cis-ubuntu1604-v200-3-2-2

name	cis-ubuntu1604-v200-3-2-2
description	Ensure IP forwarding is disabled
category	cis-networking
version	2.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-16.04","networking"]
cis_id	3.2.2
cis_benchmark	CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 3.2.2

Description

The net.ipv4.ip_forward and net.ipv6.conf.all.forwarding flags are used to tell the system whether it can forward packets or not.

Rationale

Setting the flags to 0 ensures that a system with multiple interfaces (for example, a hard proxy), will never be able to forward packets, and therefore, never serve as a router.

Impact

None.

Audit Procedure

Command Line

Run the following commands and verify output matches:

sysctl net.ipv4.ip_forward

grep -E -s "^\s*net\.ipv4\.ip_forward\s*=\s*1" /etc/sysctl.conf /etc/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /run/sysctl.d/*.conf

No value should be returned.

IF IPv6 is enabled:

Run the following commands and verify output matches:

sysctl net.ipv6.conf.all.forwarding

grep -E -s "^\s*net\.ipv6\.conf\.all\.forwarding\s*=\s*1" /etc/sysctl.conf /etc/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /run/sysctl.d/*.conf

No value should be returned.

OR verify IPv6 is disabled:

Run the following script. Output will confirm if IPv6 is disabled on the system.

#!/usr/bin/bash
output=""
grubfile="$(find -L /boot -name 'grub.cfg' -type f)"
[ -f "$grubfile" ] && ! grep "^\s*linux" "$grubfile" | grep -vq ipv6.disable=1 && output="ipv6 disabled in grub config"
grep -Eqs "^\s*net\.ipv6\.conf\.all\.disable_ipv6\s*=\s*1\b" /etc/sysctl.conf /etc/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /run/sysctl.d/*.conf && grep -Eqs "^\s*net\.ipv6\.conf\.default\.disable_ipv6\s*=\s*1\b" /etc/sysctl.conf /etc/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /run/sysctl.d/*.conf && sysctl net.ipv6.conf.all.disable_ipv6 | grep -Eq "^\s*net\.ipv6\.conf\.all\.disable_ipv6\s*=\s*1\b" && sysctl net.ipv6.conf.default.disable_ipv6 | grep -Eq "^\s*net\.ipv6\.conf\.default\.disable_ipv6\s*=\s*1\b" && output=
[ -n  ] &&  -e  ||  -e