DNS (Domain Name System) plays an indispensable role in web hosting and domain management. DNS is like a phonebook for the internet that translates human-readable domain names (e.g., example.com) into IP addresses that computers can understand. In this post, we will delve into a common question related to DNS records – why can’t you have an MX record and a CNAME record for the same domain?
What Are CNAME and MX Records?
Before addressing the question, let’s quickly understand what CNAME and MX records are.
CNAME Records
CNAME stands for Canonical Name. A CNAME record is used to point one domain or subdomain to another domain name. Instead of pointing to an IP address, it points to another domain. For example, you can have `blog.example.com` point to `example.wordpress.com`, which means when someone types in `blog.example.com`, they’ll be directed to your WordPress blog.
MX Records
MX stands for Mail Exchange. MX records are used to route emails and determine the mail servers that are responsible for receiving emails on behalf of a domain. For example, if you have a domain `example.com` and you want to use Google’s mail servers, you will set up MX records pointing to Google’s mail servers.
The CNAME and MX Conflict
Now, let’s address the core of our topic. In DNS, if you have a CNAME record for a domain, you cannot have any other records of a different type for the same domain. This includes MX records.
Why is this the case?
A CNAME record indicates that the domain is an alias for another domain. All DNS queries, regardless of their type (A, MX, TXT, etc.), for this domain should be resolved by referring to the DNS records of the target domain to which the CNAME record points.
This creates a conflict if an MX record (or any other type of record) is defined for the same domain because the CNAME record is effectively stating that the domain shouldn’t have any records of its own.
Standards and Guidelines
This restriction is not arbitrary but is based on the DNS standards defined in RFC 1034. The RFC states:
> If a CNAME RR is present at a node, no other data should be present; this ensures that the data for a canonical name and its aliases cannot be different.
In simpler terms, this ensures consistency and integrity in the DNS records.
Conclusion
DNS can be a little tricky, but understanding the basic records and how they interact is essential for effective domain management. Always remember that CNAME records should be used carefully to avoid conflicts with other record types. When in doubt, refer to the DNS standards or consult a domain management expert.
WORK AROUNDS:
When you want a subdomain to act as an alias and also receive emails, you can employ two different approaches as workarounds. Here are both solutions explained in detail:
Workaround 1: Using CNAME and MX Records
This approach uses a combination of CNAME and MX records to achieve the desired functionality.
Step 1: Set up the CNAME record
First, create a CNAME record for the subdomain you want to act as an alias. For example, if you want `prod-webapi.cordero.me` to be an alias for `webapi.prod.cordero.me`, your DNS record will be:
prod-webapi.cordero.me. 300 IN CNAME webapi.prod.cordero.me.
Step 2: Set up the MX record on the target domain
Next, set up an MX record on the target domain of the CNAME (in this case, `webapi.prod.cordero.me`) to specify the mail server that will handle the emails.
webapi.prod.cordero.me. IN MX 10 inbound.postmarkapp.com.
How it works
When an email is sent to `prod-webapi.cordero.me`, the mail server first resolves the CNAME record to `webapi.prod.cordero.me`. It then uses the MX record of `webapi.prod.cordero.me` to deliver the email. Essentially, `prod-webapi.cordero.me` inherits the mail functionality from `webapi.prod.cordero.me`.
Workaround 2: Using Separate A and MX Records
This approach avoids the use of CNAME records entirely and sets up separate A and MX records for the subdomain.
Step 1: Set up the A records
Set up an A record for the subdomain pointing to the desired IP address. In this example, both subdomains point to the same IP address.
webapi.prod.cordero.me. IN A 10.10.10.10 prod-webapi.cordero.me. IN A 10.10.10.10
Step 2: Set up the MX records for both subdomains
Now, create MX records for both subdomains specifying the mail server.
webapi.prod.cordero.me. IN MX 10 inbound.postmarkapp.com. prod-webapi.cordero.me. IN MX 10 inbound.postmarkapp.com.
How it works
In this approach, both subdomains have their own set of A and MX records. When an email is sent to either of the subdomains, the mail server directly uses the respective MX records without any alias redirection.
Conclusion
Both approaches will allow `prod-webapi.cordero.me` to receive emails. The choice between them can depend on various factors such as administrative preferences, DNS resolution times, or the simplicity of DNS records management. It’s also important to ensure that the mail server specified in the MX records is configured correctly to handle emails for the subdomains.
IMPORTANT FOR WORKAROUND 1
Note on Email Handling: The MX record I’ve specified (inbound.postmarkapp.com) must be configured to accept and handle emails for the domains in question. Simply setting the MX record doesn’t guarantee that emails will be accepted unless the server specified in the MX record is configured to do so.
Suppose Postmark (or any mail service specified in an MX record) is not configured to accept emails for the domain specified in the MX record (`webapi.prod.cordero.me` in this example). In that case, emails sent to that domain, or any domain aliasing it through a CNAME record (such as `prod-webapi.cordero.me`), will not be successfully processed.
In the scenario, I describe:
1. An email is sent to an address at `prod-webapi.cordero.me`.
2. The DNS system resolves the CNAME record and sees that `prod-webapi.cordero.me` is an alias for `webapi.prod.cordero.me`.
3. The DNS system then looks for the MX records for `webapi.prod.cordero.me` and sees that it should use Postmark for email processing.
4. Postmark receives the incoming email but does not recognize `webapi.prod.cordero.me` as a domain it is configured to handle.
5. Postmark rejects the email because it is not configured to accept emails for this domain.
To make it work, you must ensure that the mail service (in this case, Postmark) is configured to accept emails for the domain specified in the MX records. This usually involves adding and verifying the domain in the mail service’s control panel and possibly configuring additional DNS records (such as SPF or DKIM) for authentication purposes.
If Postmark is configured to recognize `prod-webapi.cordero.me` but not `webapi.prod.cordero.me`, then you might consider using the second workaround mentioned earlier by setting separate A and MX records for `prod-webapi.cordero.me` without using a CNAME record. This way, you can directly associate Postmark with `prod-webapi.cordero.me`:
prod-webapi.cordero.me. IN A 10.10.10.10 prod-webapi.cordero.me. IN MX 10 inbound.postmarkapp.com.
HOW IT WORKS
Using the configurations I provided in the previous response, `prod-webapi.cordero.me` will be able to receive emails. However, which method you choose affects how it’s achieved:
1. Using CNAME and MX records: In this case, `prod-webapi.cordero.me` doesn’t have its own MX record. Instead, it has a CNAME record pointing to `webapi.prod.cordero.me`. When an email is sent to `prod-webapi.cordero.me`, the mail server first resolves the CNAME record to `webapi.prod.cordero.me` and then uses the MX record of `webapi.prod.cordero.me` to deliver the email. This means that `prod-webapi.cordero.me` inherits the mail functionality from `webapi.prod.cordero.me`.
2. Using separate A and MX records: In this approach, `prod-webapi.cordero.me` has its own MX record. When an email is sent to `prod-webapi.cordero.me`, the mail server directly uses the MX record associated with it. This means that `prod-webapi.cordero.me` has its own mail functionality independent of any other domain or subdomain.
In both cases, yes, `prod-webapi.cordero.me` will be able to receive emails. However, it’s important to ensure that the mail server specified in the MX record (`inbound.postmarkapp.com` in this example) is configured correctly to handle emails for the subdomain.