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M D

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DNS Aliases

When it comes to DNS aliasing my experience has always been to limit the number of aliases in use and have a single A record per host then setup aliases using CNAMEs for secondary references.  I started researching as I have seen this method as well as setting up multiple A records pointing to a single host IP address.  I was surprised to find that there is indeed a strong argument for both sides debating management/performance/lookup functionality.

If you have experience with DNS and aliases I would be interested to know the EE community input on this topic.

PROS vs CONS:
Multiple A Records to Single IP Address Aliasing
      A Record Definition - Domain Name to IP Address Resolution
      Management - If server IP address needs to be changed all A Records need to be updated as well to the new IP address
      Performance - DNS server resolves single name to get IP address
      Reverse Lookup - If using multiple A Records to point to a single IP address there will only be 1 reverse lookup pointer

Multiple CNAMES to Single A Record Aliasing
      CNAME Record Definition - Domain Name Alias to Primary Domain Name A Record IP Address Resolution.  Canonical name records were designed to alias A records
      Management - Easier to manage server IP address changes at single A Record for a multiple CNAME references
      Performance - DNS server resolves 2 names before getting IP address putting more load on the resolver
      Reverse Lookup - Clear understanding that this is an alias and what host it is pointed to

CNAME Chaining
      Defined - Domain Name Alias -> Domain Name Alias -> Primary Domain Name A Record IP Address
      Management – Multiple records to reference.  Changes along the chain could break name resolution
      Performance - DNS server resolves many names before getting IP address

Thanks
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Alan
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Thanks Alan.  I agree and am curious when it would make sense to have multiple A records pointing to the same ip rather than using CNAMES.

--MD
Hi MD,

It would, in theory, under certain circumstances, deliver faster name resolution.

If you use a CName, and assuming that the record is not cached somewhere from your machine, through all upstream DNS servers, then the CName is resolved to the A record, and then a second query is initiated to resolve the A record to an IP address, making two queries rather than one.

I cannot imagine that is a practical issue in many cases, unless you have a very slow DNS response to start with perhaps.

I have no personal experience of having any issues with CNames, even with two 'layers' of recursion.

Alan.