Why DNS uses both TCP and UDP?

-->  DNS and some other Services uses both the TCP and UDP Protocols for working.

--> These two protocols are so different from each other. TCP is a connection-oriented protocol whereas UDP is a connection-less protocol.

--> DNS Servers need to maintain the same database between each other, This is achieved by using Zone Transfer feature.

-->  The Zone Transfer feature of DNS Server always uses TCP protocol. The connection is established between the DNS Server to transfer the zone data and Source and Destination DNS Servers will make sure that data is consistent by using TCP ACK bit.

--> This communication happens between DNS Servers only.

--> A client computer will always send a DNS Query using UDP Protocol over Port 53.

-->  If a client computer does not get the response from a DNS Server, it must re-transmit the DNS Query using the TCP after 3-5 seconds of interval.

--> In simple terms,  the communication between DNS Servers done by TCP Protocol whereas the communication between client and DNS Server is done by UDP Protocol.

--> In Firewall Policies we need to allow both TCP and UDP Port 53 to Your DNS Servers.

Md.Kareemoddin

CCIE # 54759