Prismjs

Stop DHCP From Changing resolv.conf

For DHCP users, there may be times when you need to edit /etc/resolv.conf to use other nameservers. Then, after a period of time (or after a system reboot), you discover that your changes to /etc/resolv.conf have been reverted.

This tutorial shows three methods to stop DHCP from changing the /etc/resolv.conf on Debian or Ubuntu.

Method 1: Change interface settings to static

  • On a cloud vps, I do not suggest using this method.
  • If you use this method, you may find that the reboot processing (until you can login through ssh) takes longer.

First, we need to get the IP/netmask/gateway of the server. Run the following command.

ifconfig | grep "inet addr" | head -n 1 | awk '{print $2, $4}'
addr:1.2.3.4 Mask:255.255.254.0

... the server IP address is 1.2.3.4 and mask is 255.255.254.0.

To get the gateway address, run the following command.

netstat -rn | grep '^0.0.0.0' | awk '{print $2}'

In this example, I will use the gateway address 1.2.3.1.

Now that we have the IP/netmask/gateway, edit /etc/network/interfaces.

 vim /etc/network/interfaces

Make the following edits:

# Comment out this line
# iface eth0 inet dhcp

# Add these contents
iface eth0 inet static
address 1.2.3.4
mask 255.255.254.0
gateway 1.2.3.1

Remember, you must replace these values with the appropriate IP/netmask/gateway of server.

Save and exit, then reboot.

Method 2: Write protect your nameservers

Change your nameservers by editing /etc/resolv.conf. Once you have made your edits, write protect that file.

chattr +i /etc/resolv.conf

The +i option (attribute) write protects the /etc/resolv.conf file on Linux so that no one can modify it - not even the root user.

If you need to remove the write protect attribute, use the following command.

chattr -i /etc/resolv.conf

Method 3: Use DHCP hooks

This is the method that I recommend using the most.

Edit /etc/dhcp/dhclient-enter-hooks.d/nodnsupdate.

vim /etc/dhcp/dhclient-enter-hooks.d/nodnsupdate

Make the following edits:

#!/bin/sh
make_resolv_conf(){
    :
}

Save and exit.

Update the permissions on the nodnsupdate file.

chmod +x /etc/dhcp/dhclient-enter-hooks.d/nodnsupdate

Reboot your server. You can now update nameservers by editing /etc/resolv.conf without worrying about rollback.