Intro
PXE is a great solution for booting a diskless computer (or a computer without an OS installed). This method is often used for terminal stations and OS mass installation.
Stock ubuntu (16.04) in pxe-mode can mount rootfs only from NFS. But this is not a great idea: any difficulties with the network/NFS server and the user gets problems.
In my opinion, it's best to use other protocols, such as http/ftp. Once booting, you will have an independent system
You should add information about the limits of applicability of the proposed solution and what are the dependencies and restrictions.
Pxe short info
PXE (Preboot Execution Environment) is a special method for booting a computer from the bios / EFI.
How it works (simplified scheme):
- The computer bios/uefi sends ordinary dhcp request (DHCPDISCOVER)
- The dhcp server sends a response with the next-server option
- The computer sends DHCPREQUEST/DHCPINFORM request
- The dhcp server sends TFTP server address and the filename to upload
- The computer downloads this file from the tftp server. Its size is limited so, often, it's a bootloader like pxeinux
- pxelinux reads its own config and downloads Linux kernel using initramfs
- Linux kernel downloads squashfs with main rootfs
- switch_root to its squashfs
Keep in mind that TFTP is a slow protocol. It works around UDP with a small block size (512K). Of course, you can increase this value, but this is a way of unstable operation.
A better solution:
- get bootloader via tftp
- get kernel (+ initramfs) via tftp
- get main rootfs squash via http/tftp
How i do it
Steps to solve the task:
- Add modules to initramfs
- Write my own boot script and add it to initramfs
- Make new initramfs
- Create squashfs with future rootfs
- Setup pxe server
- Run it
I used squashfs for rootfs (the simplest way is to create squashfs from installed ubuntu). Overlayfs is necessary to make rootfs writable.
Supported protocols are http/ftp, but you can try to add others via curl/other software.
Customize initramfs
There are 2 places where you can customize initramfs in ubuntu:
- /etc/initramfs-tools
- /usr/share/initramfs-tools/
I'll use /usr/share/initramfs-tools. First, I added needed support modules in initramfs:
boozlachu@comp:~$ cat /usr/share/initramfs-tools/modules.d/pxe
overlayfs
squashfsNext, I wrote a boot script that does all the work:
boozlachu@comp:~$ cat /usr/share/initramfs-tools/scripts/pxe
#!/bin/bash
mountroot()
{
maxTryCount=5
squashfsFile="/tmp/rootfs.squashfs"
squashfsMountPoint="/mnt/ro"
tmpfsMountPoint="/mnt/rw"
overlayfsUppderDir="$tmpfsMountPoint/upper"
overlayfsWorkDir="$tmpfsMountPoint/work"
overlayfsDir="/mnt/overlayfs"
tryCount="1"
# run udevadm
wait_for_udev 10
# parce kernel cmdline args. rooturl needed
for x in $(cat /proc/cmdline); do
case $x in
rooturl=*)
export rooturl=${x#rooturl=}
;;
maxTryCount=*)
export maxTryCount=${x#maxTryCount=}
;;
esac
done
log_begin_msg "Loading modules"
modprobe squashfs || panic "can't modprobe squashfs"
modprobe af_packet || panic "can't modprobe af_packet"
modprobe overlay || panic "can't modprobe overlayfs"
log_success_msg "modules loaded"
log_begin_msg "Configure network"
configure_networking || panic "Can't configure network"
log_success_msg "Network configured"
log_begin_msg "Download rootfs"
while [ ! -f ${squashfsFile} ] && [ ${tryCount} -le ${maxTryCount} ]; do
wget ${rooturl} -O ${squashfsFile} || log_failure_msg "Can't download rootfs, count ${tryCount}"
tryCount=$(( ${tryCount} + 1 ))
sleep 0.5
done
if [ -f ${squashfsFile} ]
then
log_success_msg "Rootfs downloaded"
else
panic "Can't download rootfs"
fi
log_begin_msg "Mount rootfs"
mkdir -p $squashfsMountPoint
mount -t squashfs -o loop $squashfsFile $squashfsMountPoint || panic "Can't mount rootfs"
log_success_msg "Rootfs mounted"
log_begin_msg "Mount tmpfs"
mkdir -p $tmpfsMountPoint
mount -t tmpfs none $tmpfsMountPoint || panic "Tmpfs mount failed "
log_success_msg "Tmpfs mounted"
log_begin_msg "Mount overlayfs"
mkdir -p $overlayfsUppderDir $overlayfsWorkDir $overlayfsDir
mount -t overlay overlay -o lowerdir=$squashfsMountPoint,upperdir=$overlayfsUppderDir,workdir=$overlayfsWorkDir $overlayfsDir \
|| panic "Overlayfs mount failed"
log_success_msg "Overlayfs mounted"
log_begin_msg "Move tmpfs and squashfs to new root"
mkdir -p $overlayfsDir/$tmpfsMountPoint $overlayfsDir/$squashfsMountPoint
mount --move $squashfsMountPoint $overlayfsDir/$squashfsMountPoint || panic "squashfs move failed"
mount --move $tmpfsMountPoint $overlayfsDir/$tmpfsMountPoint || panic "tmpfs move failed"
log_success_msg "Tmpfs and squashfs moved"
log_begin_msg "Move overlayfs to new root"
mount --move $overlayfsDir ${rootmnt} || panic ""
}
The script has a lot of messages for understanding how it works.
