Vulnerability Discussion

Routing protocol daemons are typically used on routers to exchange network topology information with other routers. If this capability is used when not required, system network information may be unnecessarily transmitted across the network.

The sysctl --system command will load settings from all system configuration files. All configuration files are sorted by their filename in lexicographical order, regardless of the directories in which they reside. If multiple files specify the same option, the entry in the file with the lexicographically latest name will take precedence. Files are read from directories in the following list from top to bottom. Once a file of a given filename is loaded, any file of the same name in subsequent directories is ignored.

/etc/sysctl.d/*.conf
/run/sysctl.d/*.conf
/usr/local/lib/sysctl.d/*.conf
/usr/lib/sysctl.d/*.conf
/lib/sysctl.d/*.conf
/etc/sysctl.conf

Check

Verify RHEL 9 is not performing IPv4 packet forwarding unless the system is a router.

Check that "net.ipv4.conf.all.forwarding" is disabled using the following command:

$ sudo sysctl net.ipv4.conf.all.forwarding
net.ipv4.conf.all.forwarding = 0

If "net.ipv4.conf.all.forwarding" is not set to "0" and is not documented with the information system security officer (ISSO) as an operational requirement or is missing, this is a finding.

Fix

Configure RHEL 9 to not allow IPv4 packet forwarding, unless the system is a router.

Create a configuration file if it does not already exist:

$ sudo vi /etc/sysctl.d/ipv4_forwarding.conf

Add the following line to the file:
net.ipv4.conf.all.forwarding = 0

Reload settings from all system configuration files with the following command:

$ sudo sysctl --system