Domain Name System (DNS) Glossary

Recently PJ Web Solutions have had a large number of queries from our existing client base about technical terms associated with their Domain Name System (DNS). It is for this reason why the PJ Web Solutions team have put together jargon free, straight to the point DNS Glossary of terms:

A record

An “A Record” (short for “Address Record”) is a DNS record type which translates (or “resolves”) a hostname ordomain name into an IP address.

Alias

In the context of DNS, alias usually means CNAME records which resolve to some other hostname. In thecontext of websites, “alias” will mean “A second hostname that a website folder will respond to or servecontent for”, for example many web hosting setups will be configured for “example.com” but will also alias to”www.example.com”.

Caching

Caching is the process where DNS servers or applications (such as your web browser) will store a copy of DNSrecords. The cached copy should be identical to what is currently published on the authoritative nameserver,however this is not always the case as the authoritative nameserver could update its records and a cachingnameserver will not immediately know about the change. For this reason DNS records set a maximum age onrecords, known as TTL (time to live). Hostnames which change IP addresses frequently or which need to loadbalancevia DNS will usually have a very low TTL (30 to 300 seconds).

CNAME

CNAME (also: “Canonical Name”) is a DNS term for a type of alias. A CNAME record does not provide the finalanswer (such as an IP address), but instructs applications to conduct an additional query against this otherhostname (specified by the CNAME) and use that result as the answer.CNAMEs usually work well for web hostnames, FTP, and other services but should not be used for certainemail configurations. You can for example, CNAME “webmail.example.com” to “mail.example.com” and thisworks because it is well supported by web browsers and servers. On the other hand, a CNAME inside an MXrecord is invalid and will result in email delivery problems (both sending and receiving).

Domain Name System (DNS)

The Domain Name System (DNS) is the address system people use on the Internet. DNS is responsible forresolving hostnames into IP addresses (and in the case of ‘reverse DNS’, resolving IP addresses back intodomain names). Think of it like a mapping table that maps a hostname to an IP address or another hostname.

Hostname

Hostname (or “host”) is the DNS name (address) of a server or network on the Internet. Hostname sometimesimplies the “short host” name, such as “www” in “www.example.com”. It is more accurate to refer to systemsby their Fully Qualified Domain Name (FQDN) such as “www.example.com”.

IP Address

IP address represents the numeric address of a computer on a given network, most commonly referring to theInternet IP address (or WAN address, provided by your ISP). Less commonly, IP address can also indicate acomputer’s Local Area Network (LAN) or “private” IP address. If unsure which context of IP address is needed,it is always useful to provide both the WAN IP and the LAN IP.

Mail eXchanger (MX)

MX is a DNS record type which indicates a domain’s mailserver address(es). MX record values are fullyqualifiedhostname addresses (for which a hostname must actually exist in DNS). For example, the domainzone “pjweb.co.uk” contains an MX record whose value is “mail.pjweb.co.uk”. When mail is sent tosomeone@pjweb.co.uk, the sending mailserver will first resolve the MX record, then resolve the host A recordfor that MX, and direct the mail to that network address.

Nameserver

An authoritative nameserver (also referred to as “authoritative-only name server”) is a server which onlyreturns answers to DNS queries for specific domains which the server has been configured to resolve.The authoritative nameserver(s) are specified by the Domain Registration account for that domain, and thedomain’s WHOIS record will state which nameservers are authoritative.

Priority

A value to indicate which order to try and use multiple MX records (lower = more priority) ā€“ if the first returnsa host that is unavailable, it tries the next one etc.

Subdomain

A subdomain is a hostname which depends on (belongs with) a domain name. For example, if the FullyQualified Domain Name (FQDN) is “www.example.com”, the domain name really is “example.com”, and”www” is the short hostname ie. the subdomain

TTL

TTL is an acronym for “Time To Live”. TTL specifies how long the outside DNS servers (or your router) shouldcache this DNS record. TTL is sometimes called “cache time”. This tends to be set to one day, so that serversaren’t constantly coming back to check the DNS. If a site is due to be moved, we’ll often set the TTL to 5minutes about 2 days ahead so that the DNS change propogates a lot quicker, because we’re telling serversnot to cache it longer than 5 minutes.

TXT

TXT records are record types which are containers for free-form plain text. TXT record are most commonlyused as Sender Policy Framework (SPF) or DomainKeys, which are created using special syntax inside the TXTrecord.

We hope that our glossary will help answer some of our frequently asked questions and feel free to leave us some comments or feedback!