cis-ubuntu1804-v220-3-4-2-5

DevOps & Security

Ensure nftables base chains exist

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CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0 - Control 3.4.2.5

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 matching network traffic will be unaffected. Without base chains, no network traffic will be filtered.

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 to verify that base chains exist for INPUT, FORWARD, and OUTPUT:

nft list ruleset | grep 'hook input'
nft list ruleset | grep 'hook forward'
nft list ruleset | grep 'hook output'

Expected Result

type filter hook input priority 0;
type filter hook forward priority 0;
type filter hook output priority 0;

Remediation

Command Line

Run the following commands to create base chains:

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 \; }

Default Value

No base chains exist by default.

References

NIST SP 800-53 Rev. 5: CA-9, SC-7
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