What is Port 53?

by Colin Cohen | Published on January 26, 2024

Quick Definition: The Internet Assigned Numbers Authority (IANA) assigned port 53 to DNS. Web clients communicate with DNS servers over this port to translate domains to device-reachable IP addresses.

Why is Port 53 Important?

Port 53 is important because it facilitates DNS. Without DNS, you couldn’t enter domains such as google.com into web browsers, as DNS translates them into the IP addresses required by devices on the Internet.

Port 53’s Role in Internet Communication

Devices on the Internet do not understand domains such as google.com. They only understand IP addresses, which are strings of numbers that uniquely identify every device on the Internet.

When someone types google.com into their browser's address bar, the browser contacts a DNS server over port 53 to translate this human-readable address into a device-reachable IP address, which it then uses to communicate with Google’s web server.

You can look at DNS as the phonebook of the Internet.

Distinction Between UDP Port 53 and TCP Port 53

The default transport protocol for port 53 is UDP. This is because UDP is faster than TCP. Devices, though, will usually use TCP as a fallback if they can’t use UDP for some reason, such as if the packet size is too large for UDP communication.

While the use of UDP in DNS is more typical, the use of TCP is becoming increasingly more common.

How is DNS Related to Port 53?

Port 53 is the default port for DNS. It is the port most web applications expect to find DNS servers, which they use to translate domains into IP addresses.

Explanation of DNS

DNS lies at the foundation of the web as we know it. As devices only understand IP addresses, and humans have trouble remembering IP addresses, DNS sits between the two. It translates a domain that humans can easily remember into an IP address that devices can easily use.

The Significance of Port 53 in DNS

Most applications running on the Internet expect DNS servers to operate over port 53. If you wanted to use another port, both the DNS servers and the applications that connect to them would have to know and use this other port.

How DNS Queries and Responses Work Through Port 53

This is how DNS queries and responses work over port 53:

When an application needs to resolve a domain into an IP address, it sends the domain to a server known as a DNS recursive resolver.
The resolver queries a DNS root nameserver, which responds with the IP address of a top-level domain DNS server. A top-level domain is the highest domain level, such as .com.
The resolver queries the top-level domain DNS server with the domain, which responds with the IP address of the domain’s nameserver.
The resolver queries the nameserver with the domain, translating it into an IP address.
The resolver sends the IP address to the application to communicate with the server.

What are the Security Risks and Considerations for Port 53?

Distributed denial-of-service (DDoS) attacks are a common security risk associated with port 53. But there are ways you can mitigate this risk.

Common Security Threats Related to Port 53

Port 53 is often subjected to DDoS attacks. This is due to the following:

The heavy use of UDP in DNS
The ability to easily spoof the source packets sent to DNS servers
The abundance of open DNS resolvers

Best Practices for Securing Port 53

You can mitigate DDoS attacks on your DNS servers by doing a combination of the following on your network:

Implementing network ingress filtering
Eliminating open DNS resolvers
Using random ports instead of port 53

The Impact of DNS Attacks on Port 53

DDoS attacks on DNS can prevent users from accessing the servers they want. A variety of DNS DDoS attacks can occur over port 53. These include the following:

Flood attacks such as TCP SYN Flood and NXDOMAIN Flood
Reflection attacks, such as Reflection Amplification
Botnet attacks

How Does Port 53 Apply in Networking?

Devices on a network rely on DNS to communicate with servers. It is what allows web clients to find the servers.

Role of Port 53 in Networking Infrastructure

Port 53 plays a crucial role in networking infrastructure. Without DNS over this port, devices on a network wouldn’t be able to translate human-readable domains into the IP addresses devices use to communicate with servers.

How Port 53 Facilitates Communication Between Devices

Devices on a network need IP addresses to communicate with servers, as these addresses uniquely identify each device so that communication between them can occur. Port 53 facilitates this by enabling communication to the DNS servers that make domain-to-IP address translation possible.

Examples of Services and Protocols Using Port 53

While DNS is the main protocol used over port 53, it’s not the only one. The following other protocols use port 53 as well:

How to Configure and Manage Port 53

To enable DNS on a device, you must first open port 53 on the device’s firewall. How you do this depends on whether you are running Windows or Linux.

Configuring Firewalls for Port 53

To allow access to port 53 on a device, you must configure your firewall to open the port. But you should only do this if the device provides DNS services.

Managing Port 53 in Different Operating Systems

To configure port 53 on a firewall under Windows, you must establish inbound and outbound rules for the port in the Firewall Control Panel. In Linux, you do so using the iptables command.

Troubleshooting Common Issues Related to Port 53

One of the biggest issues related to using port 53 is when the port is unreachable on a given device. To resolve this, you must ensure the port has been properly opened on your firewall.

Conclusion

Web applications use port 53 to communicate with DNS servers. These servers allow applications such as web browsers to translate domains into computer-reachable IP addresses. You can look at these servers as the Internet’s phonebook.