Network ports act as the digital gateways that allow applications and services to communicate across devices. Where the internet is the information highway, the ports are the off-ramps. Each port has a unique identification number, and different services communicate through different ports. When a port is blocked, the services that use the port no longer communicate with the internet.
Port 53 acts as the foundation for an organization’s digital infrastructure by supporting connectivity with the Domain Name System (DNS) service. When this port loses functionality, organizations face service outages and business disruptions. Attackers recognize the important role that Port 53 plays in maintaining service availability and often target it.
According to Vercara’s DDoS Analysis Report for October 2025, 68.93% of all DDoS attacks targeted Port 53. By understanding what Port 53 is and how its availability impacts business operations, organizations can implement robust security measures to mitigate risk.
What is Port 53?
Port 53 is the well-known port number officially designated by the Internet Assigned Numbers Authority (IANA) for the Domain Name System (DNS) service. In networking, a port is a virtual endpoint in an operating system, and the combination of an IP address and a port number creates a unique destination for network traffic.
What Is Port 53 Mainly Used For?
Port 53 acts as the dedicated listening post for all DNS-related communications, making it the channel through which domain name-to-IP address translations occur. When a user types a website URL like www.vercara.digicert.com into their browser, the computer uses Port 53 to send the query to a DNS server that will find the corresponding IP address.
Is Opening Port 53 Safe?
While opening Port 53 is a requirement for internet access, it also introduces security risks. For a typical user or client machine, organizations must allow outbound traffic on Port 53 to send DNS queries. For a DNS server to receive and answer the queries, organizations must allow inbound traffic on Port 53. However, because DNS is a publicly accessible service, Port 53 is a prominent target for cyberattacks.
Is Port 53 TCP or UDP?
Port 53 uses both the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) as a DNS design feature that balances speed and efficiency with reliability and data integrity.
Why Does DNS Use Both TCP and UDP on Port 53?
The decision to use both TCP and UDP for DNS is a deliberate engineering trade-off designed to optimize performance and ensure reliability.
Why Use UDP?
UDP is a connectionless protocol, sending data packets without first establishing a dedicated connection. With lower overhead and faster communication, UDP works well for small, frequent, transactional standard DNS queries. When a client sends a single UDP packet with a question, the server responds with a single UDP packet containing the answer. The client’s resolver is responsible for re-transmitting queries if a packet is lost.
Why Use TCP?
TCP is a connection-oriented protocol that establishes a three-way handshake before transferring any data, guaranteeing that all packets arrive in order and without errors. While more reliable than UDP, TCP has higher overhead costs and slower initial setup.
DNS uses TCP on Port 53 for specific scenarios that require data integrity and completeness, including:
- Zone transfers: A secondary DNS server replicates the entire DNS record database from a primary server, requiring TCP’s reliable nature to manage the large zone files and ensure accurate, complete transfers.
- Large DNS Responses: Although extensions have increased the original UDP packet limit of 512 bytes, responses exceeding the allowed size require TCP, like those containing many records or large DNSSEC keys.
How Does Port 53 Work?
When a user initiates an action that requires accessing a domain, the following events occur in this order:
- Query Initiation: The user’s device, called the client, creates a DNS query to resolve a domain name.
- Sending the Query: The client’s operating system sends this query as a UDP packet from a random high-numbered port to a pre-configured DNS resolver on its destination Port 53.
- Recursive Resolution: The resolver receives the query on Port 53, using either answer from the cache or performing a recursive lookup that queries other DNS servers that all communicate over Port 53.
- Receiving the Response: Once the authoritative server provides the IP address, the resolver sends the answer back to the client to the original source port.
- Connection Establishment: With the IP address now known, the user’s browser or application can establish a direct TCP connection to the web server on its respective port, like Port 443 for HTTPS.
What are the Common Attacks on Port 53?
Port 53 poses a Distributed Denial of Service (DDoS) attack risk as threat actors recognize the important role it plays in maintaining service and operations. Some typical attack types include:
DNS Amplification and Reflection Attacks
Malicious actors spoof the victim’s IP address and send a small DNS query to numerous open DNS resolvers that amplifies the initial bandwidth. After the resolvers receive the query on Port 53, they send the large response to the victim’s spoofed IP address which overwhelms the network with unwanted DNS traffic.
NXDOMAIN DDoS Attacks (DNS NXDOMAIN Flood)
Trying to overwhelm the authoritative DNS server, attackers send high query volumes for non-existent subdomains, requiring the server to perform a full lookup process. Processing the invalid requests consumes CPU and memory, slowing down or crashing the server and preventing it from responding to legitimate queries.
TCP Port 53 Connection Exhaustion Attacks
In a TCP SYN flood attack, the attacker sends a large number of TCP SYN packets to the DNS server on Port 53. The server allocates resources for each incoming connection and waits for the final ACK packet, which never arrives. With a large volume of half-open connections exhausting the server’s connection table, it can no longer accept new, legitimate TCP connections.
DNS Flood Attack
Attackers use a botnet to send an overwhelming volume of legitimate-looking DNS query packets to a target DNS server. The sheer volume of traffic uses up the server’s network bandwidth and processing capacity, making it unable to respond to valid queries from real users.
5 Best Practices for Securing Port 53
Since Port 53 is critical to using the internet and web-based applications, organizations need to leave it open, rather than closing it, and implement the appropriate security controls.
Lock Down DNS Recursion and Restrict Who Can Query
The “open recursion” configuration creates risks since anyone can use the server to bounce traffic around. Organizations should start by restricting DNS queries to known, trust IP addresses.
Rate-Limit, Filter, and Monitor DNS Query Patterns
Rate limiting prevents too many requests from hitting the system at once while filtering blocks suspicious traffic before it becomes a problem. Security teams should monitor DNS logs to identify spikes or unusual requests.
Use Protective DNS and DDoS Mitigation Upstream
To implement protections before traffic reaches the DNS server, organizations can implement:
- Protective DNS service that blocks known malicious domains.
- DDoS mitigation layer to absolute waves of attack traffic before they hit the network.
Build Redundancy and Global Distribution
Spreading DNS servers across multiple regions and networks enables organizations to maintain service availability even if one area experiences an outage or an attack. By implementing failover and redundancy, organizations enhance resilience and can meet service level agreements (SLAs).
Create Response Plans and Test Regularly
Organizations should incorporate DNS security and mitigation into their incident response plans. They should also consider running tabletop exercises to test their DNS incident response capabilities.
UltraDNS and UltraDDoS Protect: Improve Port 53 Security
UltraDNS provides globally distributed, high-performance authoritative DNS that resists single points of failure, while UltraDNS² adds a second, fully independent DNS network for true redundancy. Together, they ensure fast resolution and continuous uptime, even during large-scale attacks or outages.
To further reduce risk, UltraDDoS Protect filters and scrubs malicious traffic before it reaches the network, stopping volumetric floods at the edge. For intelligent failover and load balancing, UltraDNS gives organizations the visibility, control, and stability needed to keep services running smoothly.
