Port number used by DNS/nslookup command

Port number used by DNS (53 for TCP/UDP)

　DNS primarily uses UDP for communication. The reason is that the data exchanged in name resolution is small and we want to reduce overhead.

　When using TCP, data cannot be sent unless a session is established with a three-way handshake. In addition, processing such as acknowledgment and retransmission is performed to ensure reliability, which increases overhead.

　Name resolution queries require immediacy, and above all, it is inefficient to use TCP to exchange small amounts of data.

　Name resolution by DNS is designed so that both inquiries and responses can be performed in 1 packet, up to 512 bytes, due to the limitations of UDP. This 512 bytes limit eliminates problems such as fragmentation and packet arrival order changes.

　TCP port number 53 is used for zone transfer performed by the secondary DNS server to replicate the data held by the primary DNS server.

Your firewall should be configured so that DNS packets are not filtered.

DNS (nslookup command)

　nslookup is an old command that has been used since the beginning of DNS development on UNIX. This is a command to manually run the DNS name resolution function and can be relied upon when troubleshooting.

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