How can I configure my own DNS server that gets updates from other DNS servers?

Hey guys I have a question. I have 20 remote locations that I need to have VERY limited access to the internet. There are only about 10 sites that I want them to be allowed to access. I have been using the HOSTS file so far and it works albeit very painfull to log into 20 remote locations to add site to the HOSTS file but now I have to resolve certain subdomain names from dyndns for their updater tool to work and they are resolving to a DYNAMIC ip address, so I can't keep using the HOSTS file to hardcode these ip addresses. At the same time I do not want to open up the internet by giving them full DNS. I want to be able to allow only a handful of sites that resolve using my own DNS server that will be accessible to them through a static IP address from my head office. How difficult is it to set up a DNS server that has very few domains that resolve or is there a better way to do this?

Thanks