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Current issue : #15 | Release date : 1987-08-07 | Editor : Taran King
Phrack XV IntroShooting Shark
More Stupid Unix TricksShooting Shark
Making Free Local Payfone CallsKiller Smurf
Advanced Carding XIVThe Disk Jockey
Gelled Flame FuelsElric of Imrryr
PWN I: The Scoop on Dan The OperatorKnight Lightning
PWN II: The July BustsKnight Lightning
PWN III: The Affidavitunknown
Title : More Stupid Unix Tricks
Author : Shooting Shark
                ===== Phrack Magazine presents Phrack 15 =====

                           ===== File 2 of 8 =====

I thought I had written everything there is to write about the Unix operating
system until I was recently asked to put out yet another file... so I said
"I'll try, but don't publish my file along with an article by The Radical
Rocker this time!"  These demands having been met, I booted up the PC and
threw together...

                 --- ---- ---- ------ ------ -- -- ---- -----
               % Yet Even More Stupid Things to Do With Unix! $
                 --- ---- ---- ------ ------ -- -- ---- -----

                              By Shooting Shark.
                          Submitted August 26, 1987

These two topics are methods of annoying other users of the system and
generally being a pest.  But would you want to see a file on *constructive*
things to do with Unix?  Didn't think so...

--  ------- ----- --- --- ------
1.  Keeping Users Off The System
--  ------- ----- --- --- ------

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

Block is essentially an endless loop : it keeps checking for the occurrence 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

----- cut here -----

/* 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/"

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]);

for (;;) {

if ((fp = open(UTMP,0)) == -1) {
printf("fatal error!  cannot open %s.\n",UTMP);

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);
if (access(buf,W_OK) == -1) {
printf("failed - program aborting.\n");
else {
if ((target = open(buf,O_WRONLY)) != EOF) {
sprintf(buf2,"stty 0 > %s",buf);

} /* else */
} /* if strcmp */
} /* if isprint */
} /* while */

/*sleep(SLEEP);  */

} /* for */


----- cut here -----

--  ------------- ----- ----- ---- ------  --- ------
2.  Impersonating other users with 'write' and 'talk'
--  ------------- ----- ----- ---- ------  --- ------

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]


[or a similarly confusing or rude message...]


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.

As I said, there was no great amount of genius in the above discourse, but
it's a pastime I enjoy occasionally...

-- Shooting Shark

"the first fact to face is that unix was not developed with security, in any
realistic sense, in mind..."

-- Dennis M. Ritchie

"Oryan QUEST couldn't hack his way out of a UNIX system, let alone into one."

-- Tharrys Ridenow
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