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kexec-remote-debian-install

Work in progress tools to remotely re-install Debian or Ubuntu using kexec and SSH.

build.py

Before running build.py you will need the linux and initrd.gz files from the netboot installer of the distro you prefer.

Add non-free fimrware debian :

[ -f initrd.gz.orig ] || cp -p initrd.gz initrd.gz.orig
[ -f firmware.cpio.gz ] || wget http://cdimage.debian.org/cdimage/unofficial/non-free/firmware/stable/current/firmware.cpio.gz
cat initrd.gz.orig firmware.cpio.gz > initrd.gz

If you're upgrading from Debian Jessie or Ubuntu Trusty to something newer you may not know what "eth0" is going to become inside the newer installer, as newer distros by default use systemd's new interface naming scheme. In order to predict what "eth0" will become once you reboot, you can run this which will attempt to guess:

source <(udevadm test /sys/class/net/eth0 2>/dev/null | grep ID_NET_NAME); for name in "$ID_NET_NAME_ONBOARD" "$ID_NET_NAME_SLOT" "$ID_NET_NAME_PATH" "$ID_NET_NAME_MAC" ; do [[ -n $name ]] || continue; echo "$name"; break; done

If the script doesn't seem to bring up networking during a Jessie or Trusty upgrade then it would be a good idea to try the other possible names in the output of udevadm.

Currently build.py must be run as root or with fakeroot, as it unpacks the cpio archive and re-packs it, and this allows the permissions of various device files to be preserved.

$ fakeroot ./build.py initrd.gz eth0 192.0.2.5 255.255.255.0 192.0.2.1 8.8.8.8

d-i anna/choose_modules string network-console
d-i preseed/early_command string anna-install network-console

d-i network-console/password password c96f4ef5edef6a2daa7b63eb81db5f66
d-i network-console/password-again password c96f4ef5edef6a2daa7b63eb81db5f66

d-i netcfg/choose_interface select eth0
d-i netcfg/disable_dhcp boolean true
d-i netcfg/get_ipaddress string 192.0.2.5
d-i netcfg/get_netmask string 255.255.255.0
d-i netcfg/get_gateway string 192.0.2.1
d-i netcfg/get_nameservers string 8.8.8.8
d-i netcfg/confirm_static boolean true

d-i debian-installer/locale string en_NZ.UTF-8
d-i console-keymaps-at/keymap select us
    
Unpacking initrd...
105824 blocks

Re-packing initrd...
105825 blocks

Once initrd.gz has been re-packed the new kernel can be launched directly with kexec:

sudo kexec --command-line="auto=true priority=critical mirror/http/hostname=deb.debian.org" --initrd=initrd.gz linux

Once the machine has booted the new kernel, the installer will be available over SSH on the IP address given on the command line, with the username installer and the password shown during the build output. The password is generated randomly every time the script is run.

Tested distributions and releases

This script is known to be able to install:

  • Debian Jessie
  • Debian Stretch
  • Ubuntu Trusty
  • Ubuntu Xenial
  • Ubuntu Bionic

And will probably work on other releases too.

Caveats

Presently the mirror used after rebooting may default to one that is not optimal for your location, so the initial bringup of SSH will take a worryingly long time. Don't worry, if you can ping it it's probably going to get there in the end.

The method this uses to take over the initrd is really a horrible hack. One day I'll make something more structured.