Router Configuration for DNS service

When we open the port in Router for DNS service,we should open port 53 to accept connection and enable support of both TCP and UDP protocal for DNS service . When I enable only TCP protocal support for DNS in Router,the DNS that runs in my server dosen't response for the DNS query made by the client.Then I have enabled both UDP/TCP protocal support for DNS in router,this makes DNS server to work properly.This is because DNS service uses both UDP and TCP protocal . DNS uses UDP for reciving query from the client and TCP for transfering the zone information of the Domain .
       UDP is used for transfering the packets which is less than 512 bytes (DNS query from client usally less than 512 bytes) and TCP is used for transferring the packets that are only greater than 512 bytes of size (zone information transfered from the server will be usally greater than 512 bytes) .This makes the DNS service to use both UDP and TCP,so make sure that your router supports both UDP and TCP for DNS service.