DNS: Why Can't I have a TXT Record ( Or any other record) alongside my CNAME record?

TLDR; Because that's how DNS works. https://www.ietf.org/rfc/rfc1912.txt

I've run across this issue various times in the last...we'll I won't tell you how long, but it's been a long time. Every time that I see this issue pop up I scramble and learn the same thing, in hopes that the lessons learned will stick I have decided to create a blog post. 

We are all accustomed to nice domain names (i.e. google.com, facebook.com), and as an end-user the backend inner workings are abstracted. What we do know is when I type in my domain on the browser, some magic happens. While I don't understand the complete magic I will do my best to explain why you can't have any other record alongside a CNAME record. 

What's a CNAME record, that's true let's take a step back. Let's take store.mydomain.com as an example, a DNS server is responsible for telling browsers how to traverse the internet and locate the server that is hosting your desired store. Other types of services that have dedicated records are Mail (email) servers they get their own MX record. There are various other records in the DNS scheme, we won't go through all of them but I've selected a sample to go over:

• A Record
• This record points store.mysite.com to an IP Address 5.5.5.5, this means that all traffic destined to your store will get forwarded to the IP address. 
• TXT Record
• Think of this as a text file that you can use to confirm ownership or management of a domain. This file is readable by the internet, in essence, if you can write to this file we can construe that you own the domain. 
• CNAME Record
• This stands for Canonical Name, the easiest way to think of this is an alias or a nickname. www.store.mysite.com can be a nickname for store.mysite.com. But let's take it a step further store.mysite.com can be a nickname to store.BIGCompany.server.hosted.com. That big company server can be Google, AWS, Oracle, or any company offering you a hosted service.
   

That's all great but why use CNAME vs A records if they both point to the same place? As an administrator, I can change records for mydomain.com at will without waiting for anyone else, on the flip side the administrators for the BIGComapny can update their records whenever they feel like it. In the A record above let's say that 5.5.5.5 needs to be updated to 7.7.7.7, if CNAMES were in use that change would be transparent to mydomain.com. Since we are using A records BIGComapny needs to let MyDomain.com know of the change and plan accordingly. For small mom-and-pop shops, it would be fine to coordinate and schedule time, but when dealing with thousands and possibly millions of domains and/or DNS Records it does not scale well. 

I figure giving a rundown on various types of records and why they are used is important to lay down the foundation. Don't get upset, but the reason you can't have CNAME records mixed with any others is that you can't :). DNS was built with this constraint in mind, why? That goes beyond the scope of this article. Taking an excerpt from https://www.ietf.org/rfc/rfc1912.txt, section 2.4 states "A CNAME record is not allowed to coexist with any other data.".