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When my CentOS virtual machine boots it uses DHCP to get an IP address. It also overwrites resolv.conf with the DNS settings provided by the DHCP server. The DHCP server doesn't supply any search domains so I would like to get dhclient to put in a list of search domains when it writes it. How can I configure dhclient to do this?

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10 Answers 10

Reset to default
36

None of those worked, but the last one was the closest. For Red Hat 6, Use DOMAIN instead of SEARCH as in the example above and the file location is different.

I modified the file /etc/sysconfig/network-scripts/ifcfg-eth0

and changed

DOMAIN=domain.com

to

DOMAIN="domain.com sub.domain.com"

and it all worked.

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2
  • 4
    You should be able to use both DOMAIN and SEARCH github.com/mcr/isc-dhcp/blob/master/common/resolv.c#L68
    – Radek Simko
    Jan 14, 2014 at 21:49
  • This technique of setting DOMAIN also worked for me on CentOS 6.5, even though my resolve.conf is generated by NetworkManager rather than dhclient. The generated resolve.conf file combined my entries with those from the DHCP server.
    – PolyTekPatrick
    Aug 13, 2015 at 2:51
17

Also you can add string to /etc/dhcp3/dhclient.conf like this

prepend domain-search "domain1.com", "domain2.com";

Note, that this method works with Debian Lenny and Squeeze, too.

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1
  • This works, however "append" works the same as "prepend" on CentOS 7, is this a known bug? How do I append?
    – puravidaso
    May 20, 2021 at 19:30
8

I managed to work this out in the end. I added a line like the following to /etc/dhclient-eth0.conf

append domain-name "example.com";
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6

This is mostly a note for RHEL7 to reduce trial and error. Dean's answer of using DOMAIN="domain1.exmaple.com domain2.example.com" in /etc/sysconfig/network-scripts/ifcfg-device.conf works. An interesting note is the host's domain that the connection gets from DHCP is always prepended to the search path, even if you leave it out of DOMAIN= or put it later in a list for DOMAIN=. It looks like /sbin/dhclient-script has a bunch of logic related to this.

In my testing, I found that Philip's suggestion of using /etc/dhcp/dhclient-device.conf also works, although there is some strange behavior with that, most likely due to that same logic in /sbin/dhclient-script that tries to move things around. For instance, neither supercede or prepend work as expected, the host's domain will be first. As a side note on this method, /var/lib/NetworkManager/dhclient-device.conf is the generated NetworkManager file and is used by the client. If you have a file in /etc/dhcp/ that gets read in, you'll see it pasted at the top of the file and a few extra options added below.

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1
  • AWS Linux (RHEL), DOMAIN="your.domain" didn't work for me, reset after reboot
    – radtek
    Jan 17, 2019 at 22:57
3

On CentOS 6, I'm using the following file to add my preferred DNS search domain:

# cat /etc/dhcp/dhclient-eth0.conf 
interface "eth0" {
    supersede domain-search "dns1.example.com";
}
# getenforce 
Enforcing
# ls -lZ /etc/dhcp/dhclient-eth0.conf 
-rw-r--r--. root root system_u:object_r:bin_t:s0   /etc/dhcp/dhclient-eth0.conf
#

This file is the first that's checked for in /etc/sysconfig/network-scripts/ifup-eth:

if [ -s /etc/dhcp/dhclient-${DEVICE}.conf ]; then
   DHCLIENTCONF="-cf /etc/dhcp/dhclient-${DEVICE}.conf";

See also: redhat - Configuring DHCP on RHEL 6 - Server Fault

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1
  • One note to add - if you're specifying multiple search domains to dhclient by using (option|supersede|*) domain-search ..., make sure you follow the dhcp-options(5) man page spec for quoting individual domains: option domain-search "example.com", "sales.example.com", "eng.example.com"; Many older distributions of dhclient allowed the format "example.com sales.example.com eng.example.com"; but this compatibility has been removed in the past few years due to bug fixes bugs.launchpad.net/ubuntu/+source/isc-dhcp/+bug/777785
    – Patrick
    Apr 11, 2013 at 4:42
3

The /etc/dhclient-eth0.conf answer didn't work for me. I don't have an /etc/dhcp3 directory so I didn't think that was likely to work either.

After examining the /sbin/dhclient-script file (which creates /etc/resolv.conf on my Centos 5.6 system), I added the SEARCH line below to /etc/sysconfig/networking/devices/ifcfg-eth0:

DEVICE=eth0 
BOOTPROTO=dhcp
HWADDR=08:00:24:61:17:AC 
ONBOOT=yes
TYPE=Ethernet
SEARCH="example.com sub1.example.com sub2.example.com"

Then:

# ifdown eth0
# ifup eth0
#  cat /etc/resolv.conf
; generated by /sbin/dhclient-script
search example.com sub1.example.com sub2.example.com
nameserver 10.1.0.11
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1
  • Note that DOMAIN or SEARCH apparently must be added to the ifcfg-eth0 file, and adding it to just sysconfig/network is NOT sufficient -- DESPITE the ifup-post properly sourcing that file. NetworkManager, that fridge-art of pointless D-K wheel reinvention, will go behind the ifup-post invocation and munge the resolv.conf AFTER ifup-post is done, laying down ONLY the fqdn as SEARCH . It's obviously ignoring sysconfig/network because, well #boring or something. \sigh. Gold star, kids.
    – user2066657
    Nov 18, 2021 at 16:43
2

For anyone going through Fedora / Red Hat's rather opaque pile of scripts, the answer, at least on Amazon's latest AMI, it is /etc/dhclient-eth0.conf (and not the decoy empty folder at /etc/dhcp/ ). The file is not present and will need to be created

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  • 2
    Adding SEARCH to ifcfg-eth0/etc/sysconfig/network-scripts/ifcfg-eth0 also works on the Amazon version of Fedora. In older versions of Red Hat it apparently was DOMAIN. I wpould recommend this over the above.
    – Dr David C Crooke
    Sep 20, 2013 at 20:20
1

In Fedora 19 add next lines to /etc/dhcp/dhclient.conf

# /etc/dhcp/dhclient.conf

interface "p2p1"
{
    supersede domain-name-servers 8.8.8.8, 8.8.4.4;
    append domain-name " mydomain.net example.com";
}

Work fine with NetworkManager. Details see: # man dhclient.conf

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  • does not work for aws linux (rhel)
    – radtek
    Jan 17, 2019 at 23:00
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Since I don't see this answer and it worked for me (while the others didn't), here it is: edit /etc/resolvconf/resolv.conf.d/base the same way as you would /etc/resolv.conf. You'll need resolvconf installed.

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1

Add to the DHCP server conf (ISC dhcpd): 

option domain-name "domain1.com domain2.net domain3.org";

Where domain1.com, etc.. are the domain suffixes you want added to the resolv.conf file on each client.

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