A Basic UNIX Overview
Asriel
UNIX FOR DOS ADDICTED WaReZ PuPPieZ AND THEIR PETS
Introduction
------------
One of the most common operating systems in existance is Unix. Unix
exists in many different flavors, from Berkeley BSD to AT&T System V
to SunOs. Basic working knowledge of Unix is almost essential to a
hacker, as it is the system a hacker is most likely to come across.
If you intend to use the internet at all, or to do any serious
exploration of Telenet, the ability to navigate through Unix is a
necessity. (Unix is also the single most interesting system in
existance: it's just fun to fuck with).
Unix Logins
-----------
Most Unix logins look essentially the same. A general Unix login
prompt looks something like this:
connected to five.finger.com
login:
That first line is the system identifier. Although it's not at all
essential to what you are doing, it's good to know what system you are
attempting to log on to.
The second line is what typically identifies the system you are on as
Unix. Almost all Unix systems greet a user with the same prompt:
login:.
Well, there's not much to do in Unix from the outside, and Unix
systems are typically fairly secure at this point. You may be able to
obtain a list of users, or current users, by logging in as 'who', but
other than that there are few functions available here.
Unless you are on the internet, or have accounts specifically for the
specific machine you are on, the only way on to the system is to try
the default passwords. What are the default passwords?
Unix systems come installed with certain passwords automatically. In
addition, some accounts must exist on a system. One such account is
'root'. This user is the divine Kami of the Unix system... in short,
an all access pass. Unfortunately, few systems allow root logins
remotely, and even fewer leave 'root' unpassworded. Nevertheless, it's
always worth a shot... try this:
connected to ren.stimpy.net
login: root
password: root
invalid login
well, nice try anyways... other possible passwords for root include
'sysadmin', 'sys', 'admin'... you get the idea. You may also want to
try these passwords with a single digit appended (added, idiot) to
them... meaning the password 'root' could be 'root1' or 'root2'.
An interesting tip about passwords in general... many people that use
passwords under 8 characters tend to add a digit or a non-alphanumeric
character to the password. This is done in order to hinder guessing,
and to stop password breakers (more on this later). In this case, you
may want to try adding a space before root... or even an ascii 255 to
the end.
Fortunately, there is more than one default password in a unix
system... a quick list:
sys sys
bin bin
daemon daemon
rje rje
setup setup
uucp uucp/nuucp/anonymous
nuucp uucp/nuucp/anonymous
mountfsys mountfsys
In the System
-------------
Ok, at this point, I'm going to assume you've gotten past the login...
as painful as that may sound. Although Unix may be secure from the
outside, without effort from the system administrators, the inside of
the system is not.
First off, you'll likely by asked for a terminal. vt100 serves your
purposes sufficently, and it's typically the default, so hit enter.
Now, hopefully, you have a prompt. There are many different types of
unix prompts, some of which contain current directory information,
some of which are just a single character. Just don't panic when my
examples don't look exactly like what you've got on your screen.
The first thing you *need* to do on the system is establish your tty
paramters. As eldritch and arcane sounding as this term may seem, it's
actually quite simple... you need to tell the system what keys are
going to do what.
The command to set these parameters is 'stty'. Watch:
squinkyB ] stty erase ^h
squinkyB ]
There... that wasn't so bad, was it? Well, it's also pretty
meaningless to you, unless you have the ascii table memorized and are
pretty good at on-the-spot deduction.
The tty erase parameters determines which key is to be used as a
backspace. At times, this may already be set when you log in, or it
may be set to a suitable alternate (such as delete). Most of the time
the system will tell you when you log on if this is so. In this case,
we've entered ^h in order to make the backspace key, appropriately
enough, backspace.
Another extremely important parameter is 'intr'. The 'intr' paramter
tells the Unix system what you intend to use as a break character...
you should have this set to ^c.
Getting Around
--------------
A good thing to remember about Unix is that it's alot like DOS. Files
are laid out in directories just as in DOS... in fact, the only
immediate difference in the directory structures is that Unix uses a
forward slash ("/", moron!) instead of a backwards one.
Also, the basic Unix directory navigation command is identical to DOS.
In order to change directories, you use the command 'chdir', or 'cd'.
A quick example:
1 /usr1/astoria ] cd ..
2 /usr ]
Wala. That simple. Quick notes:
ю cd / will take you to root.
ю cd /*pathname* will take you to *pathname*
ю cd home will take you to your home directory.
You can make and delete your own directories with the mkdir/rmdir
commands. Simply put, mkdir makes a subdirectory off of the current
directory, and rmdir removes a subdirectory from the current
subdirectory. Good to know if you plan to do a lot of file transfers.
An important note about Unix directories, files, and concepts:
Unix is a case-sensitive operating system. Thus, the files
ю Spleen
ю spleen
ю SPLEEN
ю SpLeEn
are all different. This rule applies to directories and command line
paramters, as well as most other Unix ideas.
Another nice thing to know about Unix: Unix files are not subject to
the normal DOS 8 character limit. Thus, you can have vast filenames,
such as "this_file_ate_my_biscuit".
Some other important commands
-----------------------------
First and foremost, you should know cp. cp is the basic Unix
equivalent of the DOS COPY command. The command line for cp is
identical to that of COPY.
