NS also in SOA doesn't get NOTIFY

[Jonathan Stewart]

Hello,

Recently I set up a group of nameservers using a hidden master,
visible slaves configuration.

ns0 - hidden master
ns1, ns2, ns3 - visible slave servers

So I set the SOA and NS records like this

zone.example  IN SOA ns1.zone.example. hostmaster.example.com (
     1            ; serial number
     3600         ; refresh   [1h]
     600          ; retry     [10m]
     86400        ; expire    [1d]
     3600 )

      IN NS  ns1.zone.example
      IN NS  ns2.zone.example
      IN NS  ns3.zone.example


Thus, the hidden master, ns0, does not appear in the SOA or NS records.

The problem is that NOTIFY messages do not get delivered to ns1,
because it's the primary server in the SOA record.  If i change the
SOA to have ns0, then NOTIFYs work, ns1 updates immediately.  I don't
like this solution because my hidden master is no longer hidden when
I'm publishing it in the SOA.

Also, is this normal/expected behaviour?  How can i get ns0 (and the
others) to NOTIFY ns1 when the serial is incremented?  Must i use an
explicit {also-notify} ?

Thanks,

-- 
     Jonathan