cis-ubuntu1204-v110-7-2-5

name	cis-ubuntu1204-v110-7-2-5
description	Enable Ignore Broadcast Requests
category	cis-os-hardening
version	1.1.0
author	cyberstrike-official
tags	["cis","ubuntu",12.04,"linux","networking","icmp","broadcast","smurf","sysctl","host-router"]
cis_id	7.2.5
cis_benchmark	CIS Ubuntu 12.04 LTS Server Benchmark v1.1.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

7.2.5 Enable Ignore Broadcast Requests (Scored)

Profile Applicability

• Level 1

Description

Setting net.ipv4.icmp_echo_ignore_broadcasts to 1 will cause the system to ignore all ICMP echo and timestamp requests to broadcast and multicast addresses.

Rationale

Accepting ICMP echo and timestamp requests with broadcast or multicast destinations for your network could be used to trick your host into starting (or participating) in a Smurf attack. A Smurf attack relies on an attacker sending large amounts of ICMP broadcast messages with a spoofed source address. All hosts receiving this message and responding would send echo-reply messages back to the spoofed address, which is probably not routable. If many hosts respond to the packets, the amount of traffic on the network could be significantly multiplied.

Audit Procedure

Using Command Line

Perform the following to determine if all ICMP echo and timestamp requests to broadcast and multicast addresses will be ignored.

/sbin/sysctl net.ipv4.icmp_echo_ignore_broadcasts

Expected Result

net.ipv4.icmp_echo_ignore_broadcasts = 1

Remediation

Using Command Line

Set the net.ipv4.icmp_echo_ignore_broadcasts parameter to 1 in /etc/sysctl.conf:

net.ipv4.icmp_echo_ignore_broadcasts=1

Modify active kernel parameters to match:

/sbin/sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=1
/sbin/sysctl -w net.ipv4.route.flush=1

Default Value

Broadcast ICMP requests are accepted by default.

References

• CIS Ubuntu 12.04 LTS Server Benchmark v1.1.0

Profile

Level 1 - Scored