# Cis Ubuntu1804 V220 3 3 1

> Ensure ip forwarding is disabled

- Skill: `cyberstrikeus/cis-ubuntu1804-v220-3-3-1` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu1804-v220-3-3-1`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-ubuntu1804-v220-3-3-1/raw
- Safety review: PASS (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: cyberstrikeus (https://skillmd.com/u/cyberstrikeus)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/cyberstrikeus/cis-ubuntu1804-v220-3-3-1

---


# 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:

```bash
#!/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:

```bash
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

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