Change host using httphdr

list Sommerdal IT ApS
Tue, 14 May 2019 09:08:08 +0200
Message-Id: <user-e10250ba09a8@xymon.invalid>

I'm trying to access an internal ressource - trying to change the host
information in the http header.

192.168.29.11 <http://s.bl-1.com/h/c4MTgLWp?url=http://192.168.29.11>;
internal-host.eu <http://s.bl-1.com/h/c4MTgRvr?url=http://internal-host.eu>;
  #conn https://internal-host.eu
<http://s.bl-1.com/h/c4MTgWJt?url=https://internal-host.eu>; httphdr="Host:
external-host.com"

I end up with a 404, as the host isn't reconized.
Any suggestion on how to imporve?

Regards
Leif