DNS Terminology

The information in this article can be divided into two parts:

  • DNS Records
  • DNS Servers

DNS Records

A Record

An A record points a domain or subdomain to an Ipv4 address:

example.com. IN A 192.0.0.1

AAAA Record

An AAAA record points a domain or subdomain to an IPv6 address.

example.com. IN AAAA 2001:db8:10::1

CNAME Record

A CNAME is a type of DNS record used to create an alias from one domain name to another:

www IN CNAME example.com.

If you create such DNS record both domain names: example.com and www. example.com will open one and the same web page. The CNAME record will redirect the sub domain www.example.com to your domain name example.com

MX Record

The MX record shows which mail server is authorized to manage the mail services of the domain name. Usually there are more than one MX records and they are ordered by priority ‐ the lowest digit designates the primary mail server. The primary mail server in the below example is mail.example.com because of its lower priority ‐ 10 against 20 for mail2.example.com.

example.com. IN MX 10 mail.example.com.

example.com. IN MX 20 mail2.example.com.

mail.example.com. IN A 192.0.0.2

mail2 IN A 192.0.0.3

SPF Record

The Sender Policy Framework (SPF) is an email authentication protocol that allows the owner of a domain to specify which mail servers are used to send mail from that domain. The list of authorized sending hosts for a domain is published in the Domain Name System (DNS) records for that domain in the form of a specially formatted TXT record. The SPF record is common practice in protecting your domain name being used by spammers.

example.com. TXT "v=spf1 mx a:mail.example.com include:aspmx.googlemail.com -all"

TXT RECORDS

TXT records hold free-form text of any type. A fully qualified domain name may have many TXT records. The most common uses for TXT records are Sender Policy Framework (SPF), DomainKeys (DK), and DomainKeys Identified E-mail (DKIM).

SRV RECORDS

A Service record (SRV record) is a specification of data in the Domain Name System defining the location (the hostname and port number) of servers for specified services:

_sip._tcp.example.com. 86400 IN SRV 0 5 5060 sipserver.example.com.

Wildcard Record

A wildcard is a DNS zone record that will match requests for non-existent domain names. A wildcard DNS record is specified by using a * as the leftmost label (part) of a domain name, e.g. *.example.com.

The below wildcard record will point all subdomains of example.com you are ever going to create automatically to the specified IP address:

*.example.com. IN A 192.0.0.1

PTR Record (Pointer Record)

While DNS connects hostnames with IP addresses, PTR records do the opposite - they associate IP addresses with hostnames. One of the most common applications of PTR records is authentication, e.g. to verify that the email is sent from a mail server whose IP address is listed in the PTR record.

DKIM Record

DKIM (Domain Keys Identified Email) is a means of verifying incoming email. It ensures that incoming messages are unmodified and are from the sender from whom they claim to be. This feature works to prevent incoming spam messages.

k=rsa; t=s; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDGMjj8MVaESl30KSPYdLaEreSYzvOVh15u9YKAmTLgk1ecr4BCRq3Vkg3Xa2QrEQWbIvQj9FNqBYOr3XIczU8gkK5Kh42P4C3DgNiBvlNNk2BlA5ITN/EvVAn/ImjoGq5IrcO+hAj2iSAozYTEpJAKe0NTrj49CIkj5JI6ibyJwIDAQAB

SOA Record

An SOA (Start of Authority) Record is the most essential part of a Zone file. The SOA record is giving out information about the domain, including the primary name server, the email of the domain administrator, the domain serial number, information on how often it is updated, when it was last updated, when to check back for more info and so on. A Zone file can contain only one SOA Record.

$ORIGIN example.com.
$TTL 86400
@	IN	SOA	dns1.example.com.	hostmaster.example.com. (
2001062501 ; serial
21600      ; refresh after 6 hours
3600       ; retry after 1 hour
604800     ; expire after 1 week
86400 )    ; minimum TTL of 1 day

DNS Servers

DNS Zone

The DNS Zone file contains all the records for a specific domain name.

dns zone file

TTL (Time to Live)

TTL (Time To Live) is the amount of time in seconds that a DNS record will be cached by an outside DNS server or resolver.

