[chef] Open Source Chef: server as workstation in dmz

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Hello community,

I'm quite new to chef and I have to set up a chef server and now I am totally
stuck. I hope I can find some help here because I found nothing about my
problem in the documentation and I'm working on this since 3 weeks :(

First of all the describtion of the situation: 

The server resides in the dmz subnet of the office lan (as a vm, Ubuntu 
14.04).
It has a private IP (192.168.0.2) and local name/fqdn (chef.dmz.loc). From the
internet the server is accessible via an external FQDN and IP (example.com,
93.184.216.119) by the appropriate firewall rules/port-forwarding.

It is also used as workstation and a special user account (chefdev) is
designated to create, modify and upload cookbooks as well as bootstrap nodes. 
This setup (dmz, special account, server = workstation) can be seen as
constraints.

The problem is that I either can't upload cookbooks or I can't bootstrap 
nodes.
If I configure everything for the local FQDN it's possible to upload cookboks,
but bootstrapping nodes does not work because from the internet the local name
is not resolveable (of course!). If I configure the server for it's external 
IP
I can't upload cookbooks because of ssl handshake failure.

Is there any solution for this under the constraints mentioned above? Thanks 
in
advance.

Below are some configurations and error messages which might be neede for you
to help me. If you need some more, please tell me.

configuration (ext. IP): http://pastebin.com/3uwMYutz

error messages
knife: http://pastebin.com/gAfsYiej
erchef: http://pastebin.com/Rc5UDvj4