cis-ubuntu1604-v200-3-2-2

DevOps & Security

Ensure IP forwarding is disabled

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Source SKILL.md: https://github.com/CyberStrikeus/CyberStrike/blob/HEAD/.cyberstrike/skill/CIS_benchmarks/Operating_Systems/Ubuntu/CIS_Ubuntu_Linux_16.04_LTS_Benchmark_v2.0.0/cis-ubuntu1604-v200-3-2-2/SKILL.md

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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" "$grubfile" | grep -vq ipv6.disable=1 && output="ipv6 disabled in grub config"
grep -Eqs "^\s*net\.ipv6\.conf\.all\.disable_ipv6\s*=\s*1\b" /etc/sysctl.conf /etc/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /run/sysctl.d/*.conf && grep -Eqs "^\s*net\.ipv6\.conf\.default\.disable_ipv6\s*=\s*1\b" /etc/sysctl.conf /etc/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /run/sysctl.d/*.conf && sysctl net.ipv6.conf.all.disable_ipv6 | grep -Eq "^\s*net\.ipv6\.conf\.all\.disable_ipv6\s*=\s*1\b" && sysctl net.ipv6.conf.default.disable_ipv6 | grep -Eq "^\s*net\.ipv6\.conf\.default\.disable_ipv6\s*=\s*1\b" && output="ipv6 disabled in sysctl config"
[ -n "$output" ] && echo -e "\n$output" || echo -e "\n*** IPv6 is enabled on the system ***"

Expected Result

net.ipv4.ip_forward = 0

No value returned from the grep command.

For IPv6 (if enabled):

net.ipv6.conf.all.forwarding = 0

No value returned from the grep command.

Remediation

Command Line

Run the following command to restore the default parameter and set the active kernel parameter:

grep -Els "^\s*net\.ipv4\.ip_forward\s*=\s*1" /etc/sysctl.conf /etc/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /run/sysctl.d/*.conf | while read filename; do sed -ri "s/^\s*(net\.ipv4\.ip_forward\s*)(=)(\s*\S+\b).*$/# *REMOVED* \1/" $filename; done; sysctl -w net.ipv4.ip_forward=0; sysctl -w net.ipv4.route.flush=1

IF IPv6 is enabled:

Run the following command to restore the default parameter and set the active kernel parameter:

grep -Els "^\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 | while read filename; do sed -ri "s/^\s*(net\.ipv6\.conf\.all\.forwarding\s*)(=)(\s*\S+\b).*$/# *REMOVED* \1/" $filename; done; sysctl -w net.ipv6.conf.all.forwarding=0; sysctl -w net.ipv6.route.flush=1

Default Value

IP forwarding is disabled (set to 0) by default.

References

CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0

CIS Controls

Version 7

5.1 Establish Secure Configurations - Maintain documented, standard security configuration standards for all authorized operating systems and software.

Profile Applicability

Level 1 - Server
Level 1 - Workstation

Assessment Status

Automated