Creating static routes

All traffic in our network has to pass through our core-routers, additionally switch ports are configured in such a way that they only accept ethernet frames with mac addresses originating from these routers. Consequently, no layer-2 traffic is forwarded between switch ports and direct communication between servers, even when on the same subnet, is not possible. If you have multiple servers with us which use IPv4 addresses from the same subnet (usually /24), and you want these servers to exchange traffic between each other, you will have to create static routes on these servers to each other.

The following examples assume that the server with the IPv4 address 192.51.100.10 and the server with the IPv4 address 192.51.100.42 want to communicate with each other.

CentOS

In CentOS, you can add static routes to the route-file of the respective interface. In case of eth0, the file is called route-eth0 and located under /etc/sysconfig/network-scripts/. The below entry on server 192.51.100.10 adds a static to server 192.51.100.42:

#/etc/sysconfig/network-scripts/route-eth0
...
192.51.100.42/32 via 192.51.100.1 dev eth0


Vice versa, the server 192.51.100.42 needs a corresponding route to server 192.51.100.10:

#/etc/sysconfig/network-scripts/route-eth0
...
192.51.100.10/32 via 192.51.100.1 dev eth0


Alternatively, both servers can have a single route added to the whole /24 subnet:

#/etc/sysconfig/network-scripts/route-eth0
...
192.51.100.0/24 via 192.51.100.1 dev eth0


Debian / Ubuntu

Static routes in Debian and Ubuntu are saved to /etc/network/interfaces. Entries are added to the end of the file or under the iface sections of the respective interface. Assuming the interface to be eth0, the entry on server 192.51.100.10 would look like this:

#/etc/network/interfaces
...
up ip route add 192.51.100.42/32 via 192.51.100.1 dev eth0
down ip route del 192.51.100.42/32 via 192.51.100.1 dev eth0


On server 192.51.100.42, this would be the static route to 192.51.100.10:

#/etc/network/interfaces
...
up ip route add 192.51.100.10/32 via 192.51.100.1 dev eth0
down ip route del 192.51.100.10/32 via 192.51.100.1 dev eth0


It is also possible to add a static route to the whole /24 on both servers instead:

#/etc/network/interfaces
...
up ip route add 192.51.100.0/24 via 192.51.100.1 dev eth0
down ip route del 192.51.100.0/24 via 192.51.100.1 dev eth0


openSUSE

Static routes in openSUSE can be set in /etc/sysconfig/network/routes. On server 192.51.100.10, the static route to 192.51.100.42 is as follows:

#/etc/sysconfig/network/routes
...
192.51.100.42/32 192.51.100.1 - eth0


Conversely, the static route on 192.51.100.42 to 192.51.100.10:

#/etc/sysconfig/network/routes
...
192.51.100.10/32 192.51.100.1 - eth0


As an alternative, both servers can have a static route to the whole /24 subnet:

#/etc/sysconfig/network/routes
...
192.51.100.0/24 192.51.100.1 - eth0


Windows Server

To create a static route under Windows Server 2008 or 2012, please open the "Command Prompt" with administrative privileges. On server 192.51.100.10 enter the following:

route -p add 192.51.100.42 mask 255.255.255.255 192.51.100.1


The corresponding entry on server 192.51.100.42 is:

route -p add 192.51.100.10 mask 255.255.255.255 192.51.100.1


Setting the -p option makes the route persistent across reboots. If the route is meant to be temporary, you may omit -p.