When I was playing around with KeepAlived, I managed to create a few HA scenarios that mirrored actual production uses. One scenario was creating a highly available router, which would forward IPv4 traffic between interfaces. To configure a CentOS or Fedora Linux host to forward IPv4 traffic, you can set the “net.ipv4.ip_forward” sysctl to 1 in /etc/sysctl.conf:
net.ipv4.ip_forward = 1
Once the sysctl is added to /etc/sysctl.conf, you can enable it by running sysctl with the “-w” (change a specific sysctl value) option:
$ /sbin/sysctl -w net.ipv4.ip_forward=1
net.ipv4.ip_forward = 1
If routing is configured correctly on the router, packets should start flowing between the interfaces on the server. Nice!