dns-mode-soa-auto-increment-serial is a variable defined in `dns-mode.el'.
Its value is

Whether to increment the SOA serial number automatically.

If this variable is t, the serial number is incremented upon each save of
the file. If it is `ask', Emacs asks for confirmation whether it should
increment the serial upon saving. If nil, serials must be incremented
manually with M-x dns-mode-soa-increment-serial.

You can customize this variable.