The difference between ALIAS and CNAME and when to use them

The chief distinction between a CNAME document and an ALIAS document shouldn’t be within the outcome—each level to a different DNS document—however in how they resolve the goal DNS document when queried. On account of this distinction, one is secure to make use of on the zone apex (for instance, bare area reminiscent of instance.com), whereas the opposite shouldn’t be.

Let’s begin with the CNAME document kind. It merely factors a DNS identify, like www.instance.com, at one other DNS identify, like lb.instance.web.  This tells the resolver to search for the reply on the reference identify for all DNS sorts (for instance, A, AAAA, MX, NS, SOA, and others). This introduces a efficiency penalty, since at the least one further DNS lookup should be carried out to resolve the goal (lb.instance.web). Within the case of neither document ever having been queried earlier than by your recursive resolver, it’s much more costly timewise, as the total DNS hierarchy could also be traversed for each information:

You because the DNS consumer (or stub resolver) question your recursive resolver for www.instance.com.

Your recursive resolver queries the basis identify server for www.instance.com.

The foundation identify server refers your recursive resolver to the .com High-Stage Area (TLD) authoritative server.

Your recursive resolver queries the .com TLD authoritative server for www.instance.com.

The .com TLD authoritative server refers your recursive server to the authoritative servers for instance.com.

Your recursive resolver queries the authoritative servers for www.instance.com and receives lb.instance.web as the reply.

Your recursive resolver caches the reply and returns it to you.

You now difficulty a second question to your recursive resolver for lb.instance.web.

Your recursive resolver queries the basis identify server for lb.instance.web.

The foundation identify server refers your recursive resolver to the .web High-Stage Area (TLD) authoritative server.

Your recursive resolver queries the .web TLD authoritative server for lb.instance.web.

The .web TLD authoritative server refers your recursive server to the authoritative servers for instance.web.

Your recursive resolver queries the authoritative servers for lb.instance.web and receives an IP deal with as the reply.

Your recursive resolver caches the reply and returns it to you.

Every of those steps consumes at the least a number of milliseconds, usually extra, relying on community circumstances. This may add as much as a substantial period of time that you simply spend ready for the ultimate, actionable reply of an IP deal with.

Within the case of an ALIAS document, all the identical actions are taken as with the CNAME, besides the authoritative server for instance.com performs steps six by 13 for you and returns the ultimate reply as each an IPv4 and IPv6 deal with. This presents two benefits and one vital downside:

Benefits

Sooner last reply decision velocity

Usually, the authoritative servers for instance.com may have the reply cached and thus can return the reply in a short time.

The alias response will likely be A and AAAA information. Since an ALIAS document returns the reply that contains a number of IP addresses, it may be used wherever an A or AAAA document can be utilized—together with the zone apex. This makes it extra versatile than a CNAME, which can’t be used on the zone apex.  The flexibleness of the Alias document is required when your web site is posted on a number of the hottest CDNs that require using CNAME information if you’d like your customers to have the ability to entry it through the bare area reminiscent of instance.com.

Disadvantages

Geotargeting data is misplaced

Since it’s the authoritative server for instance.com that’s issuing the queries for lb.instance.web, then any clever routing performance on the lb.instance.web document will act upon the placement of the authoritative server, not in your location. The EDNS0 edns-client-subnet possibility doesn’t apply right here. This implies that you could be be doubtlessly mis-routed: for instance, if you’re in New York and the authoritative server for instance.com is in California, then lb.instance.com will imagine you to be in California and can return a solution that’s distinctly sub-optimal for you in New York.  Nonetheless, if you’re utilizing a DNS supplier with worldwide pops, then it’s probably that the authoritative DNS server will likely be positioned in your area, thus mitigating this difficulty.

One vital factor to notice is that NS1 collapses CNAME information, supplied that all of them fall throughout the NS1 system. NS1’s nameservers are authoritative for each the CNAME and the goal document. Collapsing merely implies that the NS1 nameserver will return the total chain of information, from CNAME to last reply, in a single response. This eliminates all the extra lookup steps and lets you use CNAME information, even in a nested configuration, with none efficiency penalty.

And even higher, NS1 helps a novel document kind known as a Linked Document. That is mainly a symbolic hyperlink inside our platform that acts as an ALIAS document may, besides with sub-microsecond decision velocity. To make use of a Linked Document, merely create the goal document as you normally would (it may be of any kind) after which create a second document to level to it and choose the Linked Document possibility. Notice that Linked Information can cross area (zone) boundaries and even account boundaries inside NS1 and provide a strong option to set up and optimize your DNS document construction.

CNAME, ALIAS and Linked Document Reference Chart

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