DHCP Load balancing/Failover with two Edubuntu/K12LTSP servers

Note: This document is written from an Edubuntu (Feisty or 7.04) perspective with regard to paths...simply change the paths and configuration file names to reflect those in K12LTSP (as of K12LTSP 5 it was /etc/dhcpd-k12ltsp.conf instead of /etc/ltsp/dhcpd.conf)

The idea behind setting up a DHCP failover/load-balance system for Edubuntu is to use two identical servers to spread the load and provide a little bit of a safety net in the event of the failure of one of the servers. In this situation (you can adjust as you see fit) we will assume that the Edubuntu servers are set up with a single network interface connected to the LAN and that the Edubuntu servers will be supplying the DHCP for the entire LAN. Given that, let's begin...

1.) Setup your Edubuntu servers...I recommend making them as identical as possible.

2.) Upon reboot be sure to STOP the dhcp service on both servers so that we can set up the new configuration

3.) Now...decide which server will be the "primary" and which will be the "secondary" (there is no urgency to this...you simply need to know which is which)

Requirements, caveats, and gotcha's

This will only work with two machines, a primary and a secondary. You will need to decide how to serve home directories and users. This actually works very well in a situation where the users and home directories "live" on a third server. In my case I use a Samba/LDAP server running on an Ubuntu Server 7.04 as my central point of authentication and to host and export the users home directories to the Edubuntu servers. (View a simple diagram here as part of another document: http://www.vcsvikings.org/smbldap-diagram ) The Edubuntu servers mount the exported /home to /home and thus the home directories appear as though they are a part of the Edubuntu server, but in reality they "live" on the Samba/LDAP server. The Samba/LDAP server also is a Windows primary domain controller and allows authentication from Windows servers and clients. In my case, NO users or user data lives on the Edubuntu servers, thus making upgrading of these servers very easy. You can find out more about how to set up your own Samba/LDAP server using the easy "smbldap-installer" by visiting http://www.majen.net/smbldap I wrote the original how-to several years ago, but Matt Oquist used his fantastic scripting abilites to take the original 36 page how-to and create a simple easy-to-use script that will set up a Samba/LDAP server for you in minutes! You can also set up slave LDAP servers and BDC's with the smbldap-installer script as well! So...with all that in mind...let's get started!

Let's begin with the "primary" dhcp server

We need to set up the configuration files for this server. I will provide the samples from my own installation...obviously you will need to tweak it for your situation. I also make liberal use of "includes" in an effort to make it easy to "scp" files from one server to another as I make changes. With some exceptions, these files are generally the same between the two servers.

Let's start with the dhcpd.conf file located at /etc/ltsp/dhcpd.conf

Open it with your favorite editor. For simplicity sake...we will use gedit

sudo gedit /etc/ltsp/dhcpd.conf

Below is a sample dhcpd.conf file (in my case, the primary server is 10.0.14.253...the LAN has two gateways also running DNS and they are 10.0.0.1 and 10.0.10.10)

NOTE: this file CANNOT be copied "as-is" to the "secondary" server as it is indigenous to the "primary" server ONLY

    default-lease-time            521600;
    max-lease-time                521600;
    ddns-update-style none;
    allow booting;
    authoritative;
    include "/etc/ltsp/dhcpd.failover";
    include "/etc/ltsp/dhcpd.ltspboot";
    include "/etc/ltsp/dhcpd.reserved";
    include "/etc/ltsp/dhcpd.subnet";
    option subnet-mask            255.255.240.0;
    option broadcast-address      10.0.0.255;
    option routers                10.0.0.1, 10.0.14.253, 10.0.10.10;
    option domain-name-servers    10.0.14.253, 10.0.0.1;
    option domain-name            "mydomain.com";
    option option-128 code 128 = string;
    option option-129 code 129 = text;
    option option-221 code 221 = text;
    use-host-decl-names       on;
    option log-servers        10.0.14.253;

Now for the first "include" file /etc/ltsp/dhcpd.failover...let's create the file

sudo gedit /etc/ltsp/dhcpd.failover

Below is my sample dhcpd.failover file for the "primary" server (you can copy this and put in your own IP addresses...the name "dhcp" is fine as it's not a hostname or anything...note that my secondary server is 10.0.15.253)

NOTE: this file CANNOT be copied "as-is" to the "secondary server" as this file is for the "primary" server ONLY...the one on the "secondary" server will be a little different.

