14) What should be on a (clean) boot disk?
Description
This article is from the Computer viruses FAQ, by David Harley D.Harley@icrf.icnet.uk, George Wenzel gwenzel@telusplanet.net and Bruce Burrell bpb@umich.edu with numerous contributions by others.
14) What should be on a (clean) boot disk?
A boot floppy is one which contains the basic operating system, so that
if the hard disk becomes inaccessible, you can still boot the machine
to attempt some repairs. All formatted floppies contain a boot sector,
but only floppies which contain the necessary system files can be used
as boot floppies. A clean boot disk is one which is known not to be
virus-infected. It's best to use a clean boot disk before routine
scans of your hard disk(s). Some antivirus packages will refuse to run
if there is a virus in memory. It is usually better and sometimes
mandatory to disinfect a system without the virus in memory, and an
undetected file virus may actually spread faster during a scan, since
scanners normally open all executable files in all directories.
To make an emergency bootable floppy disk, put a disk in drive A and type
FORMAT A: /S
Be careful to avoid 'cross-formatting', i.e. formatting a double-density
disk as high-density or vice versa, if you system allows this. (You should
avoid this all the time, not just when creating a boot disk. I'd also
recommend avoiding single-density and quad-density disks, and there may
be problems writing to double-density 5.25" disks on a different machine
to the one on which they were formatted, if one machine is an XT and the
other an AT or better.)
You can also make a pre-formatted floppy into a boot disk by typing
SYS A:
I'd suggest you also COPY these commands from C:\DOS to it: ATTRIB,
CHKDSK (or SCANDISK if you have DOS6), FDISK, FORMAT, SYS, and BACKUP and
RESTORE (or whatever backup program you use, if it will fit). They may
come in handy if you can't access the hard disk, or it won't boot up.
You may be aware that if there is a problem with your boot sequence, you
can boot from the hard disk on a DOS 6/7/Win95 system while bypassing
AUTOEXEC.BAT and CONFIG.SYS. This is not as good as a clean floppy boot:
it won't help at all if you have a boot sector/partition sector infector,
or if any or all of the basic operating system files have been infected
by a file virus.
The boot disk should have been created with the same version of DOS as
you have on your hard disk. It should also include any drivers necessary
to access your hard disk and other devices (such as a CD-ROM). If, for
some reason, you can't obtain a clean boot disk with the same version
of DOS, you can often get away with booting from a (clean) disk using
a different version, though: indeed, there are viruses which exploit a
bug in recent versions of MS-DOS which will prevent a clean boot from
DOS vs. 4-6. If you *do* use a different version, remember that you
won't be able to use many of the standard DOS system utilities on the
hard disk, which will simply return a message like 'Wrong DOS version'
when you try to run them, and avoid the use of FORMAT or FDISK.
If you become virus-infected it can be very helpful to have backup of your
hard disk's boot sector and partition sector (also known as MBR). Some
anti-virus and disk utilities can do this. Other useful tools to include are
a small DOS-based text editor (for editing AUTOEXEC.BAT, CONFIG.SYS and so
forth), a copy of the DOS commands COMP or FC (for comparing files),
FDISK and SYS (make sure they are from the same version of DOS as you are
booting). There is a school of thought that your boot disk should also
include your anti-virus software. The problem with this is that
anti-virus software should be updated frequently, and you may forget to
update (and re-write-protect) your boot disk each time. Ideally you will
have been sent a clean, write-protected copy of the latest version of your
anti-virus software by your vendor/supplier.
If you want to use the DOS program EDIT, remember that you need both
EDIT.* and QBASIC.* on the same disk.
When you have everything you need on your boot floppy and any supplementary
floppies (see below), make sure they're all *write-protected*!
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