Network Booting

Network Booting #

TODO: This page probably needs to be tidied up.

Combined DHCP responses #

Last time I changed my PXE procedure to use custom iPXE scripts and compiled iPXE binaries, I ran into the problem that the bootloop needs to be broken somehow, if you specify the iPXE binary as the boot target via DHCP and do not want to recompile your binary with new scripts embedded every time. This assumed dumb DHCP servers, which cannot react to different client classes. And while OpenWRT and the underlying dnsmasq are not exactly dumb, the necessary settings are not comfortably exposed in Luci. So I used an unused DHCP option to specify the real PXE boot target and an embedded iPXE script which reads this option and then chainloads. Whew!

Today I learned that clients can assemble responses from multiple DHCP servers and dnsmasq can act as a proxy just fine, meaning it will only serve the settings relevant to PXE booting and leave all the IP assignments, DNS settings, etc. to the main DHCP server in the network. Yay!

This means that the same server that shall act as the target for PXE booting (possibly even containing a gRPC-enabled matchbox) can also serve the relevant DHCP settings so clients will use it.

CoreOS containers #

The CoreOS team provides containers for both matchbox and dnsmasq:

Together with the sample systemd service files provided in matchbox releases in the contrib/systemd/ subdirectory, these can easily be used to bootstrap a fully functional network boot target on top of CoreOS.

Description=CoreOS matchbox Server

ExecStartPre=/usr/bin/mkdir -p /etc/matchbox
ExecStartPre=/usr/bin/mkdir -p /var/lib/matchbox/assets
ExecStart=/usr/bin/rkt run \
  --net=host \
  --inherit-env \
  --trust-keys-from-https \
  --mount volume=data,target=/var/lib/matchbox \
  --mount volume=config,target=/etc/matchbox \
  --volume data,kind=host,source=/var/lib/matchbox \
  --volume config,kind=host,source=/etc/matchbox \

Description=CoreOS dnsmasq DHCP proxy and TFTP server

# replace with %H ?

ExecStart=/usr/bin/rkt run \
  --net=host \
  --trust-keys-from-https \
  -- -d -q \
    --dhcp-range=${NETWORK},proxy, \
    --enable-tftp --tftp-root=/var/lib/tftpboot \
    --dhcp-userclass=set:ipxe,iPXE \
    --pxe-service=tag:#ipxe,x86PC,"PXE chainload to iPXE",undionly.kpxe \
    --pxe-service=tag:ipxe,x86PC,"iPXE",http://${MATCHBOX}/boot.ipxe \
    --log-queries \


Important bits are probably --net host and the userclass, pxe-service and dhcp-range stuff in dnsmasq’s options.

I also needed to build two iPXE binaries:

  • undionly.kpxe (bios -> ipxe)
  • ipxe.efi (uefi -> ipxe)

With these two files in place in /var/tftp the following seems to work:

dnsmasq -d -q --port 0 \
  --dhcp-range=,proxy --enable-tftp --tftp-root=/var/tftp \
  --dhcp-userclass=set:ipxe,iPXE \
  --pxe-service=tag:#ipxe,x86PC,"chainload bios --> ipxe",undionly.kpxe \
  --pxe-service=tag:ipxe,x86PC,"load menu", \
  --pxe-service=tag:#ipxe,BC_EFI,"chainload bc_efi --> ipxe",ipxe.efi \
  --pxe-service=tag:ipxe,BC_EFI,"load menu", \
  --pxe-service=tag:#ipxe,x86-64_EFI,"chainload efi --> ipxe",ipxe.efi \
  --pxe-service=tag:ipxe,x86-64_EFI,"load menu",
Add --log-dhcp to get more verbose information about served DHCP requests.

