# Cis GCP Cos 3.2.2

> Ensure ICMP redirects are not accepted

- Skill: `cyberstrikeus/cis-gcp-cos-3-2-2` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-gcp-cos-3-2-2`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-gcp-cos-3-2-2/raw
- Safety review: PASS (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: DevOps & Infra
- Author: cyberstrikeus (https://skillmd.com/u/cyberstrikeus)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/cyberstrikeus/cis-gcp-cos-3-2-2

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# 3.2.2 Ensure ICMP redirects are not accepted (Automated)

## Description

ICMP redirect messages are packets that convey routing information and tell your host (acting as a router) to send packets via an alternate path. It is a way of allowing an outside routing device to update your system routing tables. By setting `net.ipv4.conf.all.accept_redirects` and `net.ipv6.conf.all.accept_redirects` to 0, the system will not accept any ICMP redirect messages, and therefore, won't allow outsiders to update the system's routing tables.

## Rationale

Attackers could use bogus ICMP redirect messages to maliciously alter the system routing tables and get them to send packets to incorrect networks and allow your system packets to be captured.

## Audit Procedure

Run the following commands and verify output matches:

```bash
# sysctl net.ipv4.conf.all.accept_redirects
net.ipv4.conf.all.accept_redirects = 0
# sysctl net.ipv4.conf.default.accept_redirects
net.ipv4.conf.default.accept_redirects = 0
# sysctl net.ipv6.conf.all.accept_redirects
net.ipv6.conf.all.accept_redirects = 0
# sysctl net.ipv6.conf.default.accept_redirects
net.ipv6.conf.default.accept_redirects = 0
```

## Expected Result

All four sysctl commands should return a value of `0`, confirming that ICMP redirects are not accepted for both IPv4 and IPv6.

## Remediation

Run the following commands to set the active kernel parameters:

```bash
# sysctl -w net.ipv4.conf.all.accept_redirects=0
# sysctl -w net.ipv4.conf.default.accept_redirects=0
# sysctl -w net.ipv6.conf.all.accept_redirects=0
# sysctl -w net.ipv6.conf.default.accept_redirects=0
# sysctl -w net.ipv4.route.flush=1
# sysctl -w net.ipv6.route.flush=1
```

`/etc` is stateless on Container-Optimized OS. Therefore, `/etc` cannot be used to make these changes persistent across reboots. The steps mentioned above needs to be performed after every boot.

## CIS Controls

| Controls Version | Control                                                                                                                                                                                                                         | IG 1 | IG 2 | IG 3 |
| ---------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---- | ---- | ---- |
| v8               | **12.2 Establish and Maintain a Secure Network Architecture** - Establish and maintain a secure network architecture. A secure network architecture must address segmentation, least privilege, and availability, at a minimum. |      | X    | X    |
| v7               | **5.1 Establish Secure Configurations** - Maintain documented, standard security configuration standards for all authorized operating systems and software.                                                                     | X    | X    | X    |

## Profile

- Level 2 - Server