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CIS Ubuntu 14.04 LTS - 3.1.2 Ensure packet redirect sending is disabled

DevOps & Security
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Verify that ICMP packet redirect sending is disabled to prevent routing information disclosure

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Source SKILL.md: https://github.com/CyberStrikeus/CyberStrike/blob/HEAD/.cyberstrike/skill/CIS_benchmarks/Operating_Systems/Ubuntu/CIS_Ubuntu_Linux_14.04_LTS_Benchmark_v2.1.0/cis-ubuntu1404-v210-3-1-2/SKILL.md

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3.1.2 Ensure packet redirect sending is disabled (Scored)

Profile Applicability

  • Level 1 - Server
  • Level 1 - Workstation

Description

ICMP Redirects are used to send routing information to other hosts. As a host itself does not act as a router (in a host only configuration), there is no need to send redirects.

Rationale

An attacker could use a compromised host to send invalid ICMP redirects to other router devices in an attempt to corrupt routing and have users access a system set up by the attacker as opposed to a valid system.

Audit Procedure

Run the following commands and verify output matches:

sysctl net.ipv4.conf.all.send_redirects
# Expected: net.ipv4.conf.all.send_redirects = 0

sysctl net.ipv4.conf.default.send_redirects
# Expected: net.ipv4.conf.default.send_redirects = 0

grep "net\.ipv4\.conf\.all\.send_redirects" /etc/sysctl.conf /etc/sysctl.d/*
# Expected: net.ipv4.conf.all.send_redirects = 0

grep "net\.ipv4\.conf\.default\.send_redirects" /etc/sysctl.conf /etc/sysctl.d/*
# Expected: net.ipv4.conf.default.send_redirects= 0

Expected Result

net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0

Remediation

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

net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0

Run the following commands to set the active kernel parameters:

sysctl -w net.ipv4.conf.all.send_redirects=0
sysctl -w net.ipv4.conf.default.send_redirects=0
sysctl -w net.ipv4.route.flush=1

Default Value

Not specified. Note: It has been reported that due to Ubuntu bug #50093 this setting (and some others) can fail to apply properly on reboot requiring it to be manually re-applied. One method of accomplishing this is to add sysctl -p to run on reboot to your systems crontab.

References

  • CIS Controls: 3 - Secure Configurations for Hardware and Software on Mobile Devices, Laptops, Workstations, and Servers
  • CIS Controls: 11 - Secure Configurations for Network Devices such as Firewalls, Routers and switches

Profile

  • Level 1 - Server
  • Level 1 - Workstation