DNS MX records for subdomains

[Kevin Darcy]

Gerry George wrote:

> I have a DNS setup & working correctly (AFAIK).
>
> I want to create separate mail domains for departments for this  mail to be
> handled separately as follows:
>
> My.domain       MX      10      mail.my.domain
>                 MX      20      other.my.domain
>
> I would have expected that mail to user at sub.my.domain would automatically
> get handled without the need for a wildcard DNS record.  However, the DNS
> server would respond with a "domain unknown" type of error.
>
> I ended up doing
> My.domain       MX      10      mail.my.domain.
>                 MX      20      other.my.domain.
> sub1            MX      10      mail.sub1.my.domain.
>                 MX      20      mail.my.domain.
>                 MX      30      other.my.domain.
> sub2            MX      10      mail.sub2.my.domain.
>                 MX      20      mail.my.domain.
>                 MX      30      other.my.domain.
>
> This correctly gets the mail to the sub-domain mail server.  Is this the
> expected BIND behavior and were *my* expectations incorrect?
>
> system: Linux (SuSE 6.4) & BIND 8.x

This is the expected *DNS* (not BIND-specific) behavior. Your expectations
were incorrect. In the absence of a wildcard entry, it would be dangerous for
a DNS server to just *assume* the existence of records that weren't explicitly
added to the database. Note that even *with* a wildcard "my.domain" MX record,
if you had nodes under "sub1.my.domain" and/or "sub2.my.domain", or
non-MX records owned by those names, you'd still have to add MX records with
those names if you wanted them to be mailable, because of the way
wildcard-matching works in DNS.


- Kevin