Contents of the READ.ME file
Author: R. E. Faith
Release date: 11 Jan 88
This is special first release documentation. Better documentation
will be released later. This program was thrown together quickly
because of the recent discussions regarding viral infections of
computer software. A virus has been isolated which infects
COMMAND.COM and spreads to new disks by altering their COMMAND.COM
files. The virus is apparently stored in COMMAND.COM stack space.
The CRCDOS program will make a list of valid CRC's for a list of files
(such as your DOS files). CRCDOS will also check the current CRC of
those files against the list you made. This takes a while to do, but
is short enough to do once a day or whenever you run CHKDSK. Be sure
to use the CRCDOS compare facility BEFORE all backups, or you will
potentially backup viruses.
A list of DOS 3.1 files is in DOS.IN. Modify this file to contain a
list of files in your version of DOS. Get the pathnames correct for
your system. This should involve a few substitution commands with a
good editor. You may also use DIR to generate a list of all files in
your DOS directory, and then edit that list. DON'T FORGET IBMBIO.COM
and IBMDOS.COM (or their equivalents), and your BASIC, if you use one.
Now, install CRCDOS in a directory in your path. Use a command
similar to "CRCDOS -B DOS.CRC DOS.IN" to generate a list of current
CRC's. The CRC list is in DOS.CRC (DOS.IN is not modified, so you may
keep it around to use later). Copy DOS.CRC somewhere. Now set up a
batch file to do something like:
CRCDOS -S -C C:\YOURPATH\DOS.CRC
(Try it without the -S to see it doing the check. CRC's should be
non-zero numbers, and are given in hex).
The -S is useful, because only files which changed will be reported.
CRCDOS is not a fool-proof way to prevent viral infections, but it
can detect most (all?) modifications of your DOS files. Other
viruses may install themselves in more insidious ways, and other
programs will have to be designed to detect them.