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Useful Unix Commands

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                       *** A List Of Some OF The Most Useful UNIX **
     *** Hacking Commands, and Some Hints On Their Usage ***
     It is fun and often useful to create a file that is owned 
by someone else.  On most systems with slack security ie 99% of 
all UNIX systems, this is quite easily done.  The chown command 
will change any of your files to make someone else the owner. 
Format is as follows:
chown ownername filelist
 Where ownername is the new owner, and filelist is the list of 
files to change.  You must own the file which your are goin to 
change, unless you are a superuser....then u can change ANYTHING!
     chgrp is a similar command which will change the group 
ownership on a file.  If you are going to do both a chown and a 
chgrp on a file, then make sure you do the chgrp first!  Once the 
file is owned by someone else, you cant change anything about it!
     Sometimes just seeing who is on the system is a challenge in 
itself.  The best way is to write your own version of who in C, 
but if you can't do that then this may be of some help to you:
     who   followed by on or more of the following flags:
          -b Displays time sys as last booted.
          -H Precedes output with header.
          -l Lists lines waiting for users to logon.
          -q displays number of users logged on.
          -t displays time sys clock was last changed.
          -T displays the state field (a + indicates it is 
possible to send to terminal, a - means u cannot)
          -u Give a complete listing of those logged on.
   **who -HTu is about the best choice for the average user**
##by the way, the list of users logged on is kept in the file 
/etc/utmp.  If you want to write your own personalised version of 
who in C, you now know where to look!###
     When a users state field (see -T flag option for who 
command) says that a user has their message function on, this 
actually means that it is possible to get stuff onto their 
     Basically, every terminal on the system has a file 
corresponding to it.  These files can be found in the /dev 
directory.  You can to anything to these files, so long as you 
have access -eg you can read them, and write to them, but you 
will notice that they never change in size.  They are called 
character specific files, and are really the link between the 
system and the terminals.  Whatever you put in these files will 
go straight to the terminal it corresponds to.
     Unfortunately, on most systems, when the user logs in, the 
"mesg n" command is issued which turns off write access to that 
terminal, BUT- if you can start cating to that terminal before 
system issues the mesg n command, then you will continue to be 
able to get stuff up on that terminal! This has many varied uses.
     Check out the terminal, or terminal software being used.  
Often you will be able to remotely program another users 
terminal, simply by 'cating' a string to a users screen.  You 
might be able to set up a buffer, capturing all that is typed, or 
you may be able to send the terminal into a frenzy- (sometimes a 
user will walk away without realizing that they are sill 
effectively logged on, leaving you with access to their 
account!).  Some terminal types also have this great command 
called transmit screen. It transmits everything on the screen, 
just as if the user had typed it !
     So just say I wanted to log off a user, then I would send a 
clear screen command (usually ctrl l), followed by "exit" 
followed by a carriage return, followed by the transmit screen 
code.  Using this technique you can wipe peoples directories or 
anything.  My favourite is to set open access on all their files 
and directories so I can peruse them for deletion etc at my own 
     If you ever briefly get access to another persons account 
eg. they leave the room to go to toilet or whatever, then simply 
type the following:
chmod 777 $HOME
chmod 777 $MAIL
     Then clear the screen so they don't see what you just typed.
     Now you can go look at their directory, and their mail, and 
you can even put mail in their mail file. (just use the same 
format as any mail that is already there!). Next time they log in 
the system will automatically inform them they have new mail!
     Another way to send fake mail to people is to use the mail 
server.  This method produces mail that is slightly different to 
normal, so anyone who uses UNIX a bit may be suspiscious when 
they receive it, but it will fool the average  user!
type telnet
the following prompt will appear:
now type :
open localhost 25
some crap will come up about the mail type:
mail from: xxxxxx     Put any name you want.
some more bullshit will come up. Now type:
rcpt to: xxxxxx  Put the name of the person to receive mail here.
now type:
now you can type the letter...end it with a "."
type quit to exit once you are done.
     Here's one for any experimenters out there...
It is possible to create files which simply cannot be deleted 
from the standard shell.  To do this you will have to physically 
have to use a sequence of control characters which cannot be 
typed from the shell.  Try things like Ctrl-h (this is the 
code for the delete key).  Just a file with the name Ctrl-h would 
not be deleteable from the shell, unless you used wildcards. So, 
make it a nice long series of characters, so that to delete the 
file, the user has no choice but to individually copy all his 
files elsewhere, then delete everything in his directory, and 
then copy all his files back.....this is one of my 
favourites..gets em every time!
     The following script file is an example which will create a 
file with the name Ctrl-h.  You MUST tyoe this file in using the 
vi editor or similar.
*****If you are not very good with vi, type "man vi" and print the 
help even contains stuff that I find useful now and 
type the following in vi...
echo'' > 'a^h'
  *** get the ^h (this really means ctrl-h) from vi type:
Ctrl v
Ctrl h 
  The Ctrl v instructs vi to take the next character as a ascii 
character, and not to interpret it.
     change the access on the file you just created and now 
execute it.  It will create a file which looks like it is called 
a, but try to delete it !..use wildcards if you really want to 
delete it. 
*> Title:   Tutorial on hacking through a UNIX system

