I had to make DNS changes today to support some legacy applications we’re are replacing but will be in use for the next two months until the go live date.
To avoid going to the Network plus book and look for this info, or to avoid Googl-ing it away, I thought I would write down this small reminder to myself.
Differences between the A, CNAME, ALIAS and URL records
A, CNAME, ALIAS and URL records are all possible solutions to point a host name (name hereafter) to your site. However, they have some small differences that affect how the client will reach your site.
Before going further into the details, it’s important to know that A and CNAME records are standard DNS records, whilst ALIAS and URL records are custom DNS records provided by DNSimple’s DNS hosting. Both of them are translated internally into A records to ensure compatibility with the DNS protocol.
Understanding the differences
The A record maps a name to one or more IP addresses, when the IP are known and stable.
The CNAME record maps a name to another name. It should only be used when there are no other records on that name.
The ALIAS record maps a name to another name, but in turns it can coexist with other records on that name.
The URL record redirects the name to the target name using the HTTP 301 status code.
Some important rules to keep in mind:
The A, CNAME, ALIAS records causes a name to resolve to an IP. Vice-versa, the URL record redirects the name to a destination. The URL record is simple and effective way to apply a redirect for a name to another name, for example to redirect www.example.com to example.com.
The A name must resolve to an IP, the CNAME and ALIAS record must point to a name.
Understanding the difference between the A name and the CNAME records will help you to decide.
use an A record if you manage what IP addresses are assigned to a particular machine or if the IP are fixed/static.
use a CNAME record if you want to alias a name to another name, and you don’t need other records (such as MX records for emails) for the same name
use an ALIAS record if you are trying to alias the root domain or if you need other records for the same name
use the URL record if you want the name to redirect (change address) instead of resolving to a destination.
If you trace the request responses with an HTTP sniffer like Fiddler, you’ll see in this case a 301 Code returned from the server to the browser before the redirection happens.