# Cis Ubuntu1604 V200 3 5 3 3 2

> Ensure ip6tables loopback traffic is configured

- Skill: `cyberstrikeus/cis-ubuntu1604-v200-3-5-3-3-2` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu1604-v200-3-5-3-3-2`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-ubuntu1604-v200-3-5-3-3-2/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-ubuntu1604-v200-3-5-3-3-2

---


# CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 3.5.3.3.2

## Profile

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

## Description

Configure the loopback interface to accept traffic. Configure all other interfaces to deny traffic to the loopback network (::1).

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

Loopback traffic is generated between processes on machine and is typically critical to operation of the system. The loopback interface is the only place that loopback network (::1) traffic should be seen, all other interfaces should ignore traffic on this network as an anti-spoofing measure.

## Audit Procedure

### Command Line

Run the following commands and verify output includes the listed rules in order (packet and byte counts may differ):

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

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

OR verify IPv6 is disabled. Run the following script. Output will confirm if IPv6 is disabled on the system.

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

output=""
grubfile="$(find -L /boot -name 'grub.cfg' -type f)"
[ -f "$grubfile" ] && ! grep "^\s*linux" "$grubfile" | grep -vq ipv6.disable=1 && output="ipv6 disabled in grub config"
grep -Eqs "^\s*net\.ipv6\.conf\.all\.disable_ipv6\s*=\s*1\b" /etc/sysctl.conf /etc/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /run/sysctl.d/*.conf && grep -Eqs "^\s*net\.ipv6\.conf\.default\.disable_ipv6\s*=\s*1\b" /etc/sysctl.conf /etc/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /run/sysctl.d/*.conf && sysctl net.ipv6.conf.all.disable_ipv6 | grep -Eq "^\s*net\.ipv6\.conf\.all\.disable_ipv6\s*=\s*1\b" && sysctl net.ipv6.conf.default.disable_ipv6 | grep -Eq "^\s*net\.ipv6\.conf\.default\.disable_ipv6\s*=\s*1\b" && output="ipv6 disabled in sysctl config"
[ -n "$output" ] && echo -e "\n$output" || echo -e "\n*** IPv6 is enabled on the system ***"
```

## Expected Result

```
Chain INPUT (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all      lo     *       ::/0                 ::/0
    0     0 DROP       all      *      *       ::1                  ::/0
```

```
Chain OUTPUT (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all      *      lo      ::/0                 ::/0
```

## Remediation

### Command Line

Run the following commands to implement the loopback rules:

```bash
ip6tables -A INPUT -i lo -j ACCEPT
ip6tables -A OUTPUT -o lo -j ACCEPT
ip6tables -A INPUT -s ::1 -j DROP
```

## References

None

## CIS Controls

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.