if sysctl net.ipv6.conf.all.disable_ipv6 | grep -Pqs -- "^\h*net\.ipv6\.conf\.all\.disable_ipv6\h*=\h*1\b" \
  && \
    sysctl net.ipv6.conf.default.disable_ipv6 | grep -Pqs -- "^\h*net\.ipv6\.conf\.default\.disable_ipv6\h*=\h*1\b"; then
      l_ipv6_enabled="is not"
  fi
  echo -e " - IPv6 $l_ipv6_enabled enabled on the system"
}
```

## Expected Result

Output should show rules matching site policy for outbound and established IPv6 connections. Or IPv6 is disabled.

## Remediation

### Command Line

Configure iptables in accordance with site policy. The following commands will implement a policy to allow all outbound connections and all established connections:

```bash
ip6tables -A OUTPUT -p tcp -m state --state NEW,ESTABLISHED -j ACCEPT
ip6tables -A OUTPUT -p udp -m state --state NEW,ESTABLISHED -j ACCEPT
ip6tables -A INPUT -p tcp -m state --state ESTABLISHED -j ACCEPT
ip6tables -A INPUT -p udp -m state --state ESTABLISHED -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