Useful commands with network configuration in Linux

List network interface names:     

ls -1 /sys/class/net

Change IP/Subnet temporary:    

 sudo ifconfig eth0 192.168.0.23 netmask 255.255.255.0

Change default gateway:             

sudo route add default gw 192.168.0.1 eth0

Verify gateway                             

 ip route | grep default

Change DNS                                  

sudo nano /etc/resolv.conf   

then add DNS IPs in this file.

Change IP persistently:               

sudo nano  /etc/network/interfaces       

auto eth0  iface eth0 inet static address 192.168.1.12  netmask 255.255.255.0  gateway 192.168.1.0

Get network hardware                

sudo find /sys -name *eth*                                                      

lspci | grep -i eth*                               (get brand name)

Change the host name   

sodu nano /etc/hosts
sodu nano /etc/hostname
then enter the expected name

More ...

Reset root password on Ubuntu

1. Boot to recovery mode (Hold Ecs or Shift key to show the GRUB, choose Advance -> Recovery Mode)
2. Mount file-system with write permission: mount -rw -o remount /
3. Type command: passwd username
4. Reboot

Fix: user is not in sudoers. 

1. Login root in recovery mode (above)
2. Edit /etc/sudoers to add user has sudo privilege 
• Add a line:  user ALL=(ALL) ALL