ISC DHCPD with LDAP Part 2

If you haven’t set up your LDAP server and added the DHCP schema, then visit Part 1 of the series before moving on.

Thanks to this guy:

Based on that blogspot post, I was able to get pretty far. You should too.

Step 1: Add OU

What worked for me was adding an organizationalUnit to the LDAP tree, ou=dhcp,dc=michaelpapet,dc=com.  All the configs sit under that branch.

Step 2: /etc/dhcp/dhcpd.conf

Since we are letting Debian install the server, the slapd package maintainer allows anonymous search “out of the box.” the /etc/dhcp/dhcpd.conf doesn’t need a username and password.  If you want, you can do that.

ldap-server                 "localhost";
ldap-port                   389;
# We do an anonymous bind
# ldap-username             "cn=directorymanagerloginname";
# ldap-password             "mypassword";
ldap-base-dn                "ou=dhcp,dc=michaelpapet,dc=com";
ldap-method                 static;
ldap-debug-file             "/var/log/dhcp-ldap-startup.log";
ldap-dhcp-server-cn         "cn=server,ou=dhcp,dc=michaelpapet,dc=com"


There are two options for ldap-method.  Static reads the ldap directory and writes a config based on the directory contents.  Dynamic reads the ldap directory as needed. This has bad consequences if you are trying to debug your config with a cat /var/log/dhcp-ldap-startup.log.  Some of the config wil be “missing” on startup.  But, if a client somehow triggers a search for all settings, then the setting would be returned.

Order matters in the LDAP tree for the dhcpd server.  AFAICT, the easiest way to set it up is to adhere to the parent/child relationships mentioned below.

Step 3: Add the service.

Then, per the instructions, you add the service.  Note that the service has to correctly reference the server.  And, the server has to  correctly reference the service path.  The URL above shows that pretty clearly.

Also worth noting, your global DHCP server options go under the service.  For example, authoritative, classes, subclass and more are added using the dhcpStatements attribute.

Step 4: Add the subnet setting beneath the service

The subnet, and related settings goes in the subnet objectClass.  For example, default-lease-time would go into the dhcpOption attribute.  You will probably add a few.

Optional Step 5: Add pools as children to subnet and classes

If you want pools, then set them up under the subnet.  If you want allow/deny rules, then add something to the dhcpPermitList attribute like

allow members of "my-hosts"

Which leads us to the dhcpClass attribute.  I got it to work by using the attribute below the dhcpServer objectClass. In LDAP-ese cn=my-hosts,cn=MainServer,ou=dhcp,dc=michaelpapet,dc=com.

Then the attribute dhcpSubClassesdn points to a group of mac addresses.  For example, ou=my-macs,cn=MainServer,ou=dhcp,dc=michaelpapet,dc=com.

Under ou=my-macs, add objectClass dhcpSubClass.  As an example, attribute dhcpClassData = my-macs, cn=1:0a:1b:2c:3d:4e:5f NOTE THE 1: prefix to the mac address.  My limited understanding this means it identifies the MAC address.

Step 6: Test
dhcpd -f -d

If there were no problems, then the server is running, not forked.  You can then see it working to be sure it does what you want.

To see what config the server is running,

cat /var/log/dhcp-ldap-startup.log

Again, be sure the dhcpd server is running in static mode in /etc/dhcp/dhcpd.conf to see the config that is/isn’t running.

ISC DHCPD with LDAP Config Part 1

The average environment with one or two admins can probably do just fine with the /etc/dhcp/dhcpd.conf flat file.  Syntax isn’t the challenge.  As soon as you get into access control and many devices/subnets, the flat file gets long and that’s when mistakes happen. LDAP backend makes everything easier.

It’s important to note at the outset I’m running Debian Testing,  February 2016, Debian’s repositories.  Maybe by the time you read this, things have changed.  If you are new to LDAP, it’s a learning curve.  This first part might take a while.  I’m rushing through setup.

Get Ready

I like using an LDAP browser.  I like jxplorer because it supports the modern cn=config editing.  On Windows, the popular (and useful) ldapadmin is good, but does not support cn=config editing.

Get Packages
apt-get install slapd isc-dhcp-server-ldap isc-dhcp-server

Debian’s packages set up slapd (that’s the openldap server) for you, prompting you for an admin password.  It seems like they want you to use the command line tools ldapsearch and ldapmodify to edit cn=config. Check out what’s configured with this.

ldapsearch -Y EXTERNAL -H ldapi:/// -b "cn=config"


This is probably bad advice: I set a cn=config password.  A little later on, I can use jxplorer to change the config.

To start: use ldapsearch to find the config database.

ldapsearch -Y EXTERNAL -H ldapi:/// -b "cn=config"

Then you write the file that adds the password.

dn: olcDatabase={0}config,cn=config #be sure config is database 0.  
#You looked carefully at the output of ldapsearch, right?
changetype: modify
add: olcRootPW
olcRootPW: {SSHA}some_hashed_password #this was made with slappasswd

Then ldapmodify to add a password field. ldapmodify -c -Y EXTERNAL -H ldapi:/// -f /home/thatsme/pwd.ldif

If you aren’t familiar with ldif syntax, that should take a while to learn.


Debian’s package includes the schema file.  Openldap’s cn=config doesn’t support old-school schema files and that’s what the Debian package maintainer ships.  So, you have to find someone who has converted it for you if you don’t want to do a bunch of search/replace.

ldapadd -Y EXTERNAL -H ldapi:/// -f /etc/ldap/schema/dhcp.ldif

This is a good stopping place.  Next article, the ISC DHCP server setup.