cis-ubuntu1804-v220-3-3-1

DevOps & Security

Ensure ip forwarding is disabled

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Source SKILL.md: https://github.com/CyberStrikeus/CyberStrike/blob/HEAD/.cyberstrike/skill/CIS_benchmarks/Operating_Systems/Ubuntu/CIS_Ubuntu_Linux_18.04_LTS_Benchmark_v2.2.0/cis-ubuntu1804-v220-3-3-1/SKILL.md

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CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0 - Control 3.3.1

Description

The net.ipv4.ip_forward and net.ipv6.conf.all.forwarding flags are used to tell the system whether it can forward packets or not.

Rationale

Setting the flags to 0 ensures that a system with multiple interfaces (for example, a hard proxy), will never be able to forward packets, and therefore, never serve as a router.

Impact

None.

Audit Procedure

Command Line

Run the following script to verify IP forwarding is disabled:

#!/usr/bin/bash
{
  l_output="" l_output2=""
  a_parlist=("net.ipv4.ip_forward=0" "net.ipv6.conf.all.forwarding=0")
  l_ufwscf="$([ -f /etc/default/ufw ] && awk -F= '/^\s*IPT_SYSCTL=/ {print $2}' /etc/default/ufw)"

  kernel_parameter_chk() {
    l_krp="$(sysctl "$l_kpname" | awk -F= '{print $2}' | xargs)"
    if [ "$l_krp" = "$l_kpvalue" ]; then
      l_output="$l_output\n - \"$l_kpname\" is correctly set to \"$l_krp\" in the running configuration"
    else
      l_output2="$l_output2\n - \"$l_kpname\" is incorrectly set to \"$l_krp\" in the running configuration and should have a value of: \"$l_kpvalue\""
    fi
  }

  for l_kpe in "${a_parlist[@]}"; do
    l_kpname="$(awk -F= '{print $1}' <<< "$l_kpe")"
    l_kpvalue="$(awk -F= '{print $2}' <<< "$l_kpe")"
    kernel_parameter_chk
  done

  if [ -z "$l_output2" ]; then
    echo -e "\n- Audit Result:\n ** PASS **\n$l_output\n"
  else
    echo -e "\n- Audit Result:\n ** FAIL **\n - Reason(s) for audit failure:\n$l_output2\n"
    [ -n "$l_output" ] && echo -e "\n- Correctly set:\n$l_output\n"
  fi
}

Expected Result

- Audit Result:
 ** PASS **
 - "net.ipv4.ip_forward" is correctly set to "0" in the running configuration
 - "net.ipv6.conf.all.forwarding" is correctly set to "0" in the running configuration

Remediation

Command Line

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

net.ipv4.ip_forward = 0
net.ipv6.conf.all.forwarding = 0

Run the following commands to set the active kernel parameters:

sysctl -w net.ipv4.ip_forward=0
sysctl -w net.ipv6.conf.all.forwarding=0
sysctl -w net.ipv4.route.flush=1
sysctl -w net.ipv6.route.flush=1

Default Value

net.ipv4.ip_forward = 0, net.ipv6.conf.all.forwarding = 0

References

NIST SP 800-53 Rev. 5: CM-7, SC-5, SC-7
CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0

CIS Controls

Version 8

4.1 Establish and Maintain a Secure Configuration Process - Establish and maintain a secure configuration process for enterprise assets.

Version 7

5.1 Establish Secure Configurations - Maintain documented, standard security configuration standards for all authorized operating systems and software.

Profile Applicability

Level 1 - Server
Level 1 - Workstation

Assessment Status

Automated