cis-ubuntu2004-v300-3-3-3

DevOps & Security

Ensure bogus icmp responses are ignored

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Source SKILL.md: https://github.com/CyberStrikeus/CyberStrike/blob/HEAD/.cyberstrike/skill/CIS_benchmarks/Operating_Systems/Ubuntu/CIS_Ubuntu_Linux_20.04_LTS_Benchmark_v3.0.0/cis-ubuntu2004-v300-3-3-3/SKILL.md

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3.3.3 Ensure bogus icmp responses are ignored (Automated)

Profile

Level 1 - Server
Level 1 - Workstation

Description

Setting net.ipv4.icmp_ignore_bogus_error_responses to 1 prevents the kernel from logging bogus responses (RFC-1122 non-compliant) from broadcast reframes, keeping file systems from filling up with useless log messages.

Rationale

Some routers (and some attackers) will send responses that violate RFC-1122 and attempt to fill up a log file system with many useless error messages.

Impact

None expected.

Audit Procedure

Command Line

Run the following script to verify the following kernel parameter is set in the running configuration and correctly loaded from a kernel parameter configuration file:

net.ipv4.icmp_ignore_bogus_error_responses is set to 1

Note: kernel parameters are loaded by file and parameter order precedence. The following script observes this precedence as part of the auditing procedure. The parameters being checked may be set correctly in a file. If that file is superseded, the parameter is overridden by an incorrect setting later in that file, or in a canonically later file, that "correct" setting will be ignored both by the script and by the system during a normal kernel parameter load sequence.

Expected Result

Audit Result: ** PASS ** with net.ipv4.icmp_ignore_bogus_error_responses set to 1.

Remediation

Command Line

Set the following parameter in /etc/sysctl.conf or a file in /etc/sysctl.d/ ending in .conf:

net.ipv4.icmp_ignore_bogus_error_responses = 1

Example:

# printf '%s\n' "net.ipv4.icmp_ignore_bogus_error_responses = 1" >> /etc/sysctl.d/60-netipv4_sysctl.conf

Run the following script to set the active kernel parameters:

#!/usr/bin/env bash

{
  sysctl -w net.ipv4.icmp_ignore_bogus_error_responses=1
  sysctl -w net.ipv4.route.flush=1
}

Note: If these settings appear in a canonically later file, or later in the same file, these settings will be overwritten.

Default Value

net.ipv4.icmp_ignore_bogus_error_responses = 1

References

NIST SP 800-53 Rev. 5: CM-1, CM-2, CM-6, CM-7, IA-5

CIS Controls

Controls Version	Control	IG 1	IG 2	IG 3
v8	4.8 Uninstall or Disable Unnecessary Services on Enterprise Assets and Software		x	x
v7	9.2 Ensure Only Approved Ports, Protocols and Services Are Running		x	x