In the following file, all references made to the name Unix, may also be 
substituted to the Xenix operating system. 
Brief history:  Back in the early sixties, during the development of 
third generation computers at MIT, a group of programmers studying the 
potential of computers, discovered their ability of performing two or 
more tasks simultaneously.  Bell Labs, taking notice of this discovery, 
provided funds for their developmental scientists to investigate into this 
new frontier.  After about 2 years of developmental research, they produced 
an operating system they called "Unix".  
Sixties to Current:  During this time Bell Systems installed the Unix system 
to provide their computer operators with the ability to multitask so that 
they could become more productive, and efficient.  One of the systems they
put on the Unix system was called "Elmos". Through Elmos many tasks (i.e.
billing,and installation records) could be done by many people using the same 
Note: Cosmos is accessed through the Elmos system. 
Current:  Today, with the development of micro computers, such multitasking 
can be achieved by a scaled down version of Unix (but just as 
powerful).  Microsoft,seeing this development, opted to develop their own 
Unix like system for the IBM line of PC/XT's.  Their result they called 
Xenix (pronounced zee-nicks).  Both Unix and Xenix can be easily installed 
on IBM PC's and offer the same function (just 2 different vendors). 
Note: Due to the many different versions of Unix (Berkeley Unix, 
Bell System III, and System V the most popular) many commands 
following may/may not work. I have written them in System V routines. 
Unix/Xenix operating systems will be considered identical systems below. 
How to tell if/if not you are on a Unix system:  Unix systems are quite 
common systems across the country. Their security appears as such: 
Login;     (or login;) 
When hacking on a Unix system it is best to use lowercase because the Unix 
system commands are all done in lower- case. Login; is a 1-8 character field. It is 
usually the name (i.e. joe or fred) of the user, or initials (i.e. j.jones 
or f.wilson).  Hints for login names can be found trashing the location of 
the dial-up (use your CN/A to find where the computer is). Password: is a 1-8 character password assigned by the sysop or chosen by the user. 
   Common default logins 
   login;       Password: 
   root         root,system,etc.. 
   sys          sys,system 
   daemon       daemon 
   uucp         uucp 
   tty          tty 
   test         test 
   unix         unix 
   bin          bin 
   adm          adm 
   who          who 
   learn        learn 
   uuhost       uuhost 
   nuucp        nuucp 
If you guess a login name and you are not asked for a password, and have 
accessed to the system, then you have what is known as a non-gifted account. 
If you guess a correct login and pass- word, then you have a user account. 
And, if you get the root p/w you have a "super-user" account. 
All Unix systems have the following installed to their system: 
root, sys, bin, daemon, uucp, adm Once you are in the system, you will 
get a prompt. Common prompts are: 
But can be just about anything the sysop or user wants it to be. 
Things to do when you are in: Some of the commands that you may want to 
try follow below: 
who is on  (shows who is currently logged on the system.) 
write name (name is the person you wish to chat with) 
To exit chat mode try ctrl-D. 
EOT=End of Transfer. 
ls -a      (list all files in current   directory.) 
du -a      (checks amount of memory  your files use;disk usage) 
cd\name    (name is the name of the sub-directory you choose) 
cd\        (brings your home directory  to current use) 
cat name   (name is a filename either  a program or documentation  your username has written) 
 Most Unix programs are written  in the C language or Pascal 
 since Unix is a programmers'  environment. One of the first things done on the 
system is print up or capture (in a buffer) the file containing all user names and accounts. This can be done by doing the following command: 
cat /etc/passwd 
If you are successful you will see a list of all accounts on the system.  It 
should look like this:  
root:hvnsdcf:0:0:root dir:/: joe:majdnfd:1:1:Joe Cool:/bin:/bin/joe hal::1:2:Hal Smith:/bin:/bin/hal 
The "root" line tells the following info : 
login name=root 
hvnsdcf   = encrypted password 
0         = user group number 
0         = user number 
root dir  = name of user 
/         = root directory 
In the Joe login, the last part "/bin/joe " tells us which directory 
is his home directory (joe) is. In the "hal" example the login name is 
followed by 2 colons, that means that there is no password needed to get in 
using his name. 
Conclusion:  I hope that this file will help other novice Unix hackers 
obtain access to the Unix/Xenix systems that they may find.

