If you have ever wondered how the DNS works, this post was put together by three of us plixer dudes who wanted a DNS process overview as well as insight into many of the acronyms that accompany this protocol. Lets say for example you point your browser to www.example.com, what happens from your computer? This Overview of DNS Protocol will answer that question.
Our goal was to accomplish two things in these posts:
- Explain how the DNS process works
- Provide a glossary of terms
DNS Resolver: The client side of the DNS is called a DNS resolver. Most DNS servers are setup as a Recursive DNS or RDNS. It is responsible for initiating and sequencing the queries (e.g. what is the IP address of www.example.com) that ultimately leads to a full resolution (translation) of a domain name into an IP address.
DNS Server: Translates an often humanly-meaningful, text-based identifier (e.g. example.com) to a system-internal, often numeric identification (e.g. an IP address).
Fully Qualified Domain Name (FQDN) is sometimes also referred to as an absolute domain name. It is a domain name that specifies its exact location in the tree hierarchy of the Domain Name System (DNS). It specifies all domain levels, including the top-level domain and the root zone. A fully qualified domain name is distinguished by its lack of ambiguity: it can be interpreted only in one way. (e.g. not.myhost.example.com). A FQDN can be up to 253 characters total.
Label: This is the text between the periods ‘.’ in the FQDN. Each label can be between 1-63 characters.
Domain Suffix: This is a portion of the FQDN that a group of domains share in common. For example host1.xyz.local, host2.xyz.local and host3.xyz.local all share the Domain Suffix of .xyz.local. This is often used for internal web browsing. For example: if you type http://test into your browser with no top level domain and the Windows OS has been configured with a Domain Suffix of .xyz.local, the web browser will append the URL with the Domain Suffix automatically and point the browser to: http://test.xyz.local.
Primary DNS: The first DNS that the DNS responder will reach out to when trying to resolve a FQDN.
Secondary DNS: The second server a DNS responder will reach out to if the primary DNS fails to resolve the request for a FQDN.
Top-level Domain: This is something you may have also heard. Examples are .com .net .org, .gov .mil etc.
Second Level Domain: example.com. Also, .co is a second-level domain for .uk. The .uk is the country code top-level domain for United Kingdom (ccTLD).uk. Until 10 June 2014 it was prohibited to register a domain name directly under .uk (such as internet.uk) and a third-level domain was used (such as internet.co.uk). .co.uk – commercial and general
Third Level Domain: myhost.example.com
Fourth Level Domain: not.myhost.example.com
DNS Root Zone: is the same as a Top level domain. The root zone database is a public record.
Root Name Server: Every domain requires a zone file. For example, the Root Name Server plixer.com has a zone file that defines the A record for www.plixer.com. When someone says “we manage the plixer.com DNS, they are managing (updating) the DNS “zone file” for plixer.com.
Zone File: The zone file contains mappings between domain names and IP addresses and other resources, organized in the form of text representations of resource records (RR). Here is an example [DNS Zone File] http://corpocrat.com/2008/09/29/sample-dns-zone-file-for-bind/ .
Resource Record: A resource record (RR) is the basic data element in the domain name system. Each resource record is a single entry in a zone file and has a type (A, MX, etc.), an expiration time limit, a class, and some type-specific data. Resource records of the same type define a resource record set (RRset).
Start of Authority: The SOA is a single Record Type in the Zone File that defines the global parameters for the zone (domain). There is only one SOA record allowed in a zone file it must be the first RR in the zone.
DNS Record Types: The Resource Types are the forms or ‘types’ of resource records (database records) stored in the zone files of the Domain Name System (DNS). There are several dozen types. SOA is one Resource Type of a single Resource Record defined in the Zone File.
Part 2 of the Overview of DNS Protocol.