cis-ubuntu2004-v300-4-3-5

name	cis-ubuntu2004-v300-4-3-5
description	Ensure nftables base chains exist
category	cis-networking
version	3.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-20.04","firewall","nftables"]
cis_id	4.3.5
cis_benchmark	CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0 - Control 4.3.5

Profile

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

Description

Chains are containers for rules. They exist in two kinds, base chains and regular chains. A base chain is an entry point for packets from the networking stack, a regular chain may be used as jump target and is used for better rule organization.

Rationale

If a base chain doesn't exist with a hook for input, forward, and delete, packets that would flow through those chains will not be touched by nftables.

Impact

If configuring nftables over ssh, creating a base chain with a policy of drop will cause loss of connectivity.

Ensure that a rule allowing ssh has been added to the base chain prior to setting the base chain's policy to drop

Audit Procedure

Command Line

Run the following commands and verify that base chains exist for INPUT.

nft list ruleset | grep 'hook input'

type filter hook input priority 0;

Run the following commands and verify that base chains exist for FORWARD.

nft list ruleset | grep 'hook forward'

type filter hook forward priority 0;

Run the following commands and verify that base chains exist for OUTPUT.

nft list ruleset | grep 'hook output'

type filter hook output priority 0;

Expected Result

Base chains exist for input, forward, and output hooks.

Remediation

Command Line

Run the following command to create the base chains:

nft create chain inet <table name> <base chain name> { type filter hook <(input|forward|output)> priority 0 \; }

Example:

nft create chain inet filter input { type filter hook input priority 0 \; }
nft create chain inet filter forward { type filter hook forward priority 0 \; }
nft create chain inet filter output { type filter hook output priority 0 \; }