cis-ubuntu2004-v300-3-3-9

name	cis-ubuntu2004-v300-3-3-9
description	Ensure suspicious packets are logged
category	cis-networking
version	3.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-20.04","kernel-parameter"]
cis_id	3.3.9
cis_benchmark	CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

3.3.9 Ensure suspicious packets are logged (Automated)

Profile

• Level 1 - Server
• Level 1 - Workstation

Description

When enabled, this feature logs packets with un-routable source addresses to the kernel log.

Rationale

Setting net.ipv4.conf.all.log_martians and net.ipv4.conf.default.log_martians to 1 enables this feature. Logging these packets allows an administrator to investigate the possibility that an attacker is sending spoofed packets to their system.

Impact

None expected.

Audit Procedure

Command Line

Run the following script to verify the following kernel parameters are set in the running configuration and correctly loaded from a kernel parameter configuration file:

• net.ipv4.conf.all.log_martians is set to 1
• net.ipv4.conf.default.log_martians is set to 1

Note: kernel parameters are loaded by file and parameter order precedence. The following script observes this precedence as part of the auditing procedure. The parameters being checked may be set correctly in a file. If that file is superseded, the parameter is overridden by an incorrect setting later in that file, or in a canonically later file, that "correct" setting will be ignored both by the script and by the system during a normal kernel parameter load sequence.

Expected Result

Audit Result: ** PASS ** with both net.ipv4.conf.all.log_martians and net.ipv4.conf.default.log_martians set to 1.

Remediation

Command Line

Set the following parameters in /etc/sysctl.conf or a file in /etc/sysctl.d/ ending in .conf:

• net.ipv4.conf.all.log_martians = 1
• net.ipv4.conf.default.log_martians = 1

Example:

# printf '%s\n' "net.ipv4.conf.all.log_martians = 1" "net.ipv4.conf.default.log_martians = 1" >>
/etc/sysctl.d/60-netipv4_sysctl.conf

Run the following script to set the active kernel parameters:

#!/usr/bin/env bash

{
  sysctl -w net.ipv4.conf.all.log_martians=1
  sysctl -w net.ipv4.conf.default.log_martians=1
  sysctl -w net.ipv4.route.flush=1
}

Note: If these settings appear in a canonically later file, or later in the same file, these settings will be overwritten.