The difference between ALIAS and CNAME and when to use them

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

Let’s begin with the CNAME report kind. It merely factors a DNS title, like www.instance.com, at one other DNS title, like lb.instance.web.  This tells the resolver to search for the reply on the reference title 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 report 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 information:

1. You because the DNS shopper (or stub resolver) question your recursive resolver for www.instance.com.
2. Your recursive resolver queries the foundation title server for www.instance.com.
3. The basis title 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 foundation title server for lb.instance.web.
10. The basis title 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 tackle as the reply.
14. 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 just 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 13 for you and returns the ultimate reply as each an IPv4 and IPv6 tackle. This presents two benefits and one vital disadvantage:

Benefits

Sooner ultimate reply decision velocity

Usually, 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 information. Since an ALIAS report returns the reply that includes a number of IP addresses, it may be used anyplace 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 a number of the hottest CDNs that require using CNAME information if you need your customers to have the ability to entry it by way of the bare area resembling instance.com.

Disadvantages

Geotargeting info 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 report 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 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 might be situated 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 report. Collapsing merely signifies that the NS1 nameserver will return the complete chain of information, from CNAME to ultimate 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 singular report kind referred to as a Linked Document. That is mainly a symbolic hyperlink inside our platform that acts as an ALIAS report may, besides with sub-microsecond decision velocity. To make use of a Linked Document, merely create the goal report as you normally would (it may be of any kind) after which create a second report to level to it and choose the Linked Document choice. Be aware that Linked Information can cross area (zone) boundaries and even account boundaries inside NS1 and provide a robust option to manage and optimize your DNS report construction.

CNAME, ALIAS and Linked Document Reference Chart

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