DNS lookup

DNS is short for 'Domain Name System'. Essentially, whenever you input a domain name anywhere, this name is translated into an IP-number, identifying what remote computer you should connect to.

For instance, when you wish to access ‘google.com’, this is translated into an IP - number using the DNS-system.Then your computer, phone or other device know, which webserver to connect to. This makes the DNS-system something like ‘the internet phonebook’.

For a quick introduction, try out these DNS lookups:

DNS lookup of google.comDNS lookup of facebook.comDNS lookup of stackoverflow.comDNS lookup of wikipedia.orgDNS lookup of bbc.co.ukDNS lookup of iamroot.tech

DNS (Domain Name System) is used to translate domain names into IP-numbers – but the DNS-system is used for more applications besides this.

For instance, info on what mailserver to use for delivering mail to a certain domain, is handled by the DNS - system as well. Info on who controls the DNS - settings, and who to contact in case of abuse, is also available using DNS.

From a website visitor perspective: You connect to a DNS-server and ask for records of a certain type regarding a specific domain – and the DNS-server will respond with whatever information it has available. This is done automatically by your device (computer, phone etc), whenever it is needed – so you won’t have to deal with the hazzle about it. This way, your device knows where to connect to.

From a webmaster/admin perspective: DNS controls where traffic ends up. For instance what IP should visitors to “example.com” connect to? And what mailserver IP should receive mail for “example.com” as well. This is controlled via the DNS-records.

A DNS-record consists of nothing more than 1) a domain name, 2) a record type, 3) a value matching the record type and 4) a value specifying the suggested cache lifetime of the record (TTL – time to live).

Acceptable values are defined by the type of record – and some types of records, require the value to consist of several pieces of sub-information as well.

This is the core of the DNS-system. A domain can have several records assigned – from one to thousands if needed. Usually a domain has about five or six records though (1 x SOA, 2 x NS, 1 x A and 1 x MX).

There are a multitude of DNS record types, each with a specific purpose. There are 9 commonly used record types:

• A (Address) record: Maps a hostname to an IPv4 address. (RFC 1035)
  
• AAAA (Quad A) record: maps a hostname to an IPv6 address. (RFC 3596)
  
• CNAME (Canonical Name) record: maps an alias hostname to the real hostname. (RFC 1035)
  
• MX (Mail Exchange) record: maps a domain name to a list of mail servers responsible for accepting email messages on behalf of that domain. (RFC 1035)
  
• TXT (Text) record: used to store free-form text, such as SPF information or human-readable notes. (RFC 1035)
  
• SOA (Start of Authority) record: indicates the start of a zone of authority and specifies the primary name server and contact information for the zone. (RFC 1035)
  
• NS (Name Server) record: lists the authoritative name servers for a domain. (RFC 1035
  
• SRV (Service) record: used to identify the hostname and port number for a specific service, such as SIP or LDAP. (RFC 2782)
  
• PTR (Pointer) record: maps an IP address to a hostname. It is used for reverse DNS lookups. (RFC 1035) Reverse DNS / PTR-lookups is a bit tricky - but have a look at the Reverse DNS tool.
  

A Recursive DNS lookup is a lookup where you start from the very top of the DNS system with one of the 13 DNS Root servers, and from there it works its way down through the structure until it finds the correct answer from the Authoritative DNS server for the current domain.

A non-recursive lookup is a lookup where you create a DNS lookup against a specific DNS server and the server will return the best response it has. If the DNS server does not have a response to the query, it can send references back to other DNS servers that you can ask.

There are - roughly speaking - three types of DNS servers

A DNS Resolver (a recursive resolver) is designed to receive DNS queries and return a usable response as quickly as possible. If the DNS Resolver already has the answer in its cache, it is read from there, and otherwise it will - on behalf of the user - make a recursive lookup. The answer is stored in the cache of the DNS Resolver, and returned to the user.

A DNS Root Server is the top level DNS server. This is where all recursive queries start. There are 13 DNS Root Servers worldwide. They only hold information about Top Level Domains (TLDs), and can thus only forward queries to the DNS servers that are responsible for the individual TLDs. This is how to find out which DNS server to contact for .com domains versus .edu domains. The underlying DNS servers will typically also simply forward the query further down the structure, until the lookup finds the authoritative DNS server for the specific domain, and the correct answer is found.

An Authoritative DNS server, is responsible for DNS records on a specific domain. It is the authoritative DNS server that holds the updated DNS settings on the domain. It is the last stop on all recursive queries. This is where any changes and updates to DNS records of a domain is done.