DNS over TCP

Introduction #

DNS (Domain Name System) over TCP (Transmission Control Protocol) is an alternative mode of communication for DNS queries and responses. This documentation provides an informative overview of DNS over TCP, its characteristics, advantages, and its role in the DNS resolution process.

DNS over TCP #

DNS over TCP is an extension to the traditional DNS communication protocol that operates on the transport layer of the TCP/IP protocol stack. While DNS over UDP (User Datagram Protocol) is the default and commonly used mode, DNS over TCP provides an alternative method for transmitting DNS data.

Characteristics and Benefits #

DNS over TCP offers several characteristics and benefits:

1. Reliability: TCP provides reliable, connection-oriented communication. DNS over TCP ensures that DNS queries and responses are transmitted without loss or corruption, as TCP employs mechanisms such as sequence numbers and acknowledgments.
2. Large Packet Size: Unlike DNS over UDP, which has a maximum payload size of 65,507 bytes, DNS over TCP allows for larger DNS messages. This is particularly useful when dealing with large DNS responses or DNSSEC (DNS Security Extensions) signed responses, which can exceed the UDP size limit.
3. Handling of Fragmentation: DNS over TCP mitigates fragmentation issues that can occur with UDP. TCP automatically handles fragmentation and reassembly of DNS packets, ensuring that large DNS messages are correctly transmitted and received without the need for additional processing.
4. TCP Connection Management: TCP employs a connection-oriented model, establishing and maintaining a persistent connection between the client and server. This can be advantageous in scenarios where a reliable and continuous communication channel is required.

Use Cases #

DNS over TCP is commonly used in the following scenarios:

1. Large DNS Responses: DNS over TCP is often used when DNS responses exceed the maximum UDP packet size. This occurs with DNS responses containing a large number of resource records or DNSSEC-signed responses.
2. DNS over Transport Layer Security (TLS): DNS over TCP can be used as the underlying transport protocol for DNS over TLS (DoT). DoT provides encryption and authentication of DNS traffic, securing the communication between clients and servers.
3. Firewall Restrictions: Some network configurations or security policies may restrict or block DNS over UDP traffic. In such cases, DNS over TCP can be used as an alternative method to ensure DNS communication.

Considerations #

When deploying DNS over TCP, it is important to consider the following:

1. Increased Overhead: TCP introduces additional overhead compared to UDP due to the connection-oriented nature and reliability mechanisms. This may slightly impact the overall performance and response times of DNS queries.
2. Port Availability: DNS over TCP uses port 53, the same port traditionally used for DNS over UDP. However, it is crucial to ensure that network firewalls and security policies allow DNS over TCP traffic on port 53 to ensure seamless communication.
3. TCP Connection Setup Time: Establishing a TCP connection requires an additional step compared to UDP. The time taken to set up a TCP connection may introduce a slight delay in the DNS resolution process.

Conclusion #

DNS over TCP provides a reliable and alternative mode of DNS communication compared to the default DNS over UDP. It offers benefits such as reliability, larger packet sizes, and effective handling of fragmentation. DNS over TCP is commonly used for large DNS responses, DNS over TLS, and in scenarios where UDP traffic may be restricted. Understanding the characteristics and considerations of DNS over TCP helps in optimizing DNS infrastructure for specific use cases.