LDAP Support in DHCP

Brian Masney <masneyb@ntelos.net>

Last updated 8/16/2002

This document describes setting up the DHCP server to read it's configuration 

from LDAP. This work is based on the IETF document 

draft-ietf-dhc-ldap-schema-01.txt included in the doc directory. For the latest

version of this document, please see http://home.ntelos.net/~masneyb.

First question on most people's mind is "Why do I want to store my 

configuration in LDAP?" If you run a small DHCP server, and the configuration

on it rarely changes, then you won't need to store your configuration in LDAP.

But, if you have several DHCP servers, and you want an easy way to manage your 

configuration, this can be a solution. 

The first step will be to setup your LDAP server. I am using OpenLDAP from

www.openldap.org. Building and installing OpenLDAP is beyond the scope of this 

document. There is plenty of documentation out there about this. Once you have 

OpenLDAP installed, you will have to edit your slapd.conf file. I added the 

following 2 lines to my configuration file:

include         /etc/ldap/schema/dhcp.schema

index           dhcpHWAddress 	eq

index           dhcpClassData	eq

The first line tells it to include the dhcp schema file. You will find this 

file under the contrib directory in this distribution. You will need to copy 

this file to where your other schema files are (maybe

/usr/local/openldap/etc/openldap/schema/). The second line sets up

an index for the dhcpHWAddress parameter. The third parameter is for reading 

subclasses from LDAP every time a DHCP request comes in. Make sure you run the 

slapindex command and restart slapd to have these changes to into effect.

Now that you have LDAP setup, you should be able to use gq (http://biot.com/gq/)

to verify that the dhcp schema file is loaded into LDAP. Pull up gq, and click

on the Schema tab. Go under objectClasses, and you should see at least the 

following object classes listed: dhcpClass, dhcpGroup, dhcpHost, dhcpOptions, 

dhcpPool, dhcpServer, dhcpService, dhcpSharedNetwork, dhcpSubClass, and 

dhcpSubnet. If you do not see these, you need to check over your LDAP 

configuration before you go any further.

You should be ready to build DHCP. Edit the includes/site.h file and uncomment

the #define LDAP_CONFIGURATION. Now run configure in the base source directory.

Edit the work.os/server/Makefile and add -lldap to the LIBS= line. (replace os

with your operating system, linux-2.2 on my machine). You should be able to 

type make to build your DHCP server. 

Once you have DHCP installed, you will need to setup your initial plaintext 

config file. In my /etc/dhcpd.conf file, I have:

ldap-server "localhost";

ldap-port 389;

ldap-username "cn=DHCP User, dc=ntelos, dc=net";

ldap-password "blah";

ldap-base-dn "dc=ntelos, dc=net";

ldap-method dynamic;

All of these parameters should be self explanatory except for the ldap-method.

You can set this to static or dynamic. If you set it to static, the 

configuration is read once on startup, and LDAP isn't used anymore. But, if you

set this to dynamic, the configuration is read once on startup, and the 

hosts that are stored in LDAP are looked up every time a DHCP request comes in.

The next step is to set up your LDAP tree. Here is an example config that will

give a 10.100.0.x address to machines that have a host entry in LDAP. 

Otherwise, it will give a 10.200.0.x address to them. (NOTE: replace 

dc=ntelos, dc=net with your base dn). If you would like to convert your 

existing dhcpd.conf file to LDIF format, there is a script 

contrib/dhcpd-conf-to-ldap.pl that will convert it for you.

# You must specify the server's host name in LDAP that you are going to run

# DHCP on and point it to which config tree you want to use. Whenever DHCP 

# first starts up, it will do a search for this entry to find out which 

# config to use

dn: cn=brian.ntelos.net, dc=ntelos, dc=net

objectClass: top

objectClass: dhcpServer

cn: brian.ntelos.net

dhcpServiceDN: cn=DHCP Service Config, dc=ntelos, dc=net

# Here is the config tree that brian.ntelos.net points to. 

dn: cn=DHCP Service Config, dc=ntelos, dc=net

cn: DHCP Service Config

objectClass: top

objectClass: dhcpService

dhcpPrimaryDN: dc=ntelos, dc=net

dhcpStatements: ddns-update-style ad-hoc

dhcpStatements: default-lease-time 600

dhcpStatements: max-lease-time 7200

# Set up a shared network segment

dn: cn=WV Test, cn=DHCP Service Config, dc=ntelos, dc=net

cn: WV

objectClass: top

objectClass: dhcpSharedNetwork

# Set up a subnet declaration with a pool statement. Also note that we have

# a dhcpOptions object with this entry

dn: cn=10.100.0.0, cn=WV Test, cn=DHCP Service Config, dc=ntelos, dc=net

cn: 10.100.0.0

objectClass: top

objectClass: dhcpSubnet

objectClass: dhcpOptions

dhcpOption: domain-name-servers 10.100.0.2

dhcpOption: routers 10.100.0.1

dhcpOption: subnet-mask 255.255.255.0

dhcpOption: broadcast-address 10.100.0.255

dhcpNetMask: 24

# Set up a pool for this subnet. Only known hosts will get these IPs

dn: cn=Known Pool, cn=10.100.0.0, cn=WV Test, cn=DHCP Service Config, dc=ntelos, dc=net

cn: Known Pool

objectClass: top

objectClass: dhcpPool

dhcpRange: 10.100.0.3 10.100.0.254

dhcpPermitList: deny unknown-clients

# Set up another subnet declaration with a pool statement

dn: cn=10.200.0.0, cn=WV Test, cn=DHCP Service Config, dc=ntelos, dc=net

cn: 10.200.0.0

objectClass: top

objectClass: dhcpSubnet

objectClass: dhcpOptions

dhcpOption: domain-name-servers 10.200.0.2

dhcpOption: routers 10.200.0.1

dhcpOption: subnet-mask 255.255.255.0

dhcpOption: broadcast-address 10.200.0.255

dhcpNetMask: 24

# Set up a pool for this subnet. Only unknown hosts will get these IPs

dn: cn=Known Pool, cn=10.200.0.0, cn=WV Test, cn=DHCP Service Config, dc=ntelos, dc=net

cn: Known Pool

objectClass: top

objectClass: dhcpPool

dhcpRange: 10.200.0.3 10.200.0.254

dhcpPermitList: deny known clients

# Set aside a group for all of our known MAC addresses

dn: cn=Customers, cn=DHCP Service Config, dc=ntelos, dc=net

objectClass: top

objectClass: dhcpGroup

cn: Customers

# Host entry for my laptop

dn: cn=brianlaptop, cn=Customers, cn=DHCP Service Config, dc=ntelos, dc=net

objectClass: top

objectClass: dhcpHost

cn: brianlaptop

dhcpHWAddress: ethernet 00:00:00:00:00:00

You can use the command slapadd to load all of these entries into your LDAP 

server. After you load this, you should be able to start up DHCP. If you run

into problems reading the configuration, try running dhcpd with the -d flag. 

If you still have problems, edit the site.conf file in the DHCP source and

add the line: COPTS= -DDEBUG_LDAP and recompile DHCP. (make sure you run make 

clean and rerun configure before you rebuild).