                      On the Security of UNIX
Recently there has been much interest in the security aspects of operating
systems and software.At issue is the ability to prevent undesired disclosure of
information, destruction of information,and harm to the functioning of the
system.This paper discusses the degree of security which can be provided under
the system and offers a number of hints on how to improve security.The first
fact to face is that UNIX was not developed with security,in any realistic
sense,in mind;this fact alone guarantees a vast number of holes.(Actually the
same statement can be made with respect to most systems.)

The area of security in which is theoretically weakest is in protecting against
crashing or at least crippling the operation of the system.The problem here is
not mainly in uncritical acceptance of bad parameters to system calls (there
may be bugs in this area, but none are known)but rather in lack of checks for
excessive consumption of resources.

Most notably, there is no limit on the amount of disk storage used, either in
total space allocated or in the number of files or directories.Here is a
particularly ghastly shell sequence guaranteed to stop the system:

while : ; do
        mkdir x
        cd x

Either a panic will occur because all the i-nodes on the device are used up,
or all the disk blocks will be consumed, thus preventing anyone from writing
files on the device.In this version of the system,users are prevented from
creating more than a set number of processes simultaneously,so unless users
are in collusion it is unlikely that any one can stop the system altogether.

However, creation of 20 or so CPU or disk-bound jobs leaves few resources
available for others.Also, if many large jobs are run simultaneously,swap space
may run out, causing a panic.  It should be evident that excessive consumption
of disk space, files, swap space and processes can easily occur accidentally in
malfunctioning programs as well as at command level.In fact UNIX is essentially
defenseless against this kind of abuse,nor is there any easy fix.The best that
can be said is that it is generally fairly easy to detect what has happened
when disaster strikes ,to identify the user responsible, and take appropriate
action.In practice,we have found that difficulties in this area are rather
rare,but we have not been faced with malicious users,and enjoy a fairly
generous supply of resources which have served to cushion us against accidental
over consumption.

The picture is considerably brighter in the area of protection of information
from unauthorized perusal and destruction.Here the degree of security seems
(almost) adequate theoretically, and the problems lie more in the necessity for
care in the actual use of the system.Each UNIX file has associated with it
eleven bits of protection information together with a user identification
number and a user-group identification number (UID and GID).

Nine of the protection bits are used to specify independently permission to
read, to write, and to execute the file to the user himself, to members of the
user's group, and to all other users.Each process generated by or for a user
has associated with it an effective UID and a real UID, and an effective and
real GID. When an attempt is made to access the file for reading, writing, or
executing UID for the process is changed to the UID associated with the file;
the change persists until the process terminates or until the UID changed again
by another execution of a set-UID file.Similarly the effective group ID of a
process is changed to the GID associated with a file when that file is executed
and has the set-GID bit set.The real UID and GID of a process do not change
when any file is executed,but only as the result of a privileged system
call.The basic notion of the set-UID and set-GID bits is that one may write a
program which is executable by others and which maintains files accessible to
others only by that program.

The classical example is the game-playing program which maintains records of
the scores of its players.The program itself has to read and write the score
file,but no one but the game's sponsor can be allowed unrestricted access to
the file lest they manipulate the game to their own advantage.

The solution is to turn on the set-UID bit of the game program.  When, and only
when,it is invoked by players of the game,it may update the score file but
ordinary programs executed by others cannot access the score.  There are a
number of special cases involved in determining access permissions.  Since
executing a directory as a program is a meaningless operation,the
execute-permission bit, for directories, is taken instead to mean permission to
search the directory for a given file during the scanning of a path name; thus
if a directory has execute permission but no read permission for a given user,
he may access files with known names in the directory,but may not read (list)
the entire contents of the directory.

Write permission on a directory is interpreted to mean that the user may create
and delete files in that directory;it is impossible for any user to write
directly into any directory..Another, and from the point of view of security,
much more serious special case is that there is a ``super user'' who is able to
read any file and write any non-directory.The super-user is also able to change
the protection mode and the owner UID and GID of any file and to invoke
privileged system calls.It must be recognized that the mere notion of a
super-user is a theoretical, and usually practical, blemish on any protection

The first necessity for a secure system is of course arranging that all files
and directories have the proper protection modes.Traditionally, UNIX software
has been exceedingly permissive in this regard;essentially all commands create
files readable and writable by everyone.In the current version,this policy may
be easily adjusted to suit the needs of the installation or the individual user.

