[Linux 文章收集] How to set a default gateway on CentOS

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Preface 
A default gateway is a remote host or router that your Linux host forwards traffic to when the destination IP address of outgoing traffic does not match any route in your local routing table. Configuring a default gateway on CentOS is quite straightforward. If you wish to change a default gateway temporarily at run time, you can use ip command. 

First things first. To check what default gateway you are using currently: 

$ ip route show
192.168.91.0/24 dev eth0 proto kernel scope link src 192.168.91.128
169.254.0.0/16 dev eth0 scope link metric 1002
default via 192.168.91.2 dev eth0

According to the local routing table shown above, a default gateway is 192.168.91.2, and traffic is forwarded to the gateway via eth0. 

Change a default gateway to another IP address 
In order to change a default gateway to another IP address, try below command: 

$ sudo ip route replace default via 192.168.91.10 dev eth0 # Change default gw from 192.168.91.2 to 192.168.91.10

Obviously, a default gateway's IP address should come from the subnet associated with the interface connected to the default gateway, in this example, 192.168.91.0/24. Otherwise, the command will fail with the following error. 

RTNETLINK answers: No such process

Also, keep in mind that the default route change made by ip command will be lost after rebooting. 

Set a default gateway permanently on CentOS 
You will need to update /etc/sysconfig/network accordingly. 

$ sudo vi /etc/sysconfig/network

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1. ...  
2. GATEWAY=192.168.91.10  

Again, be aware that the IP addressed specified here should match with the subnet (192.168.91.0/24) associated with a default route interface. 

Another option to set a default gateway persistently on CentOS is to edit /etc/sysconfig/network-scripts/ifcfg-<default_interface_name>, and add "GATEWAY=" there. If the default interface is "eth0", you will need to edit /etc/sysconfig/network-scripts/ifcfg-eth0. If you choose to use this method, you need to refer to this post to get familiar with this option. 

Whether you edit /etc/sysconfig/network or /etc/sysconfig/network-scripts/ifcfg-ethX, don't forget to restart network service as follows, or reboot your CentOS for the change to take effect. 

$ sudo service network restart

Supplement 
* How to configure network interfaces in CentOS 
* How to assign multiple IP addresses to one network interface on CentOS 
* How to configure networking in CentOS Desktop with command line 
* How to run a startup script automatically after a ...work interface is up on CentOS 
* How to install dummynet on CentOS