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trojan81 asked

First off I am not a DNS guru.

I was reading about SRV records as it relates to exchange.
What is the difference between a SRV record and a CNAME? It appears that a SRV record can direct a request to a mail server and port #. When you create the SRV record in Windows, is the SERVICE a drop down menu that allows you to select a predefined service or is it anything you wish to type in?
A CNAME can direct a request to a taget host which I can make it the mail server. However, I can't control the port. Would this be the main difference between the two?
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Hi trojan81

A CNAME is more like an alias. You always have to have an A record for a CNAME. For example

the A record for example.host.com will look like

Then a CNAME called pointer.example.com will look like
pointer.example.com example.host.com

CNAME doesnt provide anything except a pointer to the A record. With SRV records (these are service location records), you can specify a target, priority, weright and a port.

More info can be obtained here

Thomas AamodtNetwork Architect
SRV records are like services yes, when you choose dns name, port number, and TCP or UDP. used with some exchange pointers and SIP (like lync/communicator).
in Lync/communicator, SRV records are used to define what company'es that are federated with your company.

As other experts have already said, a CNAME is only an alias to a HOST DNS record. This permits you to associate another DNS name to an already declared HOST. This is useful if you want to associate many names to a unique server, you create a HOST record with its IP address and you createa many CNAME associated to the HOST name. In case you'll have to change the IP address of the server you'll only have to change the HOST record.

SRV records permits client applications to locate a service (not only a server). A "service" is of course a server plus a Port number.
When you create a SRV record you can choose a predefined service in a list but you can also type what you what instead of choosing in the list. Usually, service names start with an underscore "_".
Some client application ask the DNS for a SRV records to locate their connection end point.

As an example, a Windows computer asks the DNS for a _gc SRV record when it wants to locate a valid Global Catalog in the domain.
An Outlook client asks the DNS for SRV record to locate a valid CAS server.

It's unuseful to creat SRV records if the client application is not made to ask fo it.

Have a good day.


well done paciB!