The difference between ALIAS and CNAME and when to use them

The chief distinction between a CNAME file and an ALIAS file just isn’t within the outcome—each level to a different DNS record—however in how they resolve the goal DNS file when queried. Because of this distinction, one is protected to make use of on the zone apex (for instance, bare area comparable to instance.com), whereas the opposite just isn’t.

Let’s begin with the CNAME file sort. It merely factors a DNS identify, like www.instance.com, at one other DNS identify, like lb.instance.internet.  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 at the very least one further DNS lookup have to be carried out to resolve the goal (lb.instance.internet). Within the case of neither file ever having been queried earlier than by your recursive resolver, it’s much more costly timewise, as the complete DNS hierarchy could also be traversed for each data:

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 foundation 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.internet as the reply.
7. Your recursive resolver caches the reply and returns it to you.
8. You now subject a second question to your recursive resolver for lb.instance.internet.
9. Your recursive resolver queries the basis identify server for lb.instance.internet.
10. The foundation identify server refers your recursive resolver to the .internet Prime-Degree Area (TLD) authoritative server.
11. Your recursive resolver queries the .internet TLD authoritative server for lb.instance.internet.
12. The .internet TLD authoritative server refers your recursive server to the authoritative servers for instance.internet.
13. Your recursive resolver queries the authoritative servers for lb.instance.internet and receives an IP deal with as the reply.
14. Your recursive resolver caches the reply and returns it to you.

Every of those steps consumes at the very least a number of milliseconds, usually extra, relying on community circumstances. This could 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 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 deal with. This affords two benefits and one important disadvantage:

Benefits

Quicker remaining reply decision pace

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

The alias response can be A and AAAA data. Since an ALIAS file returns the reply that includes 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 pliability of the Alias file is required when your website is posted on among the hottest CDNs that require using CNAME data if you need your customers to have the ability to entry it by way of the bare area comparable to instance.com.

Disadvantages

Geotargeting info is misplaced

Since it’s the authoritative server for instance.com that’s issuing the queries for lb.instance.internet, then any clever routing performance on the lb.instance.internet file will act upon the situation of the authoritative server, not in your location. The EDNS0 edns-client-subnet choice doesn’t apply right here. This implies that you could be be probably 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 consider you to be in California and can return a solution that’s distinctly sub-optimal for you in New York.  Nevertheless, if you’re utilizing a DNS supplier with worldwide pops, then it’s seemingly that the authoritative DNS server can be positioned in your area, thus mitigating this subject.

One necessary factor to notice is that NS1 collapses CNAME data, 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 implies that the NS1 nameserver will return the complete chain of data, from CNAME to remaining reply, in a single response. This eliminates all the extra lookup steps and means that you can use CNAME data, even in a nested configuration, with none efficiency penalty.

And even higher, NS1 helps a novel file sort referred to as a Linked File. That is mainly 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 File, merely create the goal file as you normally would (it may be of any sort) after which create a second file to level to it and choose the Linked File choice. Notice that Linked Data can cross area (zone) boundaries and even account boundaries inside NS1 and supply a strong method to arrange and optimize your DNS file construction.

CNAME, ALIAS and Linked File Reference Chart

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