Wildcards in Practice
To quote from RFC 4592, many DNS implementations diverge, in different ways, from the original definition of wildcards. Some of the variations include:
- With djbdns, in addition to checking for wildcards at the current level, the server checks for wildcards in all enclosing superdomains, all of the way up to the root. In the examples listed above, the query for _telnet._tcp.host1.example. for an MX record would match a wild card despite the domain _tcp.host1.example. existing.
- Microsoft's DNS server (if configured to do so) and MaraDNS (by default) have wildcards also match all requests for empty resource record sets, i.e. domain names for which there are no records of the desired type. In the examples listed above, the query for sub.*.example. for an MX record would match *.example., despite sub.*.example. explicitly existing with only a TXT record.
Read more about this topic: Wildcard DNS Record
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