DNS: Wildcards

Today I learned…

When you have a DNS zone (example.com) and create a wildcard like *.example.com with a CNAME to example.com every subdomain is resolved to your A/AAAA entry for example.com

If you run dig (domain information groper) you can see it in your shell:

-> % dig www.example.com

;; ANSWER SECTION:
www.example.com.	86400	IN	CNAME	example.com.
example.com.		3600	IN	A	1.2.3.256

How to break it

Today I learned, that if you add a record for a specific subdomain, e.g. www.example.com with a TXT entry, then the CNAME and therefore the A record is no longer used.

Or in other words: Wildcards work only, if there is no other more specific entry – and it doesn’t matter whether this is a MX, TXT or an SRV entry.

How to fix it

Fix is easy, if you add a TXT record for a specific subdomain, you need to add the CNAME record as well.

Source

A common mistake is thinking that a wildcard MX for a zone will apply to all hosts in the zone. A wildcard MX will apply only to names in the zone which aren’t listed in the DNS at all. e.g.,

https://datatracker.ietf.org/doc/html/rfc1912#section-2.7

More Source

Thanks Alexander Jank!

Technically, your explanation & conclusion is correct.
Just the RFC you are linking to, specifically states this for MX records – the general behavior is codified in RFC 1034 section 4.3.3

https://datatracker.ietf.org/doc/html/rfc1034#section-4.3.3

Thanks!

We had the problem with a customer of ours, it is currently 21:24 when I’m typing this and just hang up a call with Jonas Hünig – that is one reason we are hosting most of our customers (and all new one) on maxcluster. And Jonas was the one providing the source.