The difference between ALIAS and CNAME and when to use them

The chief distinction between a CNAME report and an ALIAS report just isn’t within the end result—each level to a different DNS record—however in how they resolve the goal DNS report 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 just isn’t.

Let’s begin with the CNAME report 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 sorts (for instance, A, AAAA, MX, NS, SOA, and others). This introduces a efficiency penalty, since at the very least one further DNS lookup should be carried out to resolve the goal (lb.instance.internet). Within the case of neither report 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 data:

1. You because the DNS consumer (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-Stage 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-Stage 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 tackle 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, typically 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 tackle.

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

Benefits

Quicker last reply decision pace

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

The alias response might be A and AAAA data. Since an ALIAS report returns the reply that includes a number of IP addresses, it may be used wherever an A or AAAA report 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 report is required when your website is posted on among the hottest CDNs that require using CNAME data if you’d like your customers to have the ability to entry it by way of 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.internet, then any clever routing performance on the lb.instance.internet report will act upon the placement 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, in case you are 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, in case you are utilizing a DNS supplier with worldwide pops, then it’s doubtless that the authoritative DNS server might be positioned in your area, thus mitigating this subject.

One vital factor to notice is that NS1 collapses CNAME data, offered that all of them fall inside the NS1 system. NS1’s nameservers are authoritative for each the CNAME and the goal report. Collapsing merely signifies that the NS1 nameserver will return the total chain of data, from CNAME to last 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 singular report sort referred to as a Linked Report. That is principally a symbolic hyperlink inside our platform that acts as an ALIAS report would possibly, besides with sub-microsecond decision pace. To make use of a Linked Report, merely create the goal report as you normally would (it may be of any sort) after which create a second report to level to it and choose the Linked Report choice. Observe that Linked Data can cross area (zone) boundaries and even account boundaries inside NS1 and provide a robust strategy to set up and optimize your DNS report construction.

CNAME, ALIAS and Linked Report Reference Chart

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