Using a Linux server to route packets between two private networks

Reference: http://www.tecmint.com/setup-linux-as-router/

We want to route icmp (ping) packets from dev2 to dev4 and the other way around as well (note that both client machines are on different networks). The name of each NIC, along with its corresponding IPv4 address, is given inside square brackets.
Our test environment is as follows:

Client 1: CentOS 7 [enp0s3: 192.168.0.17/24] - dev1
Router: Debian Wheezy 7.7 [eth0: 192.168.0.15/24, eth1: 10.0.0.15/24] - dev2
Client 2: openSUSE 13.2 [enp0s3: 10.0.0.18/24] - dev4

Let’s view the routing table in dev1 (CentOS box):

# ip route show

and then modify it in order to use its enp0s3 NIC and the connection to 192.168.0.15 to access hosts in the 10.0.0.0/24 network:

# ip route add 10.0.0.0/24 via 192.168.0.15 dev enp0s3

Which essentially reads, “Add a route to the 10.0.0.0/24 network through the enp0s3 network interface using 192.168.0.15 as gateway”.

Route Network in Linux

Likewise in dev4 (openSUSE box) to ping hosts in the 192.168.0.0/24 network:

# ip route add 192.168.0.0/24 via 10.0.0.15 dev enp0s3

Network Routing in Linux

Finally, we need to enable forwarding in our Debian router:

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

Now let’s ping:

Check Network Routing

and,

Route Ping Status

To make these settings persistent across boots, edit /etc/sysctl.conf on the router and make sure the net.ipv4.ip_forward variable is set to true as follows:

net.ipv4.ip_forward = 1

In addition, configure the NICs on both clients (look for the configuration file within /etc/sysconfig/network on openSUSE and /etc/sysconfig/network-scripts on CentOS – in both cases it’s called ifcfg-enp0s3).
Here’s the configuration file from the openSUSE box:

BOOTPROTO=static
BROADCAST=10.0.0.255
IPADDR=10.0.0.18
NETMASK=255.255.255.0
GATEWAY=10.0.0.15
NAME=enp0s3
NETWORK=10.0.0.0
ONBOOT=yes