To see the current routing table:
The default gateway will have the "UG" flags shown in the output. Red Hat and CentOS systems usually have the routing table stored in /etc/sysconfig/network-scripts/ by network device (e.g., route-eth1, route-eno1). To change or modify the routing table, you must be root.
To add a default gateway from the command line:
/sbin/route add default gw ip-address eth0
To add a static route, use the ip command and specify the destination and interface:
/sbin/ip route add 192.168.1.0/24 via 192.168.2.254 dev eno1
To see how packets will be routed to an ip address:
/sbin/ip route get ip-address