# Cis Ubuntu1804 V220 3 4 2 6

> Ensure nftables loopback traffic is configured

- Skill: `cyberstrikeus/cis-ubuntu1804-v220-3-4-2-6` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu1804-v220-3-4-2-6`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-ubuntu1804-v220-3-4-2-6/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-v220-3-4-2-6

---


# CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0 - Control 3.4.2.6

## Description

Configure the loopback interface to accept traffic. Configure all other interfaces to deny traffic to the loopback network.

## 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 traffic should be seen, all other interfaces should ignore traffic on this network as an anti-spoofing measure.

## Impact

None.

## Audit Procedure

### Command Line

Run the following commands to verify that the loopback interface is configured:

Run the following command to verify the loopback interface is configured to accept network traffic:

```bash
nft list ruleset | awk '/hook input/,/}/' | grep 'iif "lo" accept'
```

Example output:

```
iif "lo" accept
```

Run the following command to verify network traffic from an IPv4 loopback interface is configured to drop:

```bash
nft list ruleset | awk '/hook input/,/}/' | grep 'ip saddr'
```

Example output:

```
ip saddr 127.0.0.0/8 counter packets 0 bytes 0 drop
```

-IF- IPv6 is enabled on the system:
Run the following command to verify network traffic from an IPv6 loopback interface is configured to drop:

```bash
nft list ruleset | awk '/hook input/,/}/' | grep 'ip6 saddr'
```

Example output:

```
ip6 saddr ::1 counter packets 0 bytes 0 drop
```

## Expected Result

The loopback interface should accept traffic, and traffic from 127.0.0.0/8 (and ::1 for IPv6) should be dropped.

## Remediation

### Command Line

Run the following commands to implement the loopback rules:

```bash
nft add rule inet filter input iif lo accept
nft add rule inet filter input ip saddr 127.0.0.0/8 counter drop
```

-IF- IPv6 is enabled on the system:
Run the following command to implement the IPv6 loopback rule:

```bash
nft add rule inet filter input ip6 saddr ::1 counter drop
```

## Default Value

No loopback rules are configured by default.

## 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