FW: DNS Question

Len Conrad lconrad at Go2France.com
Fri Oct 23 15:18:20 UTC 2009

---------- Original Message ----------------------------------
From: krad <kraduk at googlemail.com>
Date:  Fri, 23 Oct 2009 15:56:40 +0100

>2009/10/23 Sean Cavanaugh <millenia2000 at hotmail.com>
>> > Date: Fri, 23 Oct 2009 08:30:08 -0400
>> > From: dave.list at pixelhammer.com
>> > To: freebsd-questions at freebsd.org
>> > Subject: DNS Question
>> >
>> > Good morning.
>> >
>> > I have been asked by my co-workers and sales why I always create a A
>> > record for new domains we host instead of a CNAME.
>> >
>> > The issue I run into lately with some domains is that a client has a
>> > website with a industry host such as frank.relator.com and he wants to
>> > have DNS point www.frank.com to frank.relator.com with a CNAME. The
>> > client does not want an A record for frank.com.
>> >
>> > Somewhere, in a class far far away, I was taught a DNS zone had to have
>> > a A record to function properly. I can't seem to locate anything in the
>> > RFCs.
>> >
>> > Am I wrong?
>> >
>> I think you are confusing basics of DNS records. you are partially correct
>> in that a DNS zone needs an initial A record to be able to translate a name
>> to an IP, but there is nothing wrong about setting up a CNAME to point to a
>> record in a different zone instead. you just cannot do a zone that has a
>> CNAME only that does not at some point to a valid A record. CNAMEs are
>> forwarders only whereas A records are actual lookups.
>> for proper way to set this up....
>> The A record would be assigned for the main name that you want to associate
>> to an IP address.
>> The CNAME record just relates a different name to that original name. this
>> allows you to change the IP address of the server and only have to update
>> the original A record instead of every DNS record for that server.
>> for small number of vhosts, this would not really be an issue, but imagine
>> if you were hosting a couple hundred vhosts from a single IP and then had to
>> change that IP because you switched your ISP. It would take you a LONG time
>> to update them if they were all A records, but only a couple of seconds if
>> you had it properly set up as CNAME's
>> www.bobshosting.com    A
>> www.vhost1.com          CNAME  www.bobshosting.com.
>> www.vhost2.com          CNAME  www.bobshosting.com.
>> www.vhost3.com          CNAME  www.bobshosting.com.
>> www.vhost4.com          CNAME  www.bobshosting.com.
>> -Sean
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>I try to use CNAMES as much as possible, for one very good reason. If say I
>have web server with 1000 vhost on it. I have one A record for the server
>and all the cnames point at that A record. Now i need to change the ip of
>the server. I update the A record and add a reverse record and im done. IF I
>had done it your way with all A records I would now have to go and edit
>another 1000 records. Even worse if some of these domains are not under my
>control I have to go and liaise with customers, or other third parties, and
>it becomes a complete mess. The chances of me convincing them all and
>coordinated it correctly are minimal 8(

domains sharing records is better handled by $INCLUDE

$INCLUDE /path/db.ttl, which contains

$TTL 6h

$INCLUDE /path/db.ns, which contains

@ ns ns1.domain.tld.
@ ns ns2.domain.tld.

$INCLUDE /path/db.www, which contains

@   a ip.ad.re.ss
www a ip.ad.re.ss


Changing an include file changes all the zone files that include it, giving enormous leverage, while removing the extra query required to resolve a CNAME to canonical.


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