Crucial Web Hosting

DNS Explained

Domain Name

A domain name usually consists of two or more parts (technically labels), separated by dots, e.g. www.crucialwebhost.com.

• The rightmost label conveys the top-level domain.
• Each label to the left specifies a subdivision or subdomain of the domain above it.
• A hostname refers to a domain name that has one or more associated IP addresses. For example, the support.crucialwebhost.net and www.crucialwebhost.com domains are both hostnames.

DNS

The Domain Name System consists of a hierarchical set of DNS servers. Each domain or subdomain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains "beneath" it. The hierarchy of authoritative DNS servers matches the hierarchy of domains. At the top of the hierarchy stand the root nameservers: the servers to query when looking up (resolving) a top-level domain name (TLD). Iterative and recursive queries:

• An iterative query is one where the DNS server may provide a partial answer to the query (or give an error). DNS servers must support non-recursive queries.
• A recursive query is one where the DNS server will fully answer the query (or give an error). DNS servers are not required to support recursive queries and both the resolver (or another DNS acting recursively on behalf of another resolver) negotiate use of recursive service using bits in the query headers.

DNS Records

CNAME or Canonical Name

(CNAME) Record is used to define an alias hostname. A CNAME record takes this format:

alias.domain.name      IN     CNAME   otherhost.domain.name.

This defines alias.domain.name as an alias for the host whose canonical (standard) name is otherhost.domain.name.

Address Record (A) Record

An A record gives you the IP address of a domain. That way, users that try to go to www.example.com will get to the right IP address. An A record maps a hostname to a 32-bit IPv4 address. An A Record takes this format (example):

Name             TTL     TYPE    DATA
ftp.domain.com   43200   A       IP Address

Mail Exchange (MX) Record

Maps a domain name to a list of mail exchange servers for that domain. A zone can have one or more MX records. These records point to hosts that accept mail messages on behalf of the host. A host can be an MX for itself. MX records need not point to a host in the same zone. An MX record takes this format:

host.domain.name       IN     MX      10 otherhost.domain.name.
                       IN     MX      20 otherhost2.domain.name.

The MX priority numbers (value 0 to 65535) signify the order in which mailers select MX records when they attempt mail delivery to the host. The lower the MX number, the higher the host is in priority.

Pointer (PTR) Record

Maps an IPv4 address to the canonical name for that host. Setting up a PTR record for a hostname in the in-addr.arpa. domain that corresponds to an IP address implements reverse DNS lookup for that address. For example (at the time of writing), www.icann.net has the IP address 192.0.34.164, but a PTR record maps 164.34.0.192.in-addr.arpa to its canonical name.

Name Server (NS) Record

Maps a domain name to a list of DNS servers authoritative for that domain. In this case, for Crucial Web Hosting purposes, would be:

ns1.crucialwebhost.com
ns2.crucialwebhost.com
ns3.crucialwebhost.com

Start of Authority (SOA) Record

Specifies the DNS server providing authoritative information about an Internet domain, the email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone.

Text (TXT) Record

The TXT record allows an administrator to insert arbitrary text into a DNS record. For example, this record is used to implement the Sender Policy Framework and DomainKeys specifications.

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