Shallow Thoughts : tags : extlinux

Extlinux on Debian Jessie

Debian "Sid" (unstable) stopped working on my Thinkpad X201 as of the last upgrade -- it's dropping mouse and keyboard events. With any luck that'll get straightened out soon -- I hear I'm not the only one having USB problems with recent Sid updates. But meanwhile, fortunately, I keep a couple of spare root partitions so I can try out different Linux distros. So I decided to switch to the current Debian stable version, "Jessie".

The mouse and keyboard worked fine there. Except it turned out I had never fully upgraded that partition to the "Jessie"; it was still on "Wheezy". So, with much trepidation, I attempted an apt-get update; apt-get dist-upgrade

After an interminable wait for everything to download, though, I was faced with a blue screen asking this:

No bootloader integration code anymore.
The extlinux package does not ship bootloader integration anymore.
If you are upgrading to this version of EXTLINUX your system will not boot any longer if EXTLINUX was the only configured bootloader.
Please install GRUB.
<Ok>

No -- it's not okay! I have good reasons for not using grub2 -- besides which, extlinux on exact machine has been working fine for years under Debian Sid. If it worked on Wheezy and works on Sid, why wouldn't it work on the version in between, Jessie?

And what does it mean not to ship "bootloader integration", anyway? That term is completely unclear, and googling was no help. There have been various Debian bugs filed but of course, no explanation from the developers for exactly what does and doesn't work.

My best guess is that what Debian means by "bootloader integration" is that there's a script that looks at /boot/extlinux/extlinux.conf, figures out which stanza corresponds to the current system, figures out whether there's a new kernel being installed that's different from the one in extlinux.conf, and updates the appropriate kernel and initrd lines to point to the new kernel.

If so, that's something I can do myself easily enough. But what if there's more to it? What would actually happen if I upgraded the extlinux package?

Of course, there's zero documentation on this. I found plenty of questions from people who had hit this warning, but most were from newbies who had no idea what extlinux was or why their systems were using it, and they were advised to install grub. I only found one hit from someone who was intentionally using extlinux. That person aborted the install, held back the package so the potentially nonbooting new version of extlinux wouldn't be installed, then updated extlinux.conf by hand, and apparently that worked fine.

It sounded like a reasonable bet. So here's what I did (as root, of course):

• Open another terminal window and run ps aux | grep apt to find the apt-get dist-upgrade process and kill it. (sudo pkill apt-get is probably an easier approach.) Ensure that apt has exited and there's a shell prompt in the window where the scary blue extlinux warning was.
• echo "extlinux hold" | dpkg --set-selections
• apt-get dist-upgrade and wait forever for all the packages to install
• aptitude search linux-image | grep '^i' to find out what kernel versions are installed. Pick one. I picked 3.14-2-686-pae because that happened to be the same kernel I was already running, from Sid.
• ls -l /boot and make sure that kernel is there, along with an initrd.img of the same version.
• Edit /boot/extlinux/extlinux.conf and find the stanza for the Jessie boot. Edit the kernel and append initrd lines to use the right kernel version.

It worked fine. I booted into jessie with the kernel I had specified. And hooray -- my keyboard and mouse work, so I can continue to use my system until Sid becomes usable again.

Configuring extlinux's auto-update on Debian

A few days ago, I wrote about how to set up and configure extlinux (syslinux) as a bootloader. But on Debian or Ubuntu, if you make changes to files like /boot/extlinux/extlinux.conf directly, they'll be overwritten.

The configuration files are regenerated by a program called extlinux-update, which runs automatically every time you update your kernel. (Specifically, it runs from the postinst script of the linux-base package: you can see it in /var/lib/dpkg/info/linux-base.postinst.)

So what's a Debian user to do if she wants to customize the menus, add a splash image or boot other operating systems?

First, if you decide you really don't want Debian overwriting your configuration files, you can change disable updates by editing /etc/default/extlinux. Just be aware you won't get your boot menu updated when you install new kernels -- you'll have to remember to update them by hand.

