Difference between A,CNAME and Alias record

What is a CNAME record?

CNAME stands for Canonical Name. CNAME records can be used to alias one name to another.

For example, if you have a server where you keep all of your documents online, it might normally be accessed through docs.example.com. You may also want to access it through documents.example.com. One way to make this possible is to add a CNAME record that points documents.example.com to docs.example.com. When someone visits documents.example.comthey will see the exact same content as docs.example.com.

To use CNAME records, select CNAME from the Add Record drop down in the advanced editor. Then enter the hostname you would like to alias from and the fully-qualified domain name you would like to alias to. You may also enter @ in the Alias for field to represent the domain itself.

For example, if the domain were example.com and you wanted www.example.com to point to example.com you could put www in the name field and @ in the alias for field.

What is an ALIAS record?

An ALIAS record is a virtual record type that we created to provide CNAME-like behavior on apex domains.

For example, if your domain is example.com and you want it to point to a host name like myapp.herokuapp.com, then you cannot use a CNAME record, but you can use an ALIAS record. The ALIAS record will automatically resolve your domain to one or more A records at resolution time and thus resolvers see your domain simply as if it had A records.

How does it work?

The DNSimple name servers are currently based on an open source Erlang DNS server that we developed along with the help of others in the DNS community. The erl-dns server provides a mechanism for plugging in custom handlers (here is a sample custom handler) and we use that extension mechanism for providing an ALIAS handler. Each time a request hits the DNSimple name servers for either an A or AAAA record type the custom handler is invoked and attempts to resolve the ALIAS into its appropriate IPv4 or IPv6 address, respectively. It does this by asking a resolver to resolve the domain. At the moment we are running PowerDNS’s resolver running locally on each system for this purpose.

If the resolution succeeds then the handler extracts the A and AAAA records and returns them to the erl-dns server process, which then goes on its merry way. It also sticks the result in an in-memory cache (you’ll see why in a second).
If the resolution fails, for example due to a timeout, then the cached response is returned, if one exists. We have the request timeout set quite low (500 milliseconds) and we may even move it lower in the future. If there is no response in the cache then the request is retried again, up to a maximum number of retries. If the request ultimately fails then we return an empty result set.

To support secondary DNS servers, especially ones that connect to us and pull zones using AXFR, we must resolve the ALIAS in a different way. Currently we resolve it as part of the secondary DNS setup process initially and then run a scheduled job to update the ALIAS record at secondary name servers by resolving them again, removing the old records and writing the new records to our zone transfer database, and then finally sending a NOTIFY message to the appropriate secondary name servers.

The actual implementation is handled by a small Go application which our Rails application calls, either from within Rails execution or in an aynchrnonous job, to handle the resolution. The Go application currently calls to Google’s public resolvers to resolve the ALIAS record.

When creating an ALIAS record you will notice than additional TXT field is created, this field is optional and it can be used for debugging purposes.

What is an A record?

An A record maps a domain name to the IP address (IPv4) of the computer hosting the domain. Simply put, an A record is used to find the IP address of a computer connected to the internet from a name.

The A in A record stands for Address. Whenever you visit a web site, send an email, connect to Twitter or Facebook or do almost anything on the Internet, the address you enter is a series of words connected with dots.

For example, to access the DNSimple website you enter www.dnsimple.com. At our name server there is an A record that points to the IP address 208.93.64.253. This means that a request from your browser to www.dnsimple.com is directed to the server with IP address 208.93.64.253.

A Records are the simplest type of DNS records, yet one of the primary records used in DNS servers.

You can actually do quite a bit more with A records, including using multiple A records for the same domain in order to provide redundancy. Additionally, multiple names could point to the same address, in which case each would have its own A record pointing to the that same IP address.

Querying A records

You can use dig to determine the A record associated to a domain name. The result is contained in the ANSWER section and it contains the fully-qualified domain name (FQDN), the remaining time-to-live (TTL) and the IP address.

$ dig A api.dnsimple.com

; <<>> DiG 9.8.3-P1 <<>> A api.dnsimple.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 5792
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;api.dnsimple.com.  IN A

;; ANSWER SECTION:
api.dnsimple.com. 59 IN A 208.93.64.253

;; Query time: 80 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Sun Jul 31 22:21:31 2016
;; MSG SIZE  rcvd: 50

A record structure

In DNSimple we represent A record with the following information:

Name	The host name for the record, without the domain name. This is generally referred as “subdomain”. We automatically append the domain name.
TTL	The time-to-leave in seconds. This is the amount of time the record is allowed to be cached by a resolver.
Address	The IPv4 address the A record points to.

Managing A records

You can create, update and delete A records for your domain using the DNSimple record editor.