# Creates static DNS entres for DHCP clients in the named DHCP server.
# Hostnames passed to DHCP are appended with the zone.

# Set the first two variables according to your installation.
:local dhcpserver "dhcp1"
:local zone "home.example.com"

# Set the TTL to the scheduler frequency for this script.
:local ttl "00:05:00"

# Clear old static DNS entries matching the zone and TTL.
/ip dns static
:foreach dnsrecord in=[find where name ~ (".*\\.".$zone) ] do={
	:local fqdn [ get $dnsrecord name ]
	:local hostname [ :pick $fqdn 0 ( [ :len $fqdn ] - ( [ :len $zone ] + 1 ) ) ]
	:local recordttl [get $dnsrecord ttl]
	:if ( $recordttl != $ttl ) do={
		:log debug ("Ignoring DNS record $fqdn with TTL $recordttl")
	} else={
		/ip dhcp-server lease
		:local dhcplease [ find where host-name=$hostname and server="$dhcpserver"]
		:if ( [ :len $dhcplease ] > 0) do={
			:log debug ("DHCP lease exists for $hostname in $dhcpserver, keeping DNS record $fqdn")
		} else={
			:log info ("DHCP lease expired for $hostname, deleting DNS record $fqdn")
			/ip dns static remove $dnsrecord
		}
	}
}

# Create or update static DNS entries from DHCP server leases.
/ip dhcp-server lease
:foreach dhcplease in=[find where server ~ ("$dhcpserver")] do={
	:local hostname [ get $dhcplease host-name ]
	:if ( [ :len $hostname ] > 0) do={
		:local dhcpip [ get $dhcplease address ]
		:local fqdn ( $hostname . "." . $zone )
		/ip dns static
		:local dnsrecord [ find where name=$fqdn ]
		:if ( [ :len $dnsrecord ] > 0 ) do={
			:local dnsip [ get $dnsrecord address ]
			:if ( $dnsip = $dhcpip ) do={
				:log debug ("DNS record for $fqdn to $dhcpip is up to date")
			} else={
				:log info ("Updating DNS record for $fqdn to $dhcpip")
				/ip dns static remove $dnsrecord
				/ip dns static add name=$fqdn address=$dhcpip ttl=$ttl
			}
		} else={
			:log info ("Creating DNS record for $fqdn to $dhcpip")
			/ip dns static add name=$fqdn address=$dhcpip ttl=$ttl
		}
	}
}