It might be worth it: the automatic update is nearly as annoying as the grub2 updater: it creates two automatic entries for every kernel you have installed. So if you have several distros installed, each with a kernel or two in your shared /boot, you'll get an entry to boot Debian Squeeze with the Ubuntu Oneiric kernel, one for Squeeze with the Natty kernel, one for Squeeze with the Fedora 16 kernel ... as well as entries for every kernel you have that's actually owned by Debian. And then for each of these, you'll also get a second entry, to boot in recovery mode. If you have several distros installed, it makes for a very long and confusing boot menu!

It's a shame that the auto-updater doesn't restrict itself to kernels managed by the packaging system, which would be easy enough to do. (Wonder if they would accept a patch?) You might be able to fudge something that works right by setting up symlinks so that the only readable kernels actually live on the root partition, so Debian can't read the kernels from the other distros. Sounds a bit complicated and I haven't tried it. For now, I've turned off automatic updating on my system.

But if your setup is simpler -- perhaps just one Debian or one Ubuntu partition plus some non-Linux entries such as BSD or Windows -- here's how to set up Debian-style automatic updating and still keep all your non-Linux boot entries and your nice menu customizations.

Debian automatic updates and themes

First, take a quick look at /etc/default/extlinux and customize anything there you might need, like the names of the kernels, kernel boot parameters or timeout. See man extlinux-update for details.

For configuring menu colors, image backgrounds and such, you'll need to make a theme. You can see a sample theme by installing the package syslinux-themes-debian -- but watch out. If you haven't configured apt not to pull in suggested packages, that may bring back grub or grub-legacy, which you probably don't want.

You can make a theme without needing that package, though. Create a directory /usr/share/syslinux/themes/mythemename (the extlinux-update man page claims you can put a theme anywhere and specify it by its full path, but it lies). Create a directory called extlinux inside it, and make a file with everything you want from extlinux.conf. For example:

default 0
prompt 1
timeout 50

ui vesamenu.c32
menu title Welcome to my Linux machine!
menu background mysplash.png
menu color title 1;36 #ffff8888 #00000000 std
menu color unsel 0    #ffffffff #00000000 none
menu color sel   7    #ff000000 #ffffff00 none

include linux.cfg
menu separator
include themes/mythemename/other.cfg

Note that last line: you can include other files from your theme. For instance, you can create a file called other.cfg with entries for other partitions you want to boot:

label oneiric
menu label Ubuntu Oneiric Ocelot
kernel /vmlinuz-3.0.0-12-generic
append initrd=/initrd.img-3.0.0-12-generic root=UUID=c332b3e2-5c38-4c50-982a-680af82c00ab ro quiet

label fedora
menu label Fedora 16
kernel /vmlinuz-3.1.0-7.fc16.i686
append initrd=/initramfs-3.1.0-7.fc16.i686.img root=UUID=47f6b1fa-eb5d-4254-9fe0-79c8b106f0d9 ro quiet

menu separator

LABEL Windows
KERNEL chain.c32
APPEND hd0 1

Of course, you could have a debian.cfg, an ubuntu.cfg, a fedora.cfg etc. if you wanted to have multiple distros all keeping their kernels up-to-date. Or you can keep the whole thing in one file, theme.cfg. You can make a theme as complex or as simple as you like.

How to install extlinux (syslinux) as a bootloader

When my new netbook arrived, I chose Debian Squeeze as the first Linux distro to install, because I was under the impression it still used grub1, and I wanted to avoid grub2. I was wrong -- Squeeze uses grub2. Uninstalling grub2, installing grub-legacy and running grub-install and update-grub didn't help; it turns out even in Debian's grub-legacy package, those programs come from grub2's grub-common package.

What a hassle! But maybe it was a blessing in disguise -- I'd been looking for an excuse to explore extlinux as a bootloader as a way out of the grub mess.

Extlinux is one of the many spinoffs of syslinux -- the bootloader used for live CDs and many other applications. It's not as commonly used as a bootloader for desktops and laptops, but it's perfectly capable of that. It's simple, well tested and has been around for years. And it supports the few things I want out of a bootloader: it has a simple configuration file that lives on the /boot partition; it can chain-load Windows, on machines with a Windows partition; it even offers pretty graphical menus with image backgrounds.

Since there isn't much written about how to use extlinux, I wrote up my experiences along with some tips for configuring it. It came out too long for a blog article, so instead I've made it its own page: How to install extlinux (syslinux) as a bootloader.