How to prevent DHCP server to override your resolv.conf script

Draft

The “problem” is caused by dhcp server that overwrite our resolv.conf script and we lose this personalization during the IP release session. Not even is a good idea to replace the dns configuration proposed by the dhcp server, if it push that to you, probably there is a reason. But some time you want to use your own configuration such as when you use your broadband router and you want your /etc/resolv.conf has been preserved fron updates.

To prevent this nasty situation we can operate in two ways, determined by which dhcp client you are using: dhcpcd ord dhclient.

In the first case, dhcpcd (as in my Slackware laptop), prevent overriding can be obtained simply add the -R option to the dhcpcd command:

# dhcpcd -R eth0

In the second case, we must read the man page about dhclient-script which is invoked any time you use dhclient.
In:

man 8 dhclient-script

at the HOOKS section we can read:

HOOKS

When it starts, the client script first defines a shell function, make_resolv_conf , which is later used to create the /etc/resolv.conf file. To override the default behaviour, redefine this function in the enter hook script.

This means that we must to create /etc/dhclient-enter-hooks and redefine the make_resolv_conf function to satisfy our needs.
If we simply wants to prevent resolv.conf updates only, the fastest way is to redefine the function to do nothing:

# cat /etc/dhclient-enter-hooks
make_resolv_conf() {
exit 0
}

Then save the file and ensure it is executable:

# chmod a+x /etc/dhclient-enter-hooks

Note that, as explained in the man page, the dhclient-script is not standard so if this configuration doesn’t work, please read the man page.

Next: How to merge dns addresses pushed by the dhcp server and my own dns.

6 thoughts on “How to prevent DHCP server to override your resolv.conf script

  1. Thanks,

    That’s works for my ubuntu 7.10.
    In my case i create that function in /etc/dhcp3/dhclient-enter-hooks.d/dhclient-enter-hooks.

    After that, /etc/resolv.conf isn’t override again.

  2. Thanks for the tip, very helpful. However there’s a slight typo.

    There is an extra ‘i’ that shouldn’t be present before make_resolv_conf in the dhclient-enter-hooks script, so it should contain:

    make_resolv_conf() {
    exit 0;
    }

    For completeness, some may like to make the file have execute permission too:
    chmod a+x dhclient-enter-hooks

  3. Just found your page when googling for a way to keep the search-list in resolv.conf independent of the dhcp server.

    For that, there is even a simpler solution, in /etc/dhcp3/dhclient.conf add:

    supersede domain-name “lan internal some.domain”;

  4. Pingback: internet very slow

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>