A gross hack.
The TELUS Wi-Fi Hub router provided by my ISP does not allow me to set a DNS server address that is within the local network to be sent in DHCP Offers. This is undocumented and if one tries to do so the router will just save the setting but ignore it and send whatever it likes (thanks Telus, for all the hours I spent wondering why the hell my split DNS didn't work).
So, solution is: disable DHCP on the ISP router and move it to the local DNS server that I'm already using. The problem is, the router doesn't allow you to disable DHCP if it isn't in bridge mode. What I can do is restrict the DHCP assignment range to a single IP.
Now I have most DHCP assignments coming from my preferred server, but occasionally one device gets an offer from the ISP router and happily takes it, creating a weird partition.
That's where this lil guy comes in, it constantly sends out DHCP requests until it gets the particular 'bad' offer and squats on it immediately forever :)
That costs money. Also I'm a bit bored.