The answer is DNS is mostly UDP Port 53, but as time progresses, DNS will rely on TCP Port 53 more heavily. DNS has always been designed to use both UDP and TCP port 53 from the start1 , with UDP being the default, and fall back to using TCP when it is unable to communicate on UDP, typically when the packet size is too large to push through in a single UDP packet.
When Does DNS Switch to TCP?
The next natural question is: when will DNS messages exceed 512 bytes? Actually, this happens quite often in today’s environment. When DNS was first implemented, the only thing that would be so large that it exceeded the 512-byte limit was a zone transfer, in which one DNS server sends every single resource record in the zone to another machine, usually another DNS server.
In modern DNS systems though, we are increasingly seeing resource record sets (or RRsets) that have a larger combined size. For example, Figure FAQ-5 illustrates querying for www.example.com may yield results such as this (AAAA are IPv6 records):
Or the same query might return the following TXT records, each providing a specific function such as spam detection or site verification as demonstrated in Figure FAQ-6:
If a zone is signed by DNSSEC, it will routinely return large responses due to the cryptographic keys and signatures as shown in Figure FAQ-7:
As more and more people adopt newer features such as IPv6, spam avoidance, and DNSSEC, DNS is more likely to switch to TCP due to the larger response size.
What Happens If TCP Is Blocked?
Whatever the case, when the message size exceeds 512 bytes, it will trigger the ‘TC’ bit (Truncation) in DNS to be set, informing the client that the message length has exceeded the allowed size. In these situations, the client needs to re-transmit over TCP for which the size limit is 64000 bytes. If DNS servers and network environment cannot support large UDP packets, it will cause retransmission over TCP; if TCP is blocked, the large UDP response will either result in IP fragmentation or be dropped completely. The end symptom to the end client is usually slow DNS resolution, or inability to resolve certain domain names at all.
Size Matters: EDNS
You might be wondering where the size limit of 512 bytes come from. The 512-byte UDP payload size is a dependency on IPv4. The IPv4 standard2 specifies that every host must be able to reassemble packets of 576 bytes or less, take away header and other options, that leaves 512 bytes for payload data. This is the reason why there are precisely 13 DNS root servers3 originally: 13 domain names and 13 IPv4 addresses fit nicely into a single UDP packet.
This size limitation was recognized long ago as a problem. In 1999, Extension Mechanism for DNS (EDNS) was proposed, and it has been updated over the years, increasing the size all the way to 4096 bytes, or 4 kilobytes. So, if you are running a reasonably up to date DNS server, the chances of it switching to TCP should be slim(mer).
However, even though EDNS has been around a long time, its support has not been as universal as it should be4 . Some network equipment, such as firewalls, might still make assumptions about DNS packet size. A firewall may drop or reject a large DNS packet, thinking it is an attack. This behavior may not have caused visible problems in the past (or it did but no one understood why), but as DNS data continues to increase in size, it is important that all network equipment is configured correctly to support large DNS packet sizes. If the network environment does not fully support large DNS messages, it may lead to the DNS message being rejected by network gear, or partially dropped during fragmentation. What this looks like to the end user is that DNS queries are going unanswered, or take a very long time, giving the impression that “DNS/network is really slow.”
While EDNS is necessary for the operation of modern-day DNS, the ability to send larger messages contributed to volumetric attacks such as Amplification and Reflection.
1 RFC 1034, written in 1987, specified the use of TCP for DNS as a requirement.
2 RFC 791, which was published in 1981
3 While there are still only 13 IPv4 addresses for DNS root today, it is actually distributed across many nodes across the world using techniques such as anycast and load balancing.
4 Recognizing this problem, the DNS community held a “DNS Flag Day” event on February 1st, 2019, declaring it the day that EDNS must be fully support going forward.
FAQs
The answer is DNS is mostly UDP Port 53, but as time progresses, DNS will rely on TCP Port 53 more heavily.
Is DNS a TCP or UDP? ›
DNS uses TCP for Zone transfer and UDP for name, and queries either regular (primary) or reverse.
What is DNS base port 53? ›
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.
