What's the meaning of '@' in a DNS zone file?

Solution 1:

RFC 1035 defines the format of a DNS zone file.

... on page 34 you'll find:

@ A free standing @ is used to denote the current origin.

This means that @ is a shortcut for the name defined with $ORIGIN.

You can find more information on $ORIGIN here, which is an excerpt from Pro DNS and BIND, published by Apress.

Solution 2:

It's the root, or in your example it's mydomain.com.

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Solution 3:

It's an alias for the zone name itself. In this case, it indicates that the zone name has that address (or mx record, or ...)