DUNDI and dynamic peers

I have a configuration in place with Asterisk servers in 4 remote locations and a fifth at a main office. I need to route “outside” calls to the main office and allow “remote to remote” calls. I am using IAX and DUNDI over the public internet which works quite well. My problem is that all offices are small and use DSL connections with dynamic addresses assigned by the different ISPs. I have assigned a dynamic dns host name for the main office server but not the remotes. I then run a dns server on the main office box and allow the remotes to update this dns database on a scheduled basis with some simple php scripts and NSUPDATE. All of this works, but it seems there must be a simpler way since with IAX registrations, the main office server knows the IP address of each remote boxes. I see a “host = dynamic” option in the dundi.conf file, but I have yet to see any reference on how this is used either stand alone or through IAX registration. Can anyone shed some light on this?