What is the TCP and UDP Port for DNS?
The TCP and UDP port for DNS is 53.
The Domain Name System (DNS) maps IP addresses to domain names, which are easy for people to remember and type into their browsers when they want to visit a website or send an email message.
Why do DNS use both TCP and UDP?
DNS lookups can be done using either TCP or UDP. There are benefits of both, but it is probably better to use TCP for DNS queries since communications failures are likely to result in lost data when using UDP. UDP can be used to exchange small packets of information, but TCP is better for larger packets. Larger packets means packets greater than 512 bytes.
DNS queries are typically sent over UDP but logins and email often use TCP, so even if there is a problem with the network connection it will still log in or send information without any problems.
DNS always transfers Zone data using UDP, but transferring using TCP is also possible and reliable and TCP makes sures zone data is consistent and transferred correctly.
Why do DNS requests sometimes fail?
DNS requests can fail if their packets are too large or too small. If the packet is larger than 512 bytes, it must be sent using TCP regardless of its contents.
If a DNS request packet is smaller than 512 bytes and contains only resource records (RRs) that fit in one UDP datagram (a single answer section), this DNS request can use either UDP or TCP without any problems.
Can a firewall block DNS?
DNS is a standard protocol, so it can operate without being hindered by firewalls. However, firewall administrators have the option of blocking DNS traffic between specific hosts or networks if they desire to do so.
If your network’s router has built-in functionality for filtering packets according to IP address and port number then you need to ensure that the DNS traffic is allowed.
DNS requests and responses are sent using UDP or TCP port 53, which means that if your firewall blocks incoming connections on this particular port then you will not be able to use DNS for looking up domain names of websites.
How do I allow DNS through the firewall?
To allow dns we may also want to allow UDP or TCP ports 53 through the firewall. If you are unsure of which ports need opening, consult with your ISP or check the documentation on their website.
In some cases, you may need to configure your firewall directly. For example, in Windows Firewall this is done by specifying an allow incoming connection on the Domain and DNS checkboxes under Inbound Connections with TCP or UDP port 53 selected as well as any other ports that are needed for receiving responses back from the servers (such as TCP 80 for web servers).
How do DNS work?
In general, the process is as follows:
The client sends a request to its configured DNS server and waits for a response from one of the configured servers.
The server verifies that it is authoritative for the zone in question (i.e., it contains an answer to the query). If not, then it refers or forwards the request to another configured server at this level of recursion – if there are any other configured name servers available which are closer to being authoritative than itself.
Not all DNS queries go through recursion; some can be resolved entirely by machines with direct knowledge of data without consulting further authorities, and these types of requests cause negative responses such as NXDOMAIN (“Name Error”).
If a domain is given but no type has been specified on the command line, then “A” records will be returned if query via UDP or TCP port 53 and “NS” records if query via TCP port 53.
In short: A request to resolve an FQDN into an IP address then If the name server doesn’t have the answer, it may be required to ask other servers for help and All responses come back via UDP or TCP port 53.
What are the contents of DNS Query?
A query that contains a QNAME, type and class fields follows this general format: [no source] [no flags] [QTYPE=qtype QCLASS=class]. If there’s no explicit match in local data (e.g., an A record for which we do not have authoritative control), then it will be sent to any one of the configured servers at level 0; otherwise, we’ll send it to all our configured name servers at level n so they can report back with responses from their own zones or queries received from other levels.
Conclusion
DNS uses UDP or TCP port 53. To allow dns we may also want to allow UDP or TCP ports 53 through the firewall, which can be configured in Windows Firewall by specifying an “allow incoming connection” on Domain and DNS checkboxes under Inbound Connections with TCP or UDP port 53 selected as well as any other ports that are needed for receiving responses back from servers.
