DNS is a foundational internet service, and as such, it operates over various protocols and ports to ensure speedy and secure domain resolution. In this section, we will delve into the different protocols that DNS can use, including DNS over UDP, DNS over TCP, DNS over HTTPS (DoH), and DNS over TLS (DoT).
DNS over UDP #
Overview #
User Datagram Protocol (UDP) is one of the core protocols used by DNS for querying and receiving responses. UDP is a connectionless protocol, which means that it doesn’t establish a connection before sending data. This makes UDP-based DNS queries fast, as there is minimal overhead.
Port #
DNS typically uses port 53 for UDP. When a DNS query is made, it is sent to the DNS server on UDP port 53.
Considerations #
While UDP is fast, it does have limitations regarding data size. The maximum size of a DNS message over UDP is 512 bytes. For DNS responses that exceed this limit, either the response must be truncated, or an alternative protocol such as TCP must be used.
DNS over TCP #
Overview #
Transmission Control Protocol (TCP) is another core protocol that DNS can use. Unlike UDP, TCP is a connection-oriented protocol, meaning it establishes a connection before sending data. This ensures that data is reliably delivered, but it introduces additional overhead.
Port #
DNS over TCP also typically uses port 53.
Considerations #
DNS over TCP is used for DNS messages that exceed the 512 byte limit of UDP. Additionally, it’s used for tasks like zone transfers (AXFR) between primary and secondary DNS servers. While TCP ensures reliable delivery, it is slower compared to UDP due to the overhead of establishing a connection.
DNS over HTTPS (DoH) #
Overview #
DNS over HTTPS (DoH) is a modern protocol that sends DNS queries and receives DNS responses over HTTP using encryption. This ensures that DNS queries are secure and cannot be easily intercepted or manipulated by third parties.
Port #
DoH usually uses port 443, which is the standard port for HTTPS traffic.
Considerations #
DoH is especially valuable for preserving user privacy, as it prevents ISPs and other network operators from seeing DNS queries. However, DoH can sometimes bypass local DNS policies and can be challenging for network administrators who need visibility into network traffic for security and management.
DNS over TLS (DoT) #
Overview #
DNS over TLS (DoT) is another modern protocol used to encrypt DNS traffic. Unlike DoH, which sends DNS queries inside HTTPS, DoT is more akin to traditional DNS, but with a TLS layer for encryption.
Port #
DoT usually uses port 853.
Considerations #
Like DoH, DoT provides enhanced security and privacy for DNS queries. It is particularly popular in scenarios where the focus is solely on securing traditional DNS, rather than embedding DNS within HTTP traffic.
This section has covered the protocols and ports that DNS uses to function effectively. As the internet evolves, DNS protocols continue to adapt, providing new methods to ensure quick and secure domain resolution. Understanding these protocols is essential for network administrators and security professionals to effectively manage and secure DNS traffic.
