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Setting up Linux DHCP Server with Dynamic DNS Support

Sunday, August 8, 2004 by digitalpeer, updated Sunday, August 8, 2004

This tutorial will explain how to setup DHCP to dynamically update a local DNS server. I use Red Hat/Fedora but these are common steps you'll have to use with just about any flavor of these Linux services.

Setting up the Server

For starters, lets configure the /etc/dhcpd.conf file. It's pretty self explanitory. If you are planning on setting up a local DNS server leave in the 3 lines pertaining to Dynamic DNS, otherwise take them out.

#
# Global Settings
#

# Turn on Dynamic DNS:
ddns-domainname "domain.lan";
ddns-update-style interim;
ddns-updates on;

# Don't allow clients to update DNS, make the server do it
# based on the hostname passed by the DHCP client:
deny client-updates;
allow unknown-clients;

#
# 10.0.0.0/255.255.255.0 Scope Settings
#
subnet 10.0.0.0 netmask 255.255.255.0 {

        # Range of DHCP assigned addresses for this scope
        range                           10.0.0.100 10.0.0.200;
        # 1 day
        default-lease-time              86400;
        # 2 days
        max-lease-time                  172800;

        # Configure the client's default Gateway:
        option subnet-mask              255.255.255.0;
        option broadcast-address        10.0.0.255;
        option routers                  10.0.0.2;

        # Configure the client's DNS settings:
        option domain-name              "domain.lan";
        option domain-name-servers      10.0.0.1;

}

Now things get a little tricky. If you have Windows clients you are going to need to modify the /etc/rc.d/init.d/dhcpd file. In the start() directive you should see a line that looks something like

daemon /usr/sbin/dhcpd ${DHCPDARGS}

Comment that out with a # and add in the following lines. If you want, add in the DHCPDARGS also, but I don't really use it.

/sbin/route add -host 255.255.255.255 dev eth0 2> /dev/null
daemon /usr/sbin/dhcpd eth0

Make sure you set the ethx to the network interface you want to serve dhcpd requests on. This is particularly vital if you have a multihomed server.

Setting up the Client

The client needs to send a hostname for this to work correctly. The DHCP server needs to know what hostname to set in DNS. Windows does it by default, however you'll need to make sure your /etc/sysconfig/network-scripts/ifcfg-ethx file looks something like this (with the DHCP_HOSTNAME directive set properly.

# make sure this line is has no settings on it!
DEVICE=eth0
BOOTPROTO=dhcp
HWADDR=01:01:01:01:01:01
ONBOOT=yes
TYPE=Ethernet
DHCP_HOSTNAME=host
USERCTL=no
PEERDNS=yes

Now, you should go have a beer and explain what you just did to your girlfriend.