Update of underlying OS

Hi there,
I'm currently using ISPConfig V. 3.1.15p2 on a server running Debian 8.11.
The websever being used is apache V. 2.4.27.
I wanted to update the system using apt-get update, apt-get upgrade and apt-get dist-upgrade, but since some time, I cannot perform apt-get dist-upgrade anymore as then, apache2 is being removed rendering my rendering my system being being unusable.
I do not want to retry the dist-upgrade with updated/patched apt.sources as I fear that this will break my system beyond any hope of repair.

I currently have several web sites running on the server with each having one or many MySQL databases.
There are some user accounts and some FTP accounts and so on.
And there are 50 (or so) primary DNS zones on the server.

How can this system be updated easily?
What are possible scenarios for the update?

My ideas:
- Set up a parallel ISPConfig system
Then transfer all the web pages (backup on one server and restore on the other one), and all the DNS zones (very complicated?)

- Set up an additional ISPConfig system (a slave ISPConfig system with the current server being the master system)
Then transfer all the web pages (one-click?) and all the DNS zones (one-click?)
Then update the primary server (does this work when only the secondary ISPConfig server is running? Can the upgraded server then be the master ISPConfig server again?)

- Full ISPConfig backup (including web sites, settings, DNS zones and all other parts of the system) which is restored on a temporary server, the upgrade the system and move the ISPConfig installation back.

- any other scenario?

Are there any suggestions on how to do this OS upgrade?

Best regards,
Thomas

 

Updated to what? Debian 9, or what?
If you have not changed /etc/apt/sources.list, and apt-get dist-upgrade removes Apache, then something conflicts with Apache causing it to be removed. You can see the reason for removal reading the stuff that dist-upgrade writes on your screen, or from dpkg.log.
If you are upgrading Debian 8 to Debian 9, do it following this: https://www.debian.org/releases/stretch/releasenotes If something goes wrong during upgrade that document should tell how to repair it.

 

Yes, I'd like to update to Debian 9.
But that didn't work. Apache2 is still uninstalled due to dependency issues.
I assume it's due to a manually installed php3 on the current ISPConfig system, but I am not sure.
Basically, I'm currently not able to update the ISPConfig installation to a new Debian version. (at least not in-place...)

 

I am sure you can upgrade to next Debian version, at least it works following the Release Notes I posted a link to.
If apache2 is removed during the upgrade, apt-get install it after the upgrade. The command shows what would get removed, so you can examine what is the conflict.
It is possible that you have installed a system where something conflicts with apache, you should be able to see what those conflicting packages are and remove those or otherwise resolve the conflict.

 

Yes, I tried to force the installation of the packages, but there were missing packages or version conflicts again and again.
I guess it's due to the manually-added php version on the server. That manually copied bits break Debians update dependencies...
I've never heard of the Migration tool or ISPCopy. I'll give it a try.