Rediscovering hosts locally via Grub2


When a host is managed via Foreman, it’s possible to download discovery image to it and create a Grub2 boot entry to quickly enter discovery mode again. This ad-hoc workflow requires the old managed host entry to be deleted prior provisioning discovered host, otherwise either “Name already taken” error can be shown or old and unused host entries will stack in the Foreman DB forever depending on the Foreman version in use.

The first thing is to expose discovery Linux kernel and init RAM disk via Apache httpd server:

server# ln -s /var/lib/tftpboot/boot/fdi-image/vmlinuz0 /var/www/html/pub/discovery-vmlinuz
server# ln -s /var/lib/tftpboot/boot/fdi-image/initrd0.img /var/www/html/pub/discovery-initrd.img

On the managed node, create new Grub2 entry which will automatically download the files if they are missing or updated and create new menu entry “Rediscover host”.

cat >/etc/grub.d/90_discovery <<EODS
wget -qNP /boot/ http://FOREMAN_URL/pub/discovery-vmlinuz
wget -qNP /boot/ http://FOREMAN_URL/pub/discovery-initrd.img
cat <<'EOM'
menuentry "Rediscover host" {
  search --file --no-floppy --set=discovery_root /discovery-vmlinuz
  linux (\$discovery_root)/discovery-vmlinuz rootflags=loop root=live:/fdi.iso rootfstype=auto ro rd.live.image acpi=force rd.luks=0 rd.md=0 rd.dm=0 rd.lvm=0 rd.bootif=0 rd.neednet=0 nomodeset proxy.url=https://FOREMAN_URL proxy.type=foreman
  initrd (\$discovery_root)/discovery-initrd.img
}
EOM
EODS

Finally, rebuild Grub2 configuration.

chmod +x /etc/grub.d/90_discovery
grub2-mkconfig -o /boot/grub2/grub.cfg

Now it’s possible to reboot the host and pick “Rediscover host” menu item to enter discovery mode. Note that discovery enters interactive mode in this case. It should be possible to automate this workflow to some degree via unattended mode since things like MAC address of primary interface is known and file 90_discovery is a simple shell script.

This could be turned into Remote Execution template as well. I am going to file a PR to our community templates repo with a proposal, this is going to be my very first Remote Execution template PR. Wish me luck!


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