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1991-07-20
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CKCOM Release 4
July 20, 1991
Checks file size of COMMAND.COM for virus attack.
Copyright (c) 1991,
Russell B. Hildreth
All Rights Reserved
Background:
One of the easiest ways to spot a virus and prevent it from
doing a great deal of damage is to check the size of the DOS
COMMAND.COM file for changes. Most virus programs
attack COMMAND.COM. After I was slimed by the Yankee
Doodle virus a couple of months ago, I put together a little
program that would compare the current size of
COMMAND.COM with a known value, it's proper, unmodified
size. This program could be called from AUTOEXEC.BAT for
a quick check each time the system is booted up. It eventually
became the more versatile version you see here. This release
allows the user to use CKCOM with *any* DOS version, by
simply providing the proper file size of COMMAND.COM as
a command line parameter.
Installation:
(1) Copy CKCOM to your boot drive (usually a hard drive), and
put it somewhere convenient, such as a UTIL directory or your
DOS directory.
(2) Figure out the proper file size of COMMAND.COM for
your DOS version. If you are fairly sure the version you are
currently using is virus free, you can use it as a basis. It's
probably a better idea to check the size of the copy of
COMMAND.COM on your original DOS distribution disks.
(3) Add a line to your AUTOEXEC.BAT file to call CKCOM.
Example: C:\UTIL\CKCOM 47845. This runs the copy of
CKCOM located in the UTIL directory, and checks for the
proper size of MSDOS 5.00.
(4) *Note: If your copy of COMMAND.COM is located
somewhere other then the root directory of the boot drive, your
AUTOEXEC.BAT file probably contains a line such as SET
COMSPEC=d:\path\file. If you have such a line in your
AUTOEXEC.BAT file, be sure to place the call to CKCOM
*after* the SET COMSPEC line. This is necessary because
CKCOM uses the COMSPEC value to locate your copy of
COMMAND.COM.
Sample COMMAND.COM file sizes:
MSDOS 3.30 25276
IBMDOS 4.00 37637
MSDOS 5.00 47845
IBMDOS 5.00 47987
These sample sizes are provided only as a basic guideline. Your
DOS version may differ, and you should check the proper size
on your original DOS distribution disks.
Operation:
Program operation is simple. If COMMAND.COM is the
proper file size, CKCOM flashes a quick COMMAND.COM
Okay! message, and that's it. On most machines it will go by so
fast you probably won't notice it, and it surely won't bother you
when everything is as it should be.
When something is wrong with COMMAND.COM, CKCOM
will stop, warn you, and pause for a key press. At that point it's
a good idea to reboot from an uninfected floppy disk and run
any one of a number of fine virus detection/clean up programs.
McAffee's SCAN is a good shareware detection program.
Central Point Software and Norton both now offer commercial
anti-virus software.
Return values:
CKCOM returns the following values to DOS:
0 Normal operation. No errors, COMMAND.COM okay.
1 Error. Improper command line parameters.
2 Error. COMMAND.COM not found. Check your
AUTOEXEC.BAT file to be sure that any SET
COMSPEC= call comes *before* the call to CKCOM.
3 Possible virus. COMMAND.COM is not the proper size.
Check for a virus. If there is no virus, check to be sure
the proper file size parameter is specified on the
command line.
Advantages: Aside from the obvious, spotting a possible virus
infection early (so weeks don't go by as every program on your
hard drive is infected--this happened to me), the CKCOM
program works well because it is NOT memory resident. Once
the COMMAND.COM file is checked, the CKCOM program
removes itself from memory. This way, you are left with the
most DOS memory.
Comments/suggestions/complaints:
All are welcome. I can be reached at the following addresses:
CompuServe (preferred) EasyPlex: 71041,2132
MCI Mail: rhildreth
US Mail: Russell Hildreth
PO BOX 72431
DAVIS CA 95617-6431
Registration:
Registration is only $15. This is used by the author to offset the
ever increasing costs of obtaining a higher education. I'll also
send you the latest release of the software. Remember,
shareware is not freeware. If you use it, pay for it. Please use
the CKCOM.REG form when registering.