Next on the scale of cosmic import is cat. cat is the Unix equivalent
of the DOS TYPE command, and once again, for simple file displaying,
the command line is identical.
Variations on the theme:
pg: displayes a file page by page. Type "pg x filename", where x is a
number of lines to display before pausing and filename is the
file you wish to display.
more: displays a file screen by screen.
Stupid pet trick:
You can use your cat to copy files, simply by using the directional
operators. To copy a file from here to there using cat, simply type:
% cat here
this is the file here
% cat there
this is the file there
% cat here > there
The operator ">" simply takes the output from the cat command and
places is in the location specified after it.
Another vital command to know is 'rm'. rm deletes a file from the
system, in the same way DEL would on a DOS system. Not to much else to
say.
Critical in your navigation of a Unix system is the ls command. ls is
DOS DIR on heroin. Simply type ls and you get a nice, neat list of
files in the directory.
DIR on controlled substances:
There are a few command line parameters that you should know...
foremost is l. ls -l gets you a list of files, and valuable
information about each file, including permissions (more on that
later), size, and linked files.
Another useful command for long file lists is C. ls -C gets you a
list of files in multiple columns, much the same as DIR /W would
merit a double column report of all existing files. A quick reminder:
ls -C is NOT the same as ls -c. Unix = case sensitive.
Another good command to know, mv will move a file from directory to
directory. For those of you without DOS 6.0 <gasp>, mv simply copies a
file to another directory and deletes the original.
quick tip for files on the lam:
if you want to rename a file (to protect the innocent), you need to
mv a file to a different file name. A quick demo:
# ls
myfile
# cat myfile
this is my file
# mv myfile my_other_file
my_other_file
# cat my_other_file
Another vastly important command is 'man'. In fact, man is probably
one of the most important commands extant for a beginning user... it
calls up the system's help files. To use man, simply type in 'man
command', where command is a Unix command you seek to gain
enlightenment regarding. It's a great way to gain an understanding of
Unix commandline parameters.
If you are interested in seeing who's been on of late, or just want a
few names to try to hack, type 'who'. You get a quick list of users
that have accessed the system lately. If you <god forbid> need to know
who you are at this point, type 'whoami'.
If you want to change your identity on the system, type 'su name'
where name is an account on the system. It'll ask you for the account
password, then, *presto*... instant transmogrification.
A Caveat for smart alec hackers:
Unix typically logs usage of the su command. While su may seem like a
great opportunity to try to hack out passwords manually without
worrying about the system hanging up after 3 attempts, it's typically
not a good idea to do this, as it may alert the administrators to
your presence.
*Numero Uno on the list of commands NEVER to use on a Unix system:
The 'passwd' command changes your password on a Unix system. Seems
innocous enough, eh? Uh-uh. If your account is active, and there's a
very strong chance that it either is or will be, there is no better
way to lose the account than to change the password, only to have the
legitimate user alert the sysadmins when he/she can't gain access to
his/her normal account (well, there are better ways... you could
simply mail the sysadmin and tell him you are trying to hack his
grandmother's life support machine through your account).
I've seen this single, quick command turn a extremely lax system
into an ironclad security compound in less than a day.
DONT-FUCK-WITH-IT.
*Numero Dos on that same list:
The 'mail' command reads and sends mail. So what? Well, unless your
account is stable (and it isn't unless you either paid for it or
killed the original owner in such a way that his body cannot claw it's
way out of it's grave to it's keyboard), the user is more likely than
not going to know if you read his mail. In addition, if you send mail
out of the system (type 'mail', and a username/address; type in your
message and end it with a ^d on it's own line), the response from your
message will likewise alert the user to your presence.
System Spelunking
-----------------
The first place you want to check out in the wild uncharted directory
tree of your friendly neighborhood Unix system is the "/etc"
directory. What's in it? The single most intensely important file on
the system (besides a world writable root owned SUID file... but don't
worry about that)... the passwd file.
What is in the passwd file?
ю a list of all accounts on the system
ю a list of the passwords for these accounts
ю a list of access levels for these accounts
ю a list of the home directories for these accounts
ю a list of information pertaining to these accounts.
Why the hell the Unix designers decided this file should be world
readable is beyond me. Be content to know that your standard everyday
run-of-the-mill-lacking-in-certified-cosmic-power 'cat' command WILL
display this file. As will pg and more. However, because most users
don't have write permissions (more on that later) to the /etc
directory, 'cat' is pretty much the only applicable command here.
However, if you need to copy the file to your own directory (for
whatever reason), just cat it there with the directional operator (>).
The catch:
Well, there are two catches here. First off, regardless of system
security, if the passwords are in the file, they are encrypted. You
can't decrypt them. Although you can get a list of accounts without
passwords this way (just look for accounts with no entry in the
password field), and a list of accounts that can't be logged onto
remotely/at all (NO LOGIN), you can't get much else. Sucks, don't it?
Notice I said 'if' the passwords are there.
<ominous soundtrack please>
Some horrible, paranoid, draconian system administrators mutilate
their passwd files in such a way that (*gasp*) the passwords don't
show up. All you get is one cold, icy X staring at you from the bowels
of Unix Shell Siberia, mocking you as you pull your hair out in
frustration (sorry, but this is a sore spot