After the modules and script, add your need to generate new initramfs:
boozlachu@comp:~$ sudo update-initramfs -c -k allCreating squashfs
The simplest method:
- install ubuntu on drive
- boot from LiveCD
- create squashfs from the installed system
I don't recommend this way for production since you'll have a very large squashfs (not the best idea for pxe)!
Setup bootloader, squashfs, and pxe server
I use pxelinux as a pxe bootloader. It's an easy way. My pxe servers are Debian 10, tftp-hpa,dhcpd, and lighttpd.
I’ll omit the installation details, but I’ll show the important info.
TFRP server file struct (/srv/tftp is root dir fot tftp-hpa):
root@debian:/srv/tftp/ubuntu# tree /srv/tftp/
/srv/tftp/
└── ubuntu
├── firmware.sq
├── initrd
├── ldlinux.c32
├── libcom32.c32
├── libutil.c32
├── menu.c32
├── pxelinux.bin
├── pxelinux.cfg
│ └── default
├── vesamenu.c32
└── vmlinuz- firmware.sq is squashfs with rootfs
- *c32 are files for pxelinux
- vmlinuz is kernel
- initrd is initramfs(which i rebuild earler)
- pxelinux.bin — main pxelinux file
- default is config for pxelinux
Bootloader config:
root@debian:/srv/tftp/ubuntu# cat /srv/tftp/ubuntu/pxelinux.cfg/default
ui menu.c32
timeout 30
default ubuntu_pxe
font UniCyr_8x16.psf
menu title PXE Special Boot Menu
menu color tabmsg 37;40 #80ffffff #00000000
menu color hotsel 30;47 #40000000 #20ffffff
menu color sel 30;47 #40000000 #20ffffff
menu color scrollbar 30;47 #40000000 #20ffffff
LABEL ubuntu_pxe
menu label Run ubuntu pxe
kernel vmlinuz
append initrd=initrd rooturl=http://192.168.56.2/ubuntu/firmware.sq boot=pxe maxTryCount=10It’s impotant to set the correct kernel parameters:
- rooturl=http://192.168.56.2/ubuntu/firmware.sq, url for rootfs
- boot=pxe, use my script for boot
- maxTryCount=10, number of tries for rootfs download (optional, default value 5)
And the last one is the dhcp config:
root@debian:/srv/tftp/ubuntu# cat /etc/dhcp/dhcpd.conf
subnet 192.168.56.0 netmask 255.255.255.0 {
range 192.168.56.10 192.168.56.45;
option routers 192.168.56.2;
option domain-name-servers 192.168.2.1 ;
option broadcast-address 192.168.56.255;
default-lease-time 3600;
max-lease-time 7200;
# Important! Set bootloader file
filename "ubuntu/pxelinux.bin";
}
The extended variant (if the dhcp and tftp servers placed on different machines) requires the next-server option for dhcp.
Conclusion
This article shows you how to change the boot mode of ubuntu without any difficulties. Use it as information and write your solutions. This can be a system in the form of firmware (with squashfs), pxe, or another solution.
