Adding DNS Records From DHCP Leases on a MikroTik Device

• Tue, Jan 17, 2017
• 3-minute read

In my personal endeavours to sharpen my development skills in Golang I’ve written a small RESTful API to take a domain name and IP address and update Route53 with these values. I’ve also had an old friend help out with code-review and bounce ideas off - it definitely still needs some polish.

This isn’t terribly ground breaking or complicated - however, when you couple this with MikroTiks powerful scripting language you can run a script to automatically replicate the hostname sent by the client in the DHCP request into DNS. The Host Name option is defined in RFC1533

The application is named DRMU (pronounced Dr. Moo) - it’s the Dynamic Route53 MikroTik Updater, although technically it doesn’t really have anything to do with MikroTik in itself. It leverages the following libraries:

• AWS SDK - interface to Route53
• Viper - very easy to use configuration library
• Goji - lightweight Web framework that has URL routing

Installing and Building DRMU

It’s as easy as the following commands:

• go get -u github.com/michaelmcallister/drmu
• go build src/github.com/michaelmcallister/drmu/drmu.go

Configuring DRMU

The config file (./config/app.yaml) must be updated with the following values:

• hostedzone - Zone ID for the Route53 zone you wish to update (eventually I’ll integrate a way to look this up within the app)
• listendaddress - what address/interface to listen on (generally 0.0.0.0 if you want to access it from outside the machine)
• listenport - the port for the listenaddress

Running and Using DRMU

Once configuration is defined you execute the binary as you would anything other binary in your OS (Root is not necessary for binding to high-range ports)

to update a record you need to hit the point as such:

http://listenaddress:listenport/drmu/update/$domain/$ipaddress

DRMU is configured to UPSERT records - so changing the IP address will update the A record if it already exists (or create it, if it doesn’t)

Integrating with MikroTik

So now we’ve established a HTTP REST endpoint we can leverage MikroTik script to update DNS records based on the leases.

This simple script running on a schedule will hit the endpoint, which will of course update records

# Domain to be added to your DHCP-clients hostname
:local topdomain;
:set topdomain "dhcp.<DOMAIN>.com";

# Set variables to use
:local hostname;
:local hostip;
:local free;

/ip dhcp-server lease ;
:foreach i in=[find] do={
 /ip dhcp-server lease ;
 :if ([:len [get $i host-name]] > 0) do={
   :set free "true";
   :set hostname ([get $i host-name] . "." . $topdomain);
   :set hostip [get $i address];
   :if ($free = true) do={
     :put ("Adding: " . $hostname . " : " . $hostip ) ;
     /tool fetch url="http://10.1.3.105:8000/drmu/update/$hostname/$hostip"
   }
 }
}