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IWETHEY Home / IWETHEY Board / Networking Forum / CNAMES are a double lookup.
Post #364,870
by folkert
Picture of folkert
10/1/12 8:06:37 PM
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New CNAMES are a double lookup.
Not as significant as in days gone by... but still it is a double lookup.
--
greg@gregfolkert.net
PGP key 1024D/B524687C 2003-08-05
Fingerprint: E1D3 E3D7 5850 957E FED0 2B3A ED66 6971 B524 687C
Post #364,872
by drook
Picture of drook
10/1/12 10:07:07 PM
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New So when would you prefer a cname?
There's got to be a reason for them, right?
--

Drew
Post #364,875
by static
10/1/12 10:56:53 PM
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New They're basically an alias.
It's been a while since I've done much in a zone file, but if you had several dozen names pointing to the same IP address, you could use CNAMEs for most of them and keep the IP address on one. This is good if the IP address is updated in the zone file much.

It's also good if you need to point a name to one in another domain.

Wade
Just Add Story http://justaddstory.wordpress.com/
Post #364,876
by folkert
Picture of folkert
10/1/12 11:23:31 PM
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New The way *I* typically use them...
A Record is the proper full name of the host (i.e. uno.gregfolkert.net) 

[greg@lanai WG3 ~]$ dig uno.gregfolkert.net



; <<>> DiG 9.2.4 <<>> uno.gregfolkert.net

;; global options:  printcmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 62625

;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0



;; QUESTION SECTION:

;uno.gregfolkert.net.		IN	A



;; ANSWER SECTION:

uno.gregfolkert.net.	1571	IN	A	208.64.37.170



;; Query time: 2 msec

;; SERVER: 192.168.204.104#53(192.168.204.104)

;; WHEN: Mon Oct  1 21:22:04 2012

;; MSG SIZE  rcvd: 53


CNAME is any other name I give it: www.gregfolkert.net, mail.gregfolkert.net or just gregfolkert.net 

[greg@lanai WG3 ~]$ dig www.gregfolkert.net



; <<>> DiG 9.2.4 <<>> www.gregfolkert.net

;; global options:  printcmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 29876

;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0



;; QUESTION SECTION:

;www.gregfolkert.net.		IN	A



;; ANSWER SECTION:

www.gregfolkert.net.	1800	IN	CNAME	uno.gregfolkert.net.

uno.gregfolkert.net.	1800	IN	A	208.64.37.170



;; Query time: 144 msec

;; SERVER: 192.168.204.104#53(192.168.204.104)

;; WHEN: Mon Oct  1 21:18:15 2012

;; MSG SIZE  rcvd: 71


CNAME for my nearly static IP Address at home, owned by comcast:

archer.gregfolkert.net is a CNAME for c-98-209-73-13.hsd1.mi.comcast.net. 

[greg@lanai WG3 ~]$ dig archer.gregfolkert.net



; <<>> DiG 9.2.4 <<>> archer.gregfolkert.net

;; global options:  printcmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 49508

;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0



;; QUESTION SECTION:

;archer.gregfolkert.net.		IN	A



;; ANSWER SECTION:

archer.gregfolkert.net.	1800	IN	CNAME	c-98-209-73-13.hsd1.mi.comcast.net.

c-98-209-73-13.hsd1.mi.comcast.net. 5278 IN A	98.209.73.13



;; Query time: 52 msec

;; SERVER: 192.168.204.104#53(192.168.204.104)

;; WHEN: Mon Oct  1 21:21:14 2012

;; MSG SIZE  rcvd: 101


If you notice there are two lookups.
--
greg@gregfolkert.net
PGP key 1024D/B524687C 2003-08-05
Fingerprint: E1D3 E3D7 5850 957E FED0 2B3A ED66 6971 B524 687C
     Any reason to prefer anames to cnames? - (drook) - (4) - Oct. 1, 2012, 05:39:17 PM EDT
         CNAMES are a double lookup. - (folkert) - (3) - Oct. 1, 2012, 08:06:37 PM EDT
             So when would you prefer a cname? - (drook) - (2) - Oct. 1, 2012, 10:07:07 PM EDT
                 They're basically an alias. - (static) - Oct. 1, 2012, 10:56:53 PM EDT
                 The way *I* typically use them... - (folkert) - Oct. 1, 2012, 11:23:31 PM EDT

Even the hydrocoptic marzelvane came up with a null...
60 ms