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Adding CNAME record to DNS

Posted on 2009-05-19
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Last Modified: 2012-08-14
Hi,

I need to add a CNAME to an existing DNS record. The CNAME is facilities.domain.com

Should I add "facilities.domain.com" or "facilities.domain.com." [nb the period at the end]

I have heard that a period must be added at the end? Or is this if I only add "facilities", so that I add "facilities."?

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Question by:kam_uk
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13 Comments
 
LVL 5

Expert Comment

by:Neranel
ID: 24425917
a FUlly Qualified Domain Name always ends with a period.  
"facilities.domain.com."
If you are using microsoft DNS, it will add the period for you, so you really dont need to worry, but the correct way is with the period.
Though you should really only need to add 'facilities' to the 'domain.com' zone.
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LVL 1

Expert Comment

by:ElloDarkstar
ID: 24425943
It may depend on where you're adding a dns record, but most registrars I know of have ways to avoid issues (they request only the host name, then fill in the domain and extension for you).
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Author Comment

by:kam_uk
ID: 24426052
Thanks guys - out of interest - why do we add the period at the end, what does it perform?
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LVL 5

Expert Comment

by:Neranel
ID: 24426066
. is the root internet domain
so com is a subdomain of .
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Author Comment

by:kam_uk
ID: 24426093
sorry, i don't understand :(

I'm not a DNS expert by any means
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Expert Comment

by:Neranel
ID: 24426510
a domain name like test.yahoo.com is actually 4 domains
first, remomber that i said all domains end with a period if they are fully qualified?
so now we have test.yahoo.com.
let me break it out
test (is a subdomain of) Yahoo (is a subdomain of) com (is a subdomain of) .
. being the root everything bucket domain of the entire internet.
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Assisted Solution

by:Neranel
Neranel earned 800 total points
ID: 24426636
So to understand, when you dns server is questioned about who is test.yahoo.com it starts at the biggest.
.
it asks a root name server, who is test.yahoo.com.
the root server says, i dont know, but i know com's nameserver is x.x.x.x (now referred to as com NS)
so the DNS server asks com NS who is test.yahoo.com.
the com NS server says i dont know but i know yahoo's NS is y.y.y.y (now yahoo NS)
so the DNS Server asks yahoo NS who is test.yahoo.com.
and the yahoo NS says test.yahoo.com. is at ip addres z.z.z.z
 
Does that help?
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LVL 3

Author Comment

by:kam_uk
ID: 24426677
Yep, helps a lot..

Just one more question :)

What is the effect of not adding the .

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Expert Comment

by:Neranel
ID: 24426717
in most cases the software will handle it for you.
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Author Comment

by:kam_uk
ID: 24426761
sure, but what if it doesn't - i'm just curious that's all :)
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Expert Comment

by:Neranel
ID: 24426918
Then you would need to.
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Accepted Solution

by:
Chris Dent earned 1200 total points
ID: 24429878

Everything in a zone file is relative to the origin (zone file name) unless explicitly terminated.

That is, if you fail to include the terminating period on a record in a DNS zone (and the software doesn't do it) the current zone name will be appended to the record.

For instance, if you have this, with an omitted period:

somezone.com.
   host1   IN CNAME   otherhost.otherdomain.com
   host2   IN CNAME   otherhostinthisdomain

You end up with this when you lookup host1:

otherhost.otherdomain.com.somezone.com.

And this when you lookup host2:

otherhostinthisdomain.somezone.com.

This occurs because of the short-hand conventions allowed when creating Zone Files for a DNS server. The same occurs with the "host1" entry. Because it doesn't have a terminating period it has the zone name appended and and becomes "host1.somezone.com.".

If all records were written in the longer form (with the correct termination this time) we would have:

somezone.com.
   host1.somezone.com.   IN CNAME   otherhost.otherdomain.com.
   host2.somezone.com.   IN CNAME   otherhostinthisdomain.somezone.com.

The zone file is less flexible, it's specific to "somezone.com", if it were all written in the short-hand form you could pick up the zone file, call it otherzone.com and it would happily answer requests. e.g.:

otherzone.com.
   host1   IN CNAME   otherhost.otherdomain.com.
   host2   IN CNAME   otherhostinthisdomain

Chris
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LVL 5

Expert Comment

by:Neranel
ID: 24477574
Did we answer your Questions?, If not what DNS Platform are you using? If we did, can you please close the question.
 
Thank You,
Neranel
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