With a CNAME resource record (CNAME for close. canonical name) is defined to an existing DNS name a pseudonym. Left in resource the record (RR) stands the pseudonym and on the right of the original name or in the DNS terminology: the canonical name. To a canonical name as many as desired Aliase can be defined.

Structure

In the following example is put on to the canonical name org nam an alias:

       org nam 1285 IN A 172.27.100.200 ali nam 1285 IN CNAME org nam 

more complex cases

To a canonical name several Aliase can be put on:

       org nam 1285 IN A 172.27.100.200 ali1-nam 1285 IN CNAME org nam ali2-nam 1285 IN CNAME org nam 

It is also permissible to define an alias on an existing alias to thus concatenate Aliase:

       org nam 1285 IN A 172.27.100.200 ali nam 1285 IN CNAME org nam bli nam 1285 IN CNAME ali nam 

It is also possible to define an alias to a canonical name from another Domain. Example:

       ali nam 1285 IN CNAME neu-nam.example.net. 

Is illegal to define an alias for a further RR-type of same name exists. The name server would not know in this case, how it should behave. In this example a A-RR and a CNAME RR exist for the name ali nam both:

       ali nam 1285 IN A 192.168.1.2 ali nam 1285 IN CNAME org nam 

Alias dissolution

If a name server receives a DNS Request, for which a CNAME RR exists, then dissolves it this. In the highest example the name server recognizes the fact that behind ali nam the canonical name org nam it hides itself and dissolves these, i.e. he determines the associated IP address 172.27.100.200.

With the answer to a DNS Request apart from the IP address also the canonical name will hand over. The resolver can recognize in such a way that its original inquiry had referred to a pseudonym.

Example

Nslookup the following answer would supply:

       C:\ >nslookup ali-nam.example.com name: org-nam.example.com ADDRESS: 172.27.100.200 Aliases: ali-nam.example.com 

It can naturally happen that the name server cannot dissolve a canonical name. In this case it omits the IP address with the answer simply, and hands over only the canonical names. The resolver can try then even to dissolve this name.

Note: A PTR-RR should point never to an alias, but in principle to a canonical name. Most name servers accept however also PTR RRs on Aliase.

Correct example (org nam is a canonical name):

       200.100.23.127.in addr.arpa. IN PTR org-nam.example.com. 

Inkorrektes example (ali nam is an alias):

       200.100.23.127.in addr.arpa. IN PTR ali-nam.example.com. 

Articles in category "CNAME resource record"

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Related Websites

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• Add a CNAME resource record to a zone
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• Add an alias (CNAME) resource record to a zone
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• DNS Resource Records
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