Which port(s) to allow through for DNS server

[Edward Ingram]

I have a DNS server inside our firewall.  People outside the firewall (Universe) need to access this DNS server to resolve requests.  I've tried opening up UDP 53 on the firewall, but requests still aren't going through.  I know the DNS server is working because it fulfills requests sent to it from clients on the inside of the firewall.  Is that the right port to use?