Although not strictly necessary for a firewall, you may need to implement routing on your RHEL7 system. Often, this will be associated with multi-homed systems with more than one network interface card; however, this is not a requirement of network routing, which allows packets to be forwarded to the correct destination network. Network routing is enabled in procfs in the /proc/sys/net/ipv4/ip_forward file. If this file contains a value of 0, then routing is disabled; if it has a value of 1, routing is enabled. This can be set using the echo command as follows:

# echo 1 > /proc/sys/net/ipv4/ip_forward

However, this is then turned on until the next reboot when the routing will revert to the configured setting. To make this setting permanent traditionally, the /etc/sysctl.conf file has been used. It's now recommended to add you own configurations to /etc/sysctl.d/. Here is an example of this:

# echo "net.ipv4.ip_forward  =  1" > /etc/sysctl.d/ipforward.conf

This will create a...