Detailed explanation of routing configuration in Linux system with multiple network cards

Routing configuration commands under Linux

1. Add host routing

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

2. Add network routing

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 a default gateway

route add default gw 192.168.1.1

4. Delete a route

route del -host 192.168.1.11 dev eth0

5. Delete the default route

route del default gw 192.168.1.1

Several ways to configure permanent routing under Linux

1. Add routing information in /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. Add in /etc/sysconfig/network

GATEWAY=[gateway IP or gateway network card name]

3. /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 opening

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

2. Permanently open

vim /etc/sysctl.conf

Modify net.ipv4.ip_forward=1

Experiment: Configuring a dual-NIC host to use both the intranet and the Internet

The background of this experiment is a problem that the author encountered in practice. This experiment tries to restore the network environment at that time. Just take a note and share it with classmates who encounter this problem.

Unfortunately, I don’t have any hardware now (T＿T)...I can only use VMware Workstation and eNSP to simulate the experimental environment.

Background

As shown in the topology below, if client node is removed, the intranet and the extranet are isolated networks.

But the actual situation is that client node needs to access both external and internal network resources, and client can only configure one default route. If the default route is configured on the external network card, client can access the resources of the 172.16.2.0/24 network and the external network, but cannot access the rest of the internal network resources; if the default route is configured on the internal network card, client can fully access the internal network resources, but cannot access the external network resources.
How to solve it?

Experimental environment

• VMware Workstation Pro
• 4 minimally installed CentOS 7.3 virtual machines
• Huawei eNSP Simulator

Experimental topology

Remove the client node, and the intranet and the extranet are isolated networks.

The external network is 10.0.0.0/16 , which represents the operator's access network;

The intranet network is 172.16.0.0/16 , which represents the intranet part (for security reasons, this part does not allow direct access to the Internet).

in:

• client is a host with two network cards, and the network segments of the two network cards are 10.0.0.0/16 and 172.16.2.0/24 respectively;
• server2 is a server in the 172.16.2.0/24 network segment;
• server3 is a server in the 172.16.3.0/24 network segment;
• server4 is a server in the 172.16.4.0/24 network segment.

Network Planning

IP Allocation