cis-ubuntu1604-v200-3-3-7

name	cis-ubuntu1604-v200-3-3-7
description	Ensure Reverse Path Filtering is enabled
category	cis-networking
version	2.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-16.04","networking"]
cis_id	3.3.7
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 3.3.7

Description

Setting net.ipv4.conf.all.rp_filter and net.ipv4.conf.default.rp_filter to 1 forces the Linux kernel to utilize reverse path filtering on a received packet to determine if the packet was valid. Essentially, with reverse path filtering, if the return packet does not go out the same interface that the corresponding source packet came from, the packet is dropped (and logged if log_martians is set).

Rationale

Setting these flags is a good way to deter attackers from sending your system bogus packets that cannot be responded to. One instance where this feature breaks down is if asymmetrical routing is employed. This would occur when using dynamic routing protocols (bgp, ospf, etc) on your system. If you are using asymmetrical routing on your system, you will not be able to enable this feature without breaking the routing.

Impact

None.

Audit Procedure

Command Line

Run the following commands and verify output matches:

sysctl net.ipv4.conf.all.rp_filter

sysctl net.ipv4.conf.default.rp_filter

grep "net\.ipv4\.conf\.all\.rp_filter" /etc/sysctl.conf /etc/sysctl.d/*

grep "net\.ipv4\.conf\.default\.rp_filter" /etc/sysctl.conf /etc/sysctl.d/*

Expected Result

net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1

And the grep commands should return:

net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1

Remediation

Command Line

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

net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1

Run the following commands to set the active kernel parameters:

sysctl -w net.ipv4.conf.all.rp_filter=1
sysctl -w net.ipv4.conf.default.rp_filter=1
sysctl -w net.ipv4.route.flush=1

Default Value

rp_filter is disabled (set to 0) by default.

References

1. 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.