# Cis Ubuntu1604 V200 3 5 1 4

> Ensure ufw loopback traffic is configured

- Skill: `cyberstrikeus/cis-ubuntu1604-v200-3-5-1-4` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu1604-v200-3-5-1-4`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-ubuntu1604-v200-3-5-1-4/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-ubuntu1604-v200-3-5-1-4

---


# CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 3.5.1.4

## 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 (127.0.0.0/8 for IPv4 and ::1/128 for IPv6).

## 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 (127.0.0.0/8 for IPv4 and ::1/128 for IPv6) 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:

```bash
ufw status verbose
```

## Expected Result

```
To                         Action      From
--                         ------      ----
Anywhere on lo             ALLOW IN    Anywhere
Anywhere                   DENY IN     127.0.0.0/8
Anywhere (v6) on lo        ALLOW IN    Anywhere (v6)
Anywhere (v6)              DENY IN     ::1

Anywhere                   ALLOW OUT   Anywhere on lo
Anywhere (v6)              ALLOW OUT   Anywhere (v6) on lo
```

## Remediation

### Command Line

Run the following commands to implement the loopback rules:

```bash
ufw allow in on lo
ufw allow out on lo
ufw deny in from 127.0.0.0/8
ufw deny in from ::1
```

## 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.