# Cis Ubuntu1604 V200 3 3 1

> Ensure source routed packets are not accepted

- Skill: `cyberstrikeus/cis-ubuntu1604-v200-3-3-1` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu1604-v200-3-3-1`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-ubuntu1604-v200-3-3-1/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-ubuntu1604-v200-3-3-1

---


# CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 3.3.1

## 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.

## Impact

None.

## Audit Procedure

### Command Line

Run the following commands and verify output matches:

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

```bash
sysctl net.ipv4.conf.default.accept_source_route
```

```bash
grep "net\.ipv4\.conf\.all\.accept_source_route" /etc/sysctl.conf /etc/sysctl.d/*
```

```bash
grep "net\.ipv4\.conf\.default\.accept_source_route" /etc/sysctl.conf /etc/sysctl.d/*
```

_IF IPv6 is enabled:_

Run the following commands and verify output matches:

```bash
sysctl net.ipv6.conf.all.accept_source_route
```

```bash
sysctl net.ipv6.conf.default.accept_source_route
```

```bash
grep "net\.ipv6\.conf\.all\.accept_source_route" /etc/sysctl.conf /etc/sysctl.d/*
```

```bash
grep "net\.ipv6\.conf\.default\.accept_source_route" /etc/sysctl.conf /etc/sysctl.d/*
```

_OR verify IPv6 is disabled._

## Expected Result

```
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
```

And the grep commands should return:

```
net.ipv4.conf.all.accept_source_route= 0
net.ipv4.conf.default.accept_source_route= 0
```

If IPv6 is enabled:

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

And the grep commands should return:

```
net.ipv4.conf.all.accept_source_route= 0
net.ipv6.conf.default.accept_source_route= 0
```

## Remediation

### Command Line

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

```
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
```

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.ipv4.route.flush=1
```

_IF IPv6 is enabled:_

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

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

Run the following commands to set the active kernel parameters:

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

## Default Value

accept_source_route is enabled (set to 1) by default.

## References

1. CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0

## CIS Controls

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