How to set Option 43 in DNSMasq (for Ubiquiti/Unifi devices)

DHCP Option 43, also known as "Vendor-specific option", is a DHCP option that allows the DHCP server to send custom or vendor-specific data to devices that match a given vendor class. The option contains encapsulated vendor-specific option.

It is defined in RFC2132#8.2, which specifies the format for the option.

Ubiquity (unifi devices) uses option 43 (sub-option 1) to send the IP address of the unifi controller to devices to enable L3 adoption. If you use an unifi device as your DHCP server, it’ll be automatically configured for you, otherwise, you have to configure it yourself.

Within DNSMasq, the option is set in the configuration file as follows:

The vendor:ubnt portion indicates that this option should be included when the DHCP request matches the ubnt vendor class. This class is sent by ubiquity devices using option 60, so you don’t have to do anything to set that up.

That’s it. Ubiquity devices will indicate that they request option 43, so they’ll receive this information. If you decide that you want to force the DHCP response to have that information even when the device doesn’t request it, simply change dhcp-option to dhcp-option-force.

If you want to send a vendor-specific encapsulated field to all clients, regardless of the vendor class, simply omit the value of the vendor class in your dhcp option: vendor:,1,value-goes-here.