The difference between ALIAS and CNAME and when to use them

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

Let’s begin with the CNAME file 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 lookup the reply on the reference identify for all DNS varieties (for instance, A, AAAA, MX, NS, SOA, and others). This introduces a efficiency penalty, since a minimum of one extra DNS lookup have to be carried out to resolve the goal (lb.instance.web). Within the case of neither file 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:

1. You because the DNS shopper (or stub resolver) question your recursive resolver for www.instance.com.
2. Your recursive resolver queries the basis identify server for www.instance.com.
3. The basis identify server refers your recursive resolver to the .com Prime-Degree Area (TLD) authoritative server.
4. Your recursive resolver queries the .com TLD authoritative server for www.instance.com.
5. The .com TLD authoritative server refers your recursive server to the authoritative servers for instance.com.
6. Your recursive resolver queries the authoritative servers for www.instance.com and receives lb.instance.web as the reply.
7. Your recursive resolver caches the reply and returns it to you.
8. You now situation a second question to your recursive resolver for lb.instance.web.
9. Your recursive resolver queries the basis identify server for lb.instance.web.
10. The basis identify server refers your recursive resolver to the .web Prime-Degree Area (TLD) authoritative server.
11. Your recursive resolver queries the .web TLD authoritative server for lb.instance.web.
12. The .web TLD authoritative server refers your recursive server to the authoritative servers for instance.web.
13. Your recursive resolver queries the authoritative servers for lb.instance.web and receives an IP handle as the reply.
14. Your recursive resolver caches the reply and returns it to you.

Every of those steps consumes a minimum of a number of milliseconds, typically extra, relying on community situations. This may add as much as a substantial period of time that you simply spend ready for the ultimate, actionable reply of an IP handle.

Within the case of an ALIAS file, 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 handle. This affords two benefits and one vital disadvantage:

Benefits

Quicker remaining reply decision pace

Generally, the authoritative servers for instance.com could 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 file returns the reply that contains a number of IP addresses, it may be used wherever an A or AAAA file 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 file is required when your web site is posted on among the hottest CDNs that require the usage of CNAME information if you need your customers to have the ability to entry it through the bare area equivalent to 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 file 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 seemingly that the authoritative DNS server will likely be positioned in your area, thus mitigating this situation.

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

And even higher, NS1 helps a novel file kind known as a Linked Document. That is principally a symbolic hyperlink inside our platform that acts as an ALIAS file would possibly, besides with sub-microsecond decision pace. To make use of a Linked Document, merely create the goal file as you often would (it may be of any kind) after which create a second file to level to it and choose the Linked Document possibility. Be aware that Linked Data can cross area (zone) boundaries and even account boundaries inside NS1 and supply a robust solution to set up and optimize your DNS file construction.

CNAME, ALIAS and Linked Document Reference Chart

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