Someone asked an easy one on Marketo Nation today:

We are adding new branding domain aliases to our Marketo Instances and we were wondering if we can use the same alias across all instances?

The answer is definitely No, and figured it was worth an official post in case others have this question down the line.

The explanation is pretty simple (as these things go!).

Branding domains are implemented via DNS Alias (CNAME) records. The left-hand-side of a CNAME record is the alias: your branding domain. The right-hand-side is the canonical name: the Marketo-controlled domain.[1]

An alias value is only allowed to appear once per DNS zone. So once you set up this record:

click.example.com.       300     IN      CNAME   mkto-sj010203.com.

You can’t also have:

click.example.com.       300     IN      CNAME   mkto-sj040506.com.

Here’s the warning from my favorite DNS server:

It’s a universally implemented rule, yet it’s kind of reading between the lines of the DNS RFC as opposed to clearly set down.

## You wouldn’t want to do it anyway

Once you step through the consequences, you realize you wouldn’t want click.vaneck.com aliases pointing to different Marketo instances even if it were possible.

Such aliases could only “work” if the client chose one at random or via round-robin. But that would never work for click tracking, because the email is only known on one of the servers! You’d just end up with rampant 404 errors.[2]

##### Notes

[1] Continuing my quest to make people understand that the branding domain is not “the CNAME”. That’s the alias. It’s the other side that’s the CNAME!

[2] In contrast, if a farm of web servers all had the same data, then multiple aliases could work. Again, in theory only. In reality, you do this by having a single alias whose CNAME is an A record, and that A record can resolve to multiple IPs. Some DNS servers additionally use load-balancing and geolocation to decide which IPs to return.