cis-gcp-cos-3-2-7

name	cis-gcp-cos-3.2.7
description	Ensure Reverse Path Filtering is enabled
category	cis-gcp-cos
version	1.2.0
author	cyberstrike-official
tags	["cis","gcp","container-optimized-os","network","sysctl","routing"]
cis_id	3.2.7
cis_benchmark	CIS Google Container-Optimized OS Benchmark v1.2.0
tech_stack	["gcp","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

3.2.7 Ensure Reverse Path Filtering is enabled (Automated)

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.

Audit Procedure

Run the following commands and verify output matches:

# sysctl net.ipv4.conf.all.rp_filter
net.ipv4.conf.all.rp_filter = 1
# sysctl net.ipv4.conf.default.rp_filter
net.ipv4.conf.default.rp_filter = 1
# grep "net\.ipv4\.conf\.all\.rp_filter" /etc/sysctl.conf /etc/sysctl.d/*
net.ipv4.conf.all.rp_filter = 1
# grep "net\.ipv4\.conf\.default\.rp_filter" /etc/sysctl.conf /etc/sysctl.d/*
net.ipv4.conf.default.rp_filter = 1

Expected Result

All sysctl commands should return a value of 1 and grep commands should confirm the settings are persisted in the sysctl configuration files.

Remediation

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

/etc is stateless on Container-Optimized OS. Therefore, /etc cannot be used to make these changes persistent across reboots. The steps mentioned above needs to be performed after every boot.

CIS Controls