#############################
#Load Balancing and Failover#
#############################

failover peer "dhcp" {
       primary;
       address 10.0.14.253;
       port 519;
       peer address 10.0.15.253;
       peer port 520;
       max-response-delay 30;
       max-unacked-updates 10;
       mclt 3600;
       split 128;
       load balance max seconds 3;
}

##############################
###End of Failover configs####
##############################

Ok, let's move on to the next "include"...the dhcpd.ltspboot file

sudo gedit /etc/ltsp/dhcpd.ltspboot

Below is my dhcpd.ltspboot file which includes the information for the thin-clients to get their boot image (if you are using Edubuntu 7.04 "Feisty Fawn" you can copy this exactly as it is)

NOTE: this file CAN be copied to the "secondary" server "as-is"

option root-path "/opt/ltsp/i386";

if substring( option vendor-class-identifier, 0, 9 ) = "PXEClient" {
        filename "/ltsp/i386/pxelinux.0";
}
else{
        filename "/ltsp/i386/nbi.img";
}

Now let's work on the next "include" file /etc/ltsp/dhcpd.reserved (this is where the reserved IP addresses are assigned)

sudo gedit /etc/ltsp/dhcpd.reserved

This file may be quite large depending on the number of "reserved" IP addresses you have. Reserved IP addresses are common for such things as servers, printers, wireless access points, and other machines that you want to have the SAME IP address at all times.

Below is a short sample of a dhcpd.reserved file

NOTE: this file CAN be copied to the "secondary" server "as-is"

host workstation001.mydomain.com {
        hardware ethernet 00:03:93:21:55:99;
        fixed-address 10.0.0.201;
    }
    host workstation002.mydomain.com {
        hardware ethernet 00:03:93:21:41:9d;
        fixed-address 10.0.0.202;
    }
    host workstation003.mydomain.com {
        hardware ethernet 00:03:93:20:f0:43;
        fixed-address 10.0.0.203;
    }
    host workstation004.mydomain.com {
        hardware ethernet 00:03:93:21:96:f9;
        fixed-address 10.0.0.204;
    }
    host workstation005.mydomain.com {
        hardware ethernet 00:03:93:21:50:dd;
        fixed-address 10.0.0.205;
    }
    host workstation006.mydomain.com {
        hardware ethernet 00:03:93:21:a8:81;
        fixed-address 10.0.0.206;
    }
    host workstation007.mydomain.com {
        hardware ethernet 00:03:93:21:b9:29;
        fixed-address 10.0.0.207;
    }
    host workstation008.mydomain.com {
        hardware ethernet 00:03:93:20:ef:39;
        fixed-address 10.0.0.208;
    }

Now let's work on the last "include" file... /etc/ltsp/dhcpd.subnet

sudo gedit /etc/ltsp/dhcpd.subnet

This file is where you declare your subnet and range for DHCP

NOTE: this file CANNOT be copied "as-is" to the "secondary server" as this file is for the "primary" server ONLY...the one on the "secondary" server will be a little different.

Below is a sample from my server

  subnet 10.0.0.0 netmask 255.255.240.0 {
     pool {
        failover peer "dhcp";
        range 10.0.7.1 10.0.9.252;
        deny dynamic bootp clients;
        }


        use-host-decl-names       on;
        option log-servers        10.0.14.253;
}

Believe it or not, that's it for the "primary" server...now let's move on to the "secondary" server

Let's set up the "secondary" dhcp server

Let's start again with the dhcpd.conf file located at /etc/ltsp/dhcpd.conf

Once again we will use gedit, however, you can use your favorite editor.

sudo gedit /etc/ltsp/dhcpd.conf

Below is a sample dhcpd.conf file (in my case, the secondary server is 10.0.15.253...the LAN has two gateways also running DNS and they are 10.0.0.1 and 10.0.10.10)

NOTE: this file CANNOT be copied "as-is" from the "primary" server as it is indigenous to the "secondary" server ONLY

default-lease-time            521600;
max-lease-time                521600;
ddns-update-style none;
allow booting;
authoritative;

include "/etc/ltsp/dhcpd.failover";
include "/etc/ltsp/dhcpd.ltspboot";
include "/etc/ltsp/dhcpd.reserved";
include "/etc/ltsp/dhcpd.subnet";

option subnet-mask            255.255.240.0;
option broadcast-address      10.0.0.255;
option routers                10.0.0.1, 10.0.15.253, 10.0.10.10;
option domain-name-servers    10.0.15.253, 10.0.0.1;
option domain-name            "mydomain.com";
option option-128 code 128 = string;
option option-129 code 129 = text;
option option-221 code 221 = text;
use-host-decl-names       on;
option log-servers        10.0.15.253;

