Cis Ubuntu1804 V220 3 3 7

---
name: cis-ubuntu1804-v220-3-3-7
description: "Ensure reverse path filtering is enabled"
category: cis-networking
version: "2.2.0"
author: cyberstrike-official
tags: [cis, ubuntu, linux, ubuntu-18.04, networking, kernel-parameter]
cis_id: "3.3.7"
cis_benchmark: "CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0"
tech_stack: [ubuntu, linux]
cwe_ids: []
chains_with: []
prerequisites: []
severity_boost: {}
---

# CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0 - Control 3.3.7

## Description

Setting `net.ipv4.conf.all.rp_filter` and `net.ipv4.conf.default.rp_filter` to 1 forces the Linux kernel to utilize reverse path filtering on a received packet to determine if the packet was valid. Essentially, with reverse path filtering, if the return packet does not go out the same interface that the corresponding source packet came from, the packet is dropped (and logged if log_martians is enabled).

## Rationale

Setting these flags is a good way to deter attackers from sending your system bogus packets that cannot be responded to. One instance where this feature breaks legitimate applications is if asymmetrical routing is employed. This would occur when using dynamic routing protocols (bgp, ospf, etc) on your system. If you are using asymmetric routing on your system, you will not be able to enable this feature without breaking the routing.

## Impact

None.

## Audit Procedure

### Command Line

Run the following commands to verify reverse path filtering is enabled:

```bash
sysctl net.ipv4.conf.all.rp_filter
sysctl net.ipv4.conf.default.rp_filter
```

```bash
grep -E '^\s*net\.ipv4\.conf\.(all|default)\.rp_filter\s*=\s*1\b' /etc/sysctl.conf /etc/sysctl.d/*.conf
```

## Expected Result

```
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1
```

## Remediation

### Command Line

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

```
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1
```

Run the following commands to set the active kernel parameters:

```bash
sysctl -w net.ipv4.conf.all.rp_filter=1
sysctl -w net.ipv4.conf.default.rp_filter=1
sysctl -w net.ipv4.route.flush=1
```

## Default Value

net.ipv4.conf.all.rp_filter = 0, net.ipv4.conf.default.rp_filter = 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