Reconfigure system (network) during boot based on HyperV information

Question

I am running a HyperV environment with Ubuntu 14.04 server running in generation 1 guests setups. It runs nicely.

I want to be able to clone the system (while shut down), give it a new name, and have the new system reconfigure itself slightly - different static IP, different LVS setup, different Apache2 setup, etc. I am doing that by copying over the appropriate configuration files (i.e. the system has multiple copies, it replaces files like /etc/network/interfaces with the right one)

I cannot find an appropriate place in upstart to do this. Doing it before local-file-systems the file system is not available read/write. Doing it after, e.g. in if-pre-up.d during network startup collides with the system's own configuration (i.e. I'm replacing /etc/network/interfaces after it has started using it).

It is not desirable to have it fully boot and run a script as it will pop up on the network with an IP conflict, potentially affecting the parent of this clone. So I want to do it during boot.

I have somewhat gotten this to work by rebooting a second time from if-pre-up.d, after moving the files, before allowing it to finish coming up. But that seems a bit wrong for various reasons, not the least of which it then stops in grub (or whatever that boot screen is) and requires an ENTER key. [I've posted a question on this elsewhere, if you can help with that, please answer there.]

Is there an appropriate place in upstart's sequence that I can overwrite network interface files (and related service files like keepaloved, apache2, haproxy) and have them processed normally?

Answer 1

I have continued to beat on this and have it working, though it is not pretty.

I put in a if-pre-up.d script, and took OUT the eth0 device from /etc/interfaces (which is the device I want to configure).

Then when this fires (which may be multiple times), I manually configure eth0 with ip command lines (e.g. ip addr, ip route, etc.). I also wrote the /etc/resolvconf file for dns servers.

I added a line at the beginning which notices if eth0 is already configured and exits, so if it runs multiple times I only do the work once. Doing command line configuration and leaving eth0 out of /etc/interfaces keeps it from conflicting with the normal configuration of eth0.

It's ugly, but I was unable to find an appropriate point in upstart between the local filesystem being configured, but before the network configuration started. At least in theory I could change the associated /etc/init files so I could insert such a step, but that seems fragile, especially during updates.

The result is a self configuring guest that can run in Hyper-V, look at its host name, and configure itself - IP addresses, other startup/config files, etc. In this way I can clone one copy to many, and get all sorts of different setups driven by a simple script.

This works... but I would sure welcome a better answer.