IT Best Practices Tip: nist_V-38511

Severity: medium

Short Description

IP forwarding for IPv4 must not be enabled, unless the system is a router.

Long Description

IP forwarding permits the kernel to forward packets from one network interface to another. The ability to forward packets between two networks is only appropriate for systems acting as routers.

How to Check Correct Configuration

The status of the "net.ipv4.ip_forward" kernel parameter can be queried by running the following command:

$ sysctl net.ipv4.ip_forward

The output of the command should indicate a value of "0". If this value is not the default value, investigate how it could have been adjusted at runtime, and verify it is not set improperly in "/etc/sysctl.conf".

$ grep net.ipv4.ip_forward /etc/sysctl.conf

The ability to forward packets is only appropriate for routers. If the correct value is not returned, this is a finding.

How to Fix

To set the runtime status of the "net.ipv4.ip_forward" kernel parameter, run the following command:

# sysctl -w net.ipv4.ip_forward=0

If this is not the system's default value, add the following line to "/etc/sysctl.conf":

net.ipv4.ip_forward = 0