cis-ubuntu2004-v300-3-3-2

name	cis-ubuntu2004-v300-3-3-2
description	Ensure packet redirect sending is disabled
category	cis-networking
version	3.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-20.04","kernel-parameter"]
cis_id	3.3.2
cis_benchmark	CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

3.3.2 Ensure packet redirect sending is disabled (Automated)

Profile

• Level 1 - Server
• Level 1 - Workstation

Description

ICMP Redirects are used to send routing information to other hosts. As a host itself does not act as a router (in a host only configuration), there is no need to send redirects.

Rationale

An attacker could use a compromised host to send invalid ICMP redirects to other router devices in an attempt to corrupt routing and have users access a system set up by the attacker as opposed to a valid system.

Impact

IP forwarding is required on systems configured to act as a router. If these parameters are disabled, the system will not be able to perform as a router.

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.send_redirects is set to 0
• net.ipv4.conf.default.send_redirects is set to 0

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 If that file is superseded, the parameter is overridden by an incorrect setting later in that file, or in a canonically later file, that "correct" setting will be ignored both by the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.send_redirects and net.ipv4.conf.default.send_redirects set to 0.

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.send_redirects = 0
• net.ipv4.conf.default.send_redirects = 0

Example:

# printf '%s\n' "net.ipv4.conf.all.send_redirects = 0" "net.ipv4.conf.default.send_redirects = 0" >>
/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.send_redirects=0
  sysctl -w net.ipv4.conf.default.send_redirects=0
  sysctl -w net.ipv4.route.flush=1
}