Associated with each process and its descendants is a mask, which is in effect
anded with the mode of every file and directory created by that process.  In
this way, users can arrange that, by default,all their files are no more
accessible than they wish.The standard mask, set by login,allows all permiss-
ions to the user himself and to his group,but disallows writing by others.

To maintain both data privacy and data integrity,it is necessary, and largely
sufficient,to make one's files inaccessible to others.  The lack of sufficiency
could follow from the existence of set-UID programs created by the user and the
possibility of total breach of system security in one of the ways discussed
below(or one of the ways not discussed below).

For greater protection,an encryption scheme is available.Since the editor is
able to create encrypted documents, and the crypt command can be used to pipe
such documents into the other text-processing programs,the length of time
during which clear text versions need be available is strictly limited.The
encryption scheme used is not one of the strongest known, but it is judged
adequate, in the sense that cryptanalysisis likely to require considerably more
effort than more direct methods of reading the encrypted files.For example, a
user who stores data that he regards as truly secret should be aware that he is
implicitly trusting the system administrator not to install a version of the
crypt command that stores every typed password in a file.  Needless to say, the
system administrators must be at least as careful as their most demanding user
to place the correct protection mode on the files under their control.

In particular,it is necessary that special files be protected from writing, and
probably reading, by ordinary users when they store sensitive files belonging
to other users. It is easy to write programs that examine and change files by
accessing the device on which the files live.

On the issue of password security,UNIX is probably better than most systems.
Passwords are stored in an encrypted form which, in the absence of serious
attention from specialists in the field,appears reasonably secure, provided its
limitations are understood.In the current version, it is based on a slightly
defective version of the Federal DES;it is purposely defective so that
easily-available hardware is useless for attempts at exhaustive
key-search.Since both the encryption algorithm and the encrypted passwords are
available,exhaustive enumeration of potential passwords is still feasible up to
a point.We have observed that users choose passwords that are easy to
guess:they are short, or from a limited alphabet, or in a dictionary.
Passwords should be at least six characters long and randomly chosen from an
alphabet which includes digits and special characters.

Of course there also exist feasible non-cryptanalytic ways of finding out
passwords.For example:  write a program which types out ``login:''on the
typewriter and copies whatever is typed to a file of your own.  Then invoke the
command and go away until the victim arrives..The set-UID (set-GID)notion must
be used carefully if any security is to be maintained.  The first thing to keep
in mind is that a writable set-UID file can have another program copied onto

For example, if the super-user command is writable,anyone can copy the shell
onto it and get a password-free version of Shell Unix.A more subtle problem can
come from set-UID programs which are not sufficiently careful of what is fed
into them.To take an obsolete example,the previous version of the mail command
was set-UID and owned by the super-user.This version sent mail to the r
ecipient's own directory.The notion was that one should be able to send mail to
anyone even if they want to protect their directories from writing.  The trouble
was that mail was rather dumb:anyone could mail someone else's private file to
himself.Much more serious is the following scenario:  make a file with a line
like one in the password file which allows one to log in as the super-user.Then
make a link named ``.mail'' to the password file in some writable directory on
the same device as the password file (say /tmp).  Finally mail the bogus login
line to /tmp/.mail;You can then login as the superuser,clean up the
incriminating evidence,and have your will.

The fact that users can mount their own disks and tapes as file systems can be
another way of gaining super-user status.Once a disk pack is mounted, the
system believes what is on it.Thus one can take a blank disk pack,put on it
anything desired,and mount it.There are obvious and unfortunate consequences.
For example:a mounted disk with garbage on it will crash the system;one of the
files on the mounted disk can easily be a password-free version of Shell Unix;
other files can be unprotected entries for special files.  The only easy fix
for this problem is to forbid the use of mount to unpriv- ileged users.A
partial solution, not so restrictive,would be to have the mount command examine
the special file for bad data,set-UID programs owned by others ,and accessible
special files,and balk at unprivileged invokers.

Scott Walters   London, CANADA
[email protected]  <CarbonBoy>
PGP 31 03 1B E1 C7 6E 3A EC  97 32 01 BA 5B 05 5D FB
finger me for public key block
MIME-mail welcome
'Beware the fury of a patient man.'