Configure Linux Routing for Dual NICs: Temporary Commands & Permanent Solutions

Linux Routing Commands

1. Add host route

route add -host 192.168.1.11 dev eth0
route add -host 192.168.1.12 gw 192.168.1.1

2. Add network route

route add -net 192.168.1.11 netmask 255.255.255.0 eth0
route add -net 192.168.1.11 netmask 255.255.255.0 gw 192.168.1.1
route add -net 192.168.1.0/24 eth0
route add -net 192.168.1.0/24 gw 192.168.1.1

3. Add default gateway

route add default gw 192.168.1.1

4. Delete route

route del -host 192.168.1.11 dev eth0

5. Delete default route

route del default gw 192.168.1.1

Ways to Configure Permanent Routes in Linux

1. Add to /etc/rc.local

route add -net 192.168.1.0/24 dev eth0
# or
route add -net 192.168.1.0 netmask 255.255.255.0 gw 192.168.1.1

2. Append to /etc/sysconfig/network

GATEWAY=[gateway IP or interface name]

3. Use /etc/sysconfig/static-routes

any net 192.168.1.0/24 gw 192.168.1.1
# or
any net 192.168.1.0 netmask 255.255.255.0 gw 192.168.1.1

4. Enable IP forwarding

1. Temporary echo "1" > /proc/sys/net/ipv4/ip_forward 2. Permanent

vim /etc/sysctl.conf
# modify or add:
net.ipv4.ip_forward=1

Experiment: Configure a Dual‑NIC Host to Use Both Internal and External Networks

The experiment reproduces a real‑world scenario where a host with two network cards must access both an external ISP network (10.0.0.0/16) and an isolated internal network (172.16.0.0/16). VMware Workstation and Huawei eNSP are used to simulate the environment.

Problem Background

When the client node is the only host connecting the two networks, the internal and external networks are isolated. Placing the default gateway on the external NIC allows access to the Internet but blocks other internal subnets; placing it on the internal NIC enables full internal access but prevents Internet connectivity. The goal is to enable simultaneous access.

Experiment Environment

VMware Workstation Pro

Four minimal CentOS 7.3 virtual machines

Huawei eNSP simulator

Experiment Topology

Key nodes:

client – dual‑NIC host with IPs 10.0.0.101/16 (external) and 172.16.2.101/24 (internal)

server2 – 172.16.2.11/24

server3 – 172.16.3.11/24

server4 – 172.16.4.11/24

Network Planning

IP Allocation

client eth0   10.0.0.101   255.255.0.0   gateway 10.0.0.1   # external
client eth1   172.16.2.101 255.255.255.0           # internal
server2 eth0   172.16.2.11  255.255.255.0  gw 172.16.2.254
server3 eth0   172.16.3.11  255.255.255.0  gw 172.16.3.254
server4 eth0   172.16.4.11  255.255.255.0  gw 172.16.4.254

VMware NIC Types

Operator network – Bridged – 10.0.0.0/16
VMnet2 – Host‑only – 172.16.2.0/24
VMnet3 – Host‑only – 172.16.3.0/24
VMnet4 – Host‑only – 172.16.4.0/24

Configure Internal Network (eNSP Router)

Set interfaces on the Huawei router:

<huawei>system-view
[int g0/0/0]
ip address 172.16.2.254 255.255.255.0
[int g0/0/1]
ip address 172.16.3.254 255.255.255.0
[int g0/0/2]
ip address 172.16.4.254 255.255.255.0

Enable IP Forwarding on the Client

Temporary: echo "1" > /proc/sys/net/ipv4/ip_forward Permanent (edit /etc/sysctl.conf):

net.ipv4.ip_forward=1

Testing Connectivity

Ping external site: ping www.baidu.com -c 4 Ping each internal server:

ping 172.16.2.11 -c 4
ping 172.16.3.11 -c 4
ping 172.16.4.11 -c 4

Access HTTP services hosted on the internal servers (Python SimpleHTTPServer on port 8080):

curl http://172.16.2.11:8080/index.html
curl http://172.16.3.11:8080/index.html
curl http://172.16.4.11:8080/index.html

Adding a Persistent Route for the Whole Internal Network

On the client, add a static route covering the remaining internal subnets: route add -net 172.16.0.0/16 gw 172.16.2.254 To make it permanent, place the same command in /etc/rc.local.

Result

After adding the route, the client can reach the Internet and all internal subnets (172.16.2.0/24, 172.16.3.0/24, 172.16.4.0/24) simultaneously. The experiment demonstrates that a single static route can solve the dual‑NIC routing problem, while also illustrating the underlying routing principles.