> From: Steve Bellovin <smb@research.att.com>
>
> I decided it would be good to have a bootable rescue CD -- one that had 
> all of /bin, /usr/bin, etc.  I wrote a script that created the file 
> system image that I wanted, then -- following the instructions in the 
> bootable CD How-To -- I ran mkhybrid.  Here I ran into my first 
> problem:  I needed a boot image.  My attempt to compile from source
> failed (no, I don't know why), so I picked up boot-big.fs from from the 
> 1.5 distribution tree on ftp.netbsd.org.  And it did indeed produce a 
> mostly-usable CD.  But...
>
> I was rather surprised, when it booted, to find myself in sysinst.  I 
> assume that this is some attribute of boot-big -- is there a way to 
> make that not happen?  I also found that root was /dev/mcd0a, which 
> isn't what I wanted -- I wanted /dev/cd0a to be root, so that /bin, /
> usr/share/man, etc., would be where they should be.  (I bypassed that 
> operationally via 'mount -t cd9660 /dev/cd0a /mnt' and 'chroot /mnt'.)

When we build boot images, the kernel we put on it has an internal
ramdisk filesystem, the idea being, it doesn't depend on anything
external. You ended up with this ramdisk root and in sysinst because of

	src/distrib/i386/floppies/ramdisk-big/*, calling sysinst is:
	src/distrib/i386/floppies/ramdisk-big/dot.profile

and because of the MEMORY_DISK* options in:

	sys/arch/i386/conf/INSTALL

(If you look carefully at your rescue FS, I think you will see that every
/bin and /usr/bin program has the same inode number, thanks to a remarkably
evil hack known as crunchgen(1).  And if you were on NetBSD/alpha, you
could boot from a kernel DIRECTLY on the CD filesystem, but that's not
netbsd, it's because the firmware is different. Since you mentioned
"boot-big", I guess you are on i386. Anyway, you wanted something else...)

If you just type 'boot -as' at the 386 boot monitor, then the standard
install kernel (off your boot-big) will ask for its root (tell it cd0a, of
course) and it will stop in single user.

The INSTALL kernel makes a fine rescue kernel, but if you want to depend
on the CD, then your existing kernel binary or GENERIC should work fine,
maybe better.  If you want the root mount to happen automatically, cp your
kernel or GENERIC config to, say, WHATEVER_CD, and patch it like so:

    -config         netbsd  root on ? type ?
    +config         netbsd  root on cd0a type cd9660

Now you need to make a filesystem, put the kernel on it, and run
installboot(8) to make it bootable. You can do this with either a vnd(4)
like src/distrib/i386/* does, or you can use a real floppy. You might test the
floppy by itself before burning any CD's, if it can't boot as a floppy it
can't boot on a CD.

> My next problem is that I would like to have /tmp as an mfs file 
> system.  But where can I mount it?  I may not have a /dev/wd0b (and, in 
> fact, I did not when I test-booted the disk on a laptop that has only a
> n EvilOS2000 partition).  /dev/cd0a is busy, and there is no disk label 
> on /dev/cd0d, for example.

You seem to already be aware that it only wants a disk to reuse the label,
as any label will do. So do the mount the "pure" way with mount_mfs -T.

If you want to do anything really fancy, consider a custom hack to init(8).
It shouldn't be necessary, but as an example see code for booting off a
MSDOS filesystem. No device nodes, but we need /dev/tty, /dev/wd*, etc.  We
can't even do console I/O without device nodes. What to do? The answer is:
it mounts an mfs and runs MAKEDEV from within init(8). Kind of cute.

// ross@netbsd.org