Is DNS zone transfer TCP or UDP? ›
DNS zone transfers use TCP port 53. DNS queries use UDP port 53.
What port is DNS? ›
A DNS server uses well-known port 53 for all its UDP activities and as its server port for TCP. It uses a random port above 1023 for TCP requests. A DNS client uses a random port above 1023 for both UDP and TCP.
Is DNS port 53 TCP or UDP? ›
The answer is DNS is mostly UDP Port 53, but as time progresses, DNS will rely on TCP Port 53 more heavily.
When DNS works on TCP? ›
The unique DNA binding mode allows the TCP domains to display broad specificity for a range of DNA sequences even shorter than 11 bp, adding further complexity to the regulatory network of plant TCP transcription factors.
Is DNS port 53 source or destination? ›
Source Ports
Since the source port for DNS responses is always 53, and since the source port for DNS queries should not be 53, source port filtering may be a viable attack mitigation technique in some situations.
Is DNS port 53 Secure? ›
Security & Compliance
UDP port 53 is used by the DNS protocol to resolve domain names to IP addresses and vice versa. If it is left open and unrestricted, it can be exploited by attackers to redirect users to malicious websites, intercept sensitive information or launch DDoS attacks.
What is the TCP IP 53 port? ›
Port 53 is the standard port for DNS traffic and allows computers to translate domain names into IP addresses through DNS queries. It uses both TCP and UDP for communications. UDP is more common for standard DNS queries, while TCP is used for zone transfers between DNS servers.
A DNS server that supports DNS over TLS listens for and accepts TCP connections on Port 853, unless it has a mutual agreement with its server to use a different port for DoT.
Should port 53 be open? ›
The only reason why you need to have port 53 (tcp and udp) open so to any IP address on the public Internet is when there is an authoritative name server running that the Internet needs to reach. If you have that going on, then focus your firewall rule to only that server.
What is the difference between TCP and UDP? ›
The main difference between TCP (transmission control protocol) and UDP (user datagram protocol) is that TCP is a connection-based protocol and UDP is connectionless. While TCP is more reliable, it transfers data more slowly. UDP is less reliable but works more quickly.
What port is secure for DNS? ›
The client resolver attempts to establish a secure connection on port 853 to the specified DNS server. If a secure connection is established, this provides privacy for the user's queries from passive observers on the path.
Is DHCP TCP or UDP? ›
The DHCP employs a connectionless service model, using the User Datagram Protocol (UDP). It is implemented with two UDP port numbers for its operations which are the same as for the bootstrap protocol (BOOTP). The server listens on UDP port number 67, and the client listens on UDP port number 68.
What is DNS protocol? ›
What is the DNS protocol? The Domain Name System (DNS) protocol is a process that allows internet users to navigate the internet using hostnames instead of numeric IP addresses. DNS is like the phonebook of the internet; it simplifies the process of searching for specific websites through web browsers.
What protocol is DNS? ›
Just like every application layer protocol, DNS uses the User Datagram Protocol (UDP) on the Transport layer of the TCP/IP model to transport data. UDP is preferred over TCP for DNS because of its speed and lightweight packets.
Is HTTP TCP or UDP? ›
Among the two most common transport protocols on the Internet, TCP is reliable and UDP isn't. HTTP therefore relies on the TCP standard, which is connection-based. Before a client and server can exchange an HTTP request/response pair, they must establish a TCP connection, a process which requires several round-trips.
Is DHCP a TCP or UDP protocol? ›
The DHCP employs a connectionless service model, using the User Datagram Protocol (UDP). It is implemented with two UDP port numbers for its operations which are the same as for the bootstrap protocol (BOOTP). The server listens on UDP port number 67, and the client listens on UDP port number 68.
Why does HTTP use TCP and DNS use UDP? ›
Simplicity - UDP is a lot simpler than TCP. TCP is optimized for long data transfers and has a bunch of complex mechanisms such as flow control and congestion control for optimizing the rate of data flow. DNS doesn't need any of these mechanisms for simple queries since the typical amount of sent data is tiny.
