Dualbooting Debian and FreeBSD

There’s no magic in dualbooting Debian and FreeBSD. It’s versions that make all the difference. Dualbooting Squeeze and FreeBSD 9.1 turned out to be a hell of a challenge, and only upgrading to Wheezy made it possible to finally get what I want. But let’s describe things in order.

Just to make sure we’re on the same wave, here’s a short recap of how GRUB2 works.

When you turn your machine on, BIOS reads Master Boot Record (MBR) of your boot device (most probably a hard drive) and executes the code stored there. The code is smart enough to load the next stage of the bootloader, the one that resides in the /boot directory of your root partition (or wherever it is you keep it). This latter stage presents you with a (pseudo)graphical selection of things to load. From there on, things might go different routes, but the ultimate destination of all is an operating system that is loaded and ready to use.

Now that we all have some mental picture of the booting process, let’s proceed to the three ways in which one can load FreeBSD with GRUB2.

The first and easiest one is chainloading. It’s simple and can be used with any other bootloader (operating systems always install one in the boot sector if their “root” partition). When this mechanism is in use, GRUB2 merely loads the code that is stored in the boot record of the specified partition, trusting it to do the right thing. GRUB2 config (/etc/grub.d/40_custom, to be specific) goes like this:

menuentry "FreeBSD 9.1 (chainloading)" {
    set root='(hd0,msdos1)'
    chainloader +1
}

Here, (hd0,msdos1) means the first partition on the first hard drive. Note that hard drives are counted from zero while partitions are counted from one.

In case of FreeBSD, this approach has a side effect of presenting you with two menus in turn: first you see GRUB2, then, after selecting FreeBSD entry there, you see FreeBSD loader. That might or might not be the way you want it, so let’s look at the other two options.

The second way is not much different from the first: we rely on FreeBSD’s own loader again, only this time we call it directly, not through executing the code in the boot sector. A nice way for Linux to pay its due to his brother of the UNIX family, but that’s pretty much it: nothing is gained compared to the first approach.

If you want to use it, change your config to look like this:

menuentry "FreeBSD 9.1 (/boot/loader)" {
    insmod ufs2
    insmod part_bsd
    set root='(hd0,msdos1,bsd1)'
    search  --no-floppy --fs-uuid --set=root 5248225c853c926d
    kfreebsd /boot/loader
}

Here we load part_bsd module to make GRUB2 see BSD partitions inside slices (counted from one), and ufs2 module to enable it to read the file system on the partition¹. By setting root to the partition you know to be the root of your FreeBSD installation, and then issuing search command to look for it again using its UUID, you make sure that you won’t get problems later on if you decide to re-partition your disk². We then proceed by specifying which partition contains /boot and then finally calling the loader.

The third, and the last, approach is the most complex: it’s GRUB2 loading FreeBSD kernel directly. Here’s how it looks in the config file:

menuentry "FreeBSD 9.1 (direct boot)" {
    insmod ufs2
    insmod part_bsd
    set root='(hd0,msdos1,bsd1)'
    search  --no-floppy --fs-uuid --set=root 5248225c853c926d
    kfreebsd /boot/kernel/kernel
    kfreebsd_loadenv /boot/device.hints
    set kFreeBSD.vfs.root.mountfrom=ufs:/dev/ufsid/5248225c853c926d
    set kFreeBSD.vfs.root.mountfrom.options=rw
}

Don’t run away yet, it’s not as bad as it looks :) The first four lines should be familiar to you by now, and the last four are all about loading the kernel and setting things up before finally trying to boot. Last two lines provide a way to specify the root partition and its mount options.

Now that we reviewed all three of them, let’s consider pros and cons of each approach. Starting from the end: the third one is unreliable because it depends on GRUB2 knowing how FreeBSD kernel wants to be loaded and initialized. Should you ever want to stick to old Debian release while keeping up with FreeBSD development, you might end up in a situation when your GRUB2 no longer knows how to boot your FreeBSD.

That’s not just a story I came up with to illustrate some possible consequences: GRUB2 that is shipped with Debian Squeeze (1.98+20100804-14+squeeze1) can’t boot FreeBSD 9.1. It just fails with errors like this (actual number might be different in your case):

address 0x94058 is out of range

To be honest, it failed with the same-looking error when I tried to apply the second technique (calling the loader directly) — some ABIs must have changed really much since the 1.97 release.

The first two approaches are much more reliable — the first one can even be called “unfailable” — because they don’t need that much knowledge about the operating system they’re loading. On the other hand, they both force you to go through FreeBSD loader, which might be undesirable — me, for example, I wanted to select an entry in GRUB2 and go right to the loading process, without need to confirm my choice again in another menu.

And now the last fact for this post: to boot FreeBSD 9.1 on amd64, you need GRUB2 version 1.99-27+deb7u2. I assume 1.99-27+deb7u1, the version shipped with Debian 7.1 (the latest point release so far), would be sufficient for any other architecture, but for amd64, they broke booting FreeBSD ≥ 9.1 and fixed it only in the u2. As of this writing, 1.99-27+deb7u2 is not in Wheezy 7.1, but it is already accepted into wheezy-proposed-updates. If you have Debian 7.2³ or above, you’ve probably got the proper version of GRUB2. For those running 7.0 and 7.1, put this line into your /etc/apt/sources.list (or into a file somewhere under /etc/apt/sources.list.d/):

deb http://ftp.debian.org/debian wheezy-proposed-updates main

You might want to change the address to the mirror closest to you (or make use of http.debian.net) and specify contrib and non-free in addition to main, but the line should be sufficient as it is.

After updating your GRUB2, you would be able to use any of the aforementioned techniquest to boot your FreeBSD.

That’s it, you’ve got a dualboot with Debian Wheezy and FreeBSD 9.1. I wish you to have as much fun with it as I did figuring it all out. Happy hacking!

Update 01.09.2013:

• link to the announcement of Debian 7.2 schedule;
• use FreBSD’s “slice” term properly;
• add a note regarding ZFS and UFS1 modules for GRUB2.

Update 13.10.2013:

• link to the announcement of Debian 7.2 release.