here’s the response:
They’re using DNS resolution as in RFC6334.
https://www.rfc-editor.org/rfc/rfc6334.html
Issue’s been brought up here:
http://forum.mikrotik.com/t/ds-lite-rfc-6333-rfc-6334/124743/1
and here:
http://forum.mikrotik.com/t/dslite-tunnel-is-not-working-fails-rfc6333-and-rfc2473/151902/1