Now for the first "include" file /etc/ltsp/dhcpd.failover...let's create the file

sudo gedit /etc/ltsp/dhcpd.failover

Below is my sample dhcpd.failover file for the "primary" server (you can copy this and put in your own IP addresses...the name "dhcp" is fine as it's not a hostname or anything...note that my "primary" server is 10.0.14.253)

NOTE: this file CANNOT be copied "as-is" from the "primary server" as this file is for the "secondary" server ONLY...the one on the "primary" server will be a little different. (note the changes in IP addresses to reflect the fact that this is the "secondary" server AND the fact that the last few lines that existed in this file on the "primary" server are now missing...they are not necessary on the "secondary" server)

#############################
#Load Balancing and Failover#
#############################

failover peer "dhcp" {
        secondary;
        address 10.0.15.253;
        port 520;
        peer address 10.0.14.253;
        peer port 519;
        max-response-delay 30;
        max-unacked-updates 10;
}

##############################
###End of Failover configs####
##############################

Ok, let's move on to the next "include"...the dhcpd.ltspboot file

sudo gedit /etc/ltsp/dhcpd.ltspboot

Below is my dhcpd.ltspboot file which includes the information for the thin-clients to get their boot image (if you are using Edubuntu 7.04 "Feisty Fawn" you can copy this exactly as it is) This file will most likely be identical on both servers.

NOTE: this file CAN be copied from the "primary" server "as-is"

option root-path "/opt/ltsp/i386";

if substring( option vendor-class-identifier, 0, 9 ) = "PXEClient" {
        filename "/ltsp/i386/pxelinux.0";
}
else{
        filename "/ltsp/i386/nbi.img";
}

Now let's work on the next "include" file /etc/ltsp/dhcpd.reserved (this is where the reserved IP addresses are assigned)

sudo gedit /etc/ltsp/dhcpd.reserved

This file will be identical between the "primary" and "secondary" servers...you can copy the one from the "primary" if you wish (in fact to avoid errors I recommend it...and I recommend ALWAYS working with the one on the "primary" server and then copying to the "secondary" server as good practice)

Below is a short sample of a dhcpd.reserved file

NOTE: this file CAN be copied from the "primary" server "as-is"

host workstation001.mydomain.com {
        hardware ethernet 00:03:93:21:55:99;
        fixed-address 10.0.0.201;
    }
    host workstation002.mydomain.com {
        hardware ethernet 00:03:93:21:41:9d;
        fixed-address 10.0.0.202;
    }
    host workstation003.mydomain.com {
        hardware ethernet 00:03:93:20:f0:43;
        fixed-address 10.0.0.203;
    }
    host workstation004.mydomain.com {
        hardware ethernet 00:03:93:21:96:f9;
        fixed-address 10.0.0.204;
    }
    host workstation005.mydomain.com {
        hardware ethernet 00:03:93:21:50:dd;
        fixed-address 10.0.0.205;
    }
    host workstation006.mydomain.com {
        hardware ethernet 00:03:93:21:a8:81;
        fixed-address 10.0.0.206;
    }
    host workstation007.mydomain.com {
        hardware ethernet 00:03:93:21:b9:29;
        fixed-address 10.0.0.207;
    }
    host workstation008.mydomain.com {
        hardware ethernet 00:03:93:20:ef:39;
        fixed-address 10.0.0.208;
    }

Now let's work on the last "include" file... /etc/ltsp/dhcpd.subnet

sudo gedit /etc/ltsp/dhcpd.subnet

This file is where you declare your subnet and range for DHCP

Below is a sample from my "secondary" server (Note the change in the log servers IP address)

  subnet 10.0.0.0 netmask 255.255.240.0 {
     pool {
        failover peer "dhcp";
        range 10.0.7.1 10.0.9.252;
        deny dynamic bootp clients;
        }


        use-host-decl-names       on;
        option log-servers        10.0.15.253;
}

Well that's it! Now you'll need to restart the dhcpd service on each server.

/etc/init.d/dhcp3-server start

If everything has been done correctly...it will start without any errors and you'll now have a really cool dhcp load-balance/failover solution!