Split DNS With External Forwarding [closed]

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I'm implementing a split DNS set up where you h开发者_开发百科ave an external and internal domain of example.com and internally the same names resolve as externally but resolve to internal addresses. This is useful for web redirects to work with the same addresses internally and externally and internal and external e-mail amongst other things. So far, so good.

However, I have a slightly more complicated requirement where I would like to resolve names that are known to the internal DNS but forward names that are not known to the external DNS (forwarder). So, mail.example.com is known to the internal DNS and resolves but smtp.example.com isn't and will forward this request to an external DNS forwarder. I'd really not like to have to replicate external DNS entries on the internal DNS system but this is problematic since the internal DNS needs to be authoritative for the domain internally.

Does anyone know how I might go about doing this, preferably with BIND? I can't do anything with slave zone transfers and alike because of restrictions.