Debian DHCP server failover

Before we start I assume you followed previous two posts: setup and dynamic-dns. Our primary dhcp server located at 192.168.10.4 and secondary at 192.168.10.5.

Primary DHCP Server

1. You need declare failover section that identifies the primary dhcp server (file /etc/dhcp/dhcpd.conf).

   failover peer "dhcp-failover" {
     primary; # declare this to be the primary server
     address 192.168.10.4;
     port 647;
     peer address 192.168.10.5;
     peer port 647;
     max-response-delay 30;
     max-unacked-updates 10;
     load balance max seconds 3;
     mclt 1800;
     split 128;
   }
   

2. Failover peer needs to be referenced by concrete subnet:

   subnet 192.168.10.0 netmask 255.255.255.0 {
     pool {
       # In order to turn off failover just comment out
       # the line below
       failover peer "dhcp-failover";
       range 192.168.10.41 192.168.10.254;
     }
     option subnet-mask 255.255.255.0;
     option broadcast-address 192.168.10.255;
     option routers gw1.dev.local;
   }
   

Secondary DHCP Server

1. Failover secondary peer declaration (file /etc/dhcp/dhcpd.conf):

   failover peer "dhcp-failover" {
     secondary; # declare this to be the secondary server
     address 192.168.10.5;
     port 647;
     peer address 192.168.10.4;
     peer port 647;
     max-response-delay 30;
     max-unacked-updates 10;
     load balance max seconds 3;
   }
   

2. subnet 192.168.10.0 netmask 255.255.255.0 {
     pool {
       # In order to turn off failover just comment out
       # the line below
       failover peer "dhcp-failover";
       range 192.168.10.41 192.168.10.254;
     }
     option subnet-mask 255.255.255.0;
     option broadcast-address 192.168.10.255;
     option routers gw1.dev.local;
   }
   

That pretty much you need to do. Read more here.