# Cis GCP Cos 3.2.1

> Ensure source routed packets are not accepted

- Skill: `cyberstrikeus/cis-gcp-cos-3-2-1` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-gcp-cos-3-2-1`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-gcp-cos-3-2-1/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-3-2-1

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# 3.2.1 Ensure source routed packets are not accepted (Automated)

## Description

In networking, source routing allows a sender to partially or fully specify the route packets take through a network. In contrast, non-source routed packets travel a path determined by routers in the network. In some cases, systems may not be routable or reachable from some locations (e.g. private addresses vs. Internet routable), and so source routed packets would need to be used.

## Rationale

Setting `net.ipv4.conf.all.accept_source_route`, `net.ipv4.conf.default.accept_source_route`, `net.ipv6.conf.all.accept_source_route` and `net.ipv6.conf.default.accept_source_route` to 0 disables the system from accepting source routed packets. Assume this system was capable of routing packets to Internet routable addresses on one interface and private addresses on another interface. Assume that the private addresses were not routable to the Internet routable addresses and vice versa. Under normal routing circumstances, an attacker from the Internet routable addresses could not use the system as a way to reach the private address systems. If, however, source routed packets were allowed, they could be used to gain access to the private address systems as the route could be specified, rather than rely on routing protocols that did not allow this routing.

## Audit Procedure

Run the following commands and verify output matches:

```bash
# sysctl net.ipv4.conf.all.accept_source_route

net.ipv4.conf.all.accept_source_route = 0
# sysctl net.ipv4.conf.default.accept_source_route

net.ipv4.conf.default.accept_source_route = 0
# sysctl net.ipv6.conf.all.accept_source_route

net.ipv6.conf.all.accept_source_route = 0
# sysctl net.ipv6.conf.default.accept_source_route

net.ipv6.conf.default.accept_source_route = 0
```

## Expected Result

All four sysctl commands should return a value of `0`, confirming that source routed packets 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_source_route=0
# sysctl -w net.ipv4.conf.default.accept_source_route=0
# sysctl -w net.ipv6.conf.all.accept_source_route=0
# sysctl -w net.ipv6.conf.default.accept_source_route=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