DNS Cache

DNS caching is the method by which any DNS server or client locally stores recently queried DNS records to re-use them in the future eliminating the need for new DNS queries and improving the efficiency of DNS.The Domain Name System implements a time-to-live (TTL) on every DNS record. When TTL get expired, the record is deleted or purged from the cache. At that point, if a query for that record is received, the DNS server has to start the resolution process again.

DNS Server

The DNS server provides answers to DNS queries.

DNS Client

The DNS Client is the client component that resolves and caches domain names. When the DNS Client service receives a request to resolve a domain name that it does not contain in its cache, it queries an assigned DNS server for an IP address for the name.

Authoritative Name Server

An authoritative name server provides actual answer to a DNS queries such as mail server IP address or web site IP address (A resource record). It provides only original and definitive answers to DNS queries and never cached answers, obtained from another name server.

Domain Name Resolver

Domain Name Resolvers are used by ISPs to resolve a domain name into an IP Address following strict protocols when contacting the authorized DNS servers.

Domain Name System (DNS)

Internet service that translates domain names into IP addresses. The Internet is based on IP addresses. Every time you use a domain name, therefore, a DNS service must translate the name into the corresponding IP address.

DNS QUERY

When a DNS client requires resolution of a host name to an IP address, it requests this resolution from a DNS server. This process is referred to as a DNS query.

IP Address

Internet Protocol (IP) address is an identifier assigned to each computer and other device (e.g., printer, router, mobile device, etc.) connected to a TCP/IP network that is used to locate and identify the node in communications with other nodes on the network. IP addresses are usually written and displayed in human-readable notations, such as:

193.192.0.0.1 in IPv4

194.2001:db8:0:1234:0:567:8:1 in IPv6

(Wikipedia)

Name Server

A name server is a specialized server on the Internet that handles queries or questions from your local computer, enabling you to enter www.example.com instead of 192.0.0.1 in the browser. It is managed by a web host that is specifically designated for managing the domain names that are associated with all of the hosting provider's accounts.

Primary and Secondary DNS

There are two basic types of DNS - primary and secondary, which are generally applied to each newly registered domain. They are name server computers where the record of your domain name is stored. The information on both servers is identical. In general, domain names can work with only one name server - the primary DNS. However, practice has shown that a domain name needs to have at least two name servers assigned in order to be available at any time.

Recursive Query

There are two types of DNS queries that can be made to your server: Recursive and Iterative Queries.

With Recursive requests, your server will attempt to find the website in question in its local cache. If it cannot find an answer it will query other DNS servers on your behalf until it finds the address. It will then respond to the original request with the results from each server's query.

Iterative Query

With Iterative requests, the DNS server will attempt to find the website in question in its local cache. If it cannot find an answer it will not ask other DNS servers but will reply back to the original request with information which server to ask next.

DNS Root

Root name servers are the servers at the root of the Domain Name System (DNS) hierarchy. DNS root servers are the first step in resolving any domain name. The authoritative name servers that the resolvers use to find top level domains (like .com domains) are the root name servers.

Fully Qualified Domain Name

A fully qualified domain name (FQDN), sometimes also referred to as an absolute domain name, is a domain name that specifies its exact location in the hierarchy of the Domain Name System (DNS). It specifies all domain levels, including the top-level domain and the root zone.

The DNS root is unnamed, expressed as the empty label terminated by the dot. This is most notable in DNS zone files in which a fully qualified domain name must be specified with a trailing dot.

For example, mail.example.com. explicitly specifies an absolute domain name that ends with the empty top level domain label:

mail.example.com. IN A 192.0.0.1

mail IN A 192.0.0.1

The two records above are identical, but the proper FQDN syntax requires dot at the end of the FQDN domain name.

#dns

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