Bootstrap Process #

This is an overview of the necessary procedures to bootstrap the entire Rechenzentrum:

  • bootstrap the network boot target ‘matchbox’
    • boot and install CoreOS to disk
    • generate tls keys for matchbox gRPC
    • enable systemd services for matchbox and dnsmasq
    • optionally add custom iPXE scripts in matchbox’ assets and tweak boot option
    • optionally download and store kernel and initramfs locally in assets
  • configure infrastructure with terraform
    • use providers for matchbox and vsphere/libvirt/…
    • configure profiles for machines that bootstrap until you can ssh in
    • terraform apply to bring them up and have them boot from pxe
  • use ansible to do proper deployments and configuration management
    • bonus points if you use terraform state as inventory for ansible

Matchbox #

Boot CoreOS into RAM through your preferred method, e.g. using the CoreOS ISO or Then install to disk with coreos-install:

curl -Lo install.json
sudo coreos-install -d /dev/sda -i install.json
sudo reboot

Append -o vmware_raw to the installation command if you’re installing on VMware and replace /dev/sda with /dev/vda if you’re installing on KVM (or use scsi-virtio disks).

Reboot twice for the autologin to take effect. Then add this matchbox server in your DNS.

Install CentOS over Serial Cable #

Simply using is not sufficient when installing from network with only a serial connection (e.g. on my X10SBA) because it does not allow you to edit the kernel commandline - which does not include console=ttyS0 by default. Thus boot into an iPXE shell and use the following script to start an interactive CentOS install over serial:

set repo
kernel ${repo}/images/pxeboot/vmlinuz
initrd ${repo}/images/pxeboot/initrd.img
imgargs vmlinuz ramdisk_size=8192 console=ttyS0,115200 text method=${repo}/

Careful when copy-pasting though. I have had incomplete pastes which lead to a missing _64 in the repo URL, etc.

Squid proxy #

Instead of mirroring multiple repositories locally, you could run a Squid proxy in your network and point all yum clients at it. With some URL rewriting you can cache the same packages from many different mirrors in a deduplicated fashion.

I used the sameersbn/squid image on a CoreOS host, started with the following arguments:

docker run -d --net host --name squid \
  -v /var/spool/squid:/var/spool/squid \
  -v /etc/squid:/etc/squid sameersbn/squid

The configuration files cache all .rpm’s from any mirrors. This is probably too open and broad to be used as a general proxy, so be careful.


# /etc/squid/squid.conf

acl safe_ports port 80 21 443
acl tls_ports  port 443

http_access deny !safe_ports
http_access deny CONNECT !tls_ports

http_access allow localhost
http_access allow all

http_port 3128

# cache yum/rpm downloads:

cache_replacement_policy heap LFUDA		# least-frequently-used
cache_dir aufs /var/spool/squid 20000 16 256	# 20GB disk cache
maximum_object_size 4096 MB			# up to 4GB files

store_id_program /usr/lib/squid/storeid_file_rewrite /etc/squid/storeid.db	# rewrite all centos mirror urls

coredump_dir /var/spool/squid

refresh_pattern -i \.(deb|rpm|tar|tar.gz|tgz)$ 10080 90% 43200 override-expire ignore-no-cache ignore-no-store ignore-private
refresh_pattern .	0	20%	4320


\/([0-9\.\-]+)\/([a-z]+)\/(x86_64|i386)\/(.*\.d?rpm)	http://rpmcache.squid.internal/$1/$2/$3/$4
The storeid.db rules need tabs as whitespace, not spaces! And during initial creation I found out that squid sends a quoted string to the helper, so using any regular expression with (.*\.rpm)$ at the end did not match. Use debug_options ALL,5 and grep for storeId if you’re having problems.

Finally, simply add proxy= in your clients’ /etc/yum.conf.

reverse proxy #

With a small addition to your squid.conf you can also make Squid act as a reverse proxy (or “accelerator” in Squid terms) for a mirror of your choice, using the same cache. Although the wiki still says that cache_peer requests do not pass through the storeid helper, it in fact seems to do just that. At the time of this writing, the latest container uses Squid Cache: Version 3.5.27 from Ubuntu’s repositories.


http_port 3128
http_port 80  accel no-vhost

cache_peer parent 80 0 no-query originserver name=mirror

acl mirror dstdomain
http_access allow mirror
cache_peer_access mirror allow mirror
cache_peer_access mirror deny all


With this configuration, you could also use Squid as a mirror to install your machines via kickstart, because also the .../os/x86_64/images/pxeboot/* files will be cached. You might want to amend your refresh_pattern rules to account for that.