Draft of how to change an OS on a Gandi VPS.
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Title: How to install any distrib on a Gandi VPS
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Date: 2021-11-18 08:51:00
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Status: Draft
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Summary: I needed to install BBB, which requires Ubuntu 18.0.4, but they only had Ubuntu 20.0.4...
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---
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Currently Gandi VPS don't provide an image with Ubuntu Bionic, which
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is needed for example to install BBB. So I started an in-memory Ubuntu
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on top of an Ubuntu to install an Ubuntu...
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If you land here from the future, and want to do the same, double
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check, Gandy may have already implemented an object storage allowing
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you to use any distrib, making the setup of alternative distribs way
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easier.
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The same process could be used to install any Debian based distrib, or
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with some more modifications any other Linux distrib, don't hesitate
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to play around.
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> ⚠ WARNING ⚠
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> The probability you'll just brick your VPS once or twice before succeding is high.
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> Don't do this on a VPS you care.
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>
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> In case of failure, just drop the VPS and start fresh.
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OK, « en route » !!
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The first step is to start a small Ubuntu (or Debian or whatever) in
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memory, so we'll be able to destroy the real one, for this I used
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[takeover.sh](https://github.com/marcan/takeover.sh), so SSH to your
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VPS and:
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First get root:
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```bash
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$ sudo su -
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```
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Then we'll create a new root, in memory, for the temporary distrib:
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```bash
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# mkdir /takeover
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# mount -t tmpfs tmpfs /takeover -o size=2G # Fully in RAM as we'll umount / later.
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# apt update; apt-get install debootstrap busybox git build-essential
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# debootstrap --variant=minbase bionic /takeover/ http://archive.ubuntu.com/ubuntu/ # Any OS would do, it's just a temporary one from which we'll install the real one.
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```
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We could already chroot in it, but we'll prepare the ground for
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[takeover.sh](https://github.com/marcan/takeover.sh):
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```bash
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# wget -O /takeover/busybox https://www.busybox.net/downloads/binaries/1.26.2-defconfig-multiarch/busybox-x86_64
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# chmod a+x /takeover/busybox
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# git clone https://github.com/marcan/takeover.sh /tmp/takeover.sh/
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# cp -a /tmp/takeover.sh/* /takeover/
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# cd /takeover
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# gcc -static fakeinit.c -o fakeinit
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# chroot /takeover /usr/bin/apt install openssh-server
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# mkdir /takeover/run/sshd
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# cp usr/bin/passwd bin/ # takover.sh need it here
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```
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OK everything is ready to "pivot" root filesystem:
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```bash
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# sh takeover.sh # Here Come The Dragons.
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```
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If everything goes well, we can now ssh to the in-memory sshd (on port
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80), and kill all remaining things of the real underlying OS, umount
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everything, format it, and start fresh:
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```bash
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$ ssh -p80 root@your_host
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# pkill -9 -f systemd # and all remaining process using kill -9 PID PID PID PID PID PID ... EXCEPT the SSH you're actually using!
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# mount | tac | grep old_root | cut -d' ' -f3 | xargs umount # Umount old_root (may need to be ran multiple times)
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# mkfs.ext4 /dev/xvda1 # A clean filesystem for the new distrib
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# mount /dev/xvda1 /old_root/
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# echo nameserver 9.9.9.9 > /etc/resolv.conf # NEIN! NEIN!! NEIN!!! NEIN!!!!
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# apt install debootstrap
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```
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OK we destroyed everything from the old distrib, let's choose a new one:
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```bash
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# debootstrap bionic /old_root/ http://archive.ubuntu.com/ubuntu/ # This is the OS and version you want to install, maybe it's not Ubuntu bionic!
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# mount -o bind /dev/ /old_root/dev/ # Prepare to chroot
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# mount -o bind /run/ /old_root/run/ # Prepare to chroot.
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# mount -o bind /sys/ /old_root/sys/ # Prepare to chroot..
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# mount -t proc none /old_root/proc/ # Prepare to chroot...
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# chroot /old_root/ # And we're back on disk!
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```
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Now we're back on disk, on the new distrib, we may have some
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configuration to do before rebooting like installing a kernel,
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changing root password, adding ssh keys, configuring grub, configuring
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network interfaces...
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```bash
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# echo nameserver 9.9.9.9 > /etc/resolv.conf # Yes, again: we're not on the same root...
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# apt update
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# apt install linux-virtual openssh-server ifupdown
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# mkdir /root/.ssh/
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# wget https://mdk.fr/id_rsa.pub -O /root/.ssh/authorized_keys # Or your own keys, your choice.
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# passwd # Set a root password, useful to connect via the emergency console.
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# printf "%s\n%s\n 'auto eth0' 'iface eth0 inet dhcp' > /etc/network/interfaces # Setup the network
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# # Setup /etc/fstab, if you want the UUID of the disk use `blkid`.
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# sync # For good measure
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# reboot now
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```
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Now you can ssh on port 22 again, yes the fingerprint will have
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changed as we replaced the root filesystem we replaced /etc/ssh/key*,
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don't worry.
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