Alberto Ferrari is looking for a way to run commands on a
system that he can't log into. What's more, he needs to do it
from different systems. The solution involves a mail filter ASCII
characters, it must be run through uuencode
before you can e-mail it. The cmd.filter script, is
shown in Part A of the Listing. After
it performs all the protocol verification, it puts command lines
in a temporary file, calls the shell to run them, and then mails
the output to the listed receiver (lines 33-35).
The chkcode program (Part
B) generates
encryption keys. If you use the -l
option, it prints a list of encryption keys on the standard
output. The number you want listed is placed after the -
l option on the command line. If chkcode is
called without arguments, it generates the current encryption key
based on a counter kept in a file.
Some setup is required for these programs. First, in
cmd.filter you need to assign the path name of the
chkcode executable file to the variable
CHKCODE (Part A, line
2). Note that it must be an absolute
path name. Second, in chkcode you must
change the define term ONETIMEPADFILE (line 7) to be the absolute path name you
want to use for the counter file. You must initialize this file
with a starting value (I recommend ``1'') before this system will
work. Next, you should redefine the random-number generator's
starting seed. Because I have published the one in
chkcode.c, it is best to pick another one (Part B, line 8). Choose any number
between one and four billion.
You must also set up a .forward
file in the home directory of the account that will receive the
execution messages. The .forward file should
contain two entries: this account's name preceded by a backslash
and a pipe command to the cmd.filter script. Part C shows the .forward
file I used while testing this application for account ray.
The cmd.filter program relies on the crypt
command, which may be missing from your system. If it is, you
can find other encryption programs on the Internet. Another
solution is the simple encryption technique of using exclusive OR
operations. An example is the mailcrypt program shown in my June
1994 column.
Part D shows the steps required to prepare a command file for insertion into an e-mail message.
One important point about preparing your mail message. When a
file is encoded with uuencode, you have to list the
file name where uudecode is to place the decoded
file. Because a daemon will
run cmd.filter, you have to know the directory where
uudecode will put the decoded script. Thus, you
need to use an absolute path name when you encode the script file
before mailing it. In Part D, I chose to put the script file in
my home directory.