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cis-ubuntu1604-v200-3-1-1

DevOps & Security
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Disable IPv6

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Source SKILL.md: https://github.com/CyberStrikeus/CyberStrike/blob/HEAD/.cyberstrike/skill/CIS_benchmarks/Operating_Systems/Ubuntu/CIS_Ubuntu_Linux_16.04_LTS_Benchmark_v2.0.0/cis-ubuntu1604-v200-3-1-1/SKILL.md

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CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 3.1.1

Description

Although IPv6 has many advantages over IPv4, not all organizations have IPv6 or dual stack configurations implemented.

Rationale

If IPv6 or dual stack is not to be used, it is recommended that IPv6 be disabled to reduce the attack surface of the system.

Impact

If IPv6 is disabled through sysctl config, SSH X11forwarding may no longer function as expected. We recommend that SSH X11fowarding be disabled, but if required, the following will allow for SSH X11forwarding with IPv6 disabled through sysctl config:

Add the following line the /etc/ssh/sshd_config file:

AddressFamily inet

Run the following command to re-start the openSSH server:

systemctl restart sshd

Audit Procedure

Command Line

Run one of the following commands to verify IPv6 is disabled:

IF IPv6 is disabled through grub:

Run the following command:

grep "^\s*linux" /boot/grub/grub.cfg | grep -v "ipv6.disable=1"

No lines should be returned.

OR

IF IPv6 is disabled through sysctl settings:

Run the following commands:

sysctl net.ipv6.conf.all.disable_ipv6

sysctl net.ipv6.conf.default.disable_ipv6

grep -E '^\s*net\.ipv6\.conf\.(all|default)\.disable_ipv6\s*=\s*1\b(\s+#.*)?
#x27; /etc/sysctl.conf /etc/sysctl.d/*.conf | cut -d: -f2

Expected Result

For grub method: no lines should be returned.

For sysctl method:

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

And the grep should return:

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

Remediation

Command Line

Use one of the two following methods to disable IPv6 on the system:

To disable IPv6 through the GRUB2 config:

Edit /etc/default/grub and add ipv6.disable=1 to the GRUB_CMDLINE_LINUX parameters:

GRUB_CMDLINE_LINUX="ipv6.disable=1"

Run the following command to update the grub2 configuration:

update-grub

OR

To disable IPv6 through sysctl settings:

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

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

Run the following commands to set the active kernel parameters:

sysctl -w net.ipv6.conf.all.disable_ipv6=1
sysctl -w net.ipv6.conf.default.disable_ipv6=1
sysctl -w net.ipv6.route.flush=1

Default Value

IPv6 is enabled by default.

References

  1. CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0

CIS Controls

Version 7

9.4 Apply Host-based Firewalls or Port Filtering - Apply host-based firewalls or port filtering tools on end systems, with a default-deny rule that drops all traffic except those services and ports that are explicitly allowed.

Profile Applicability

  • Level 2 - Server
  • Level 2 - Workstation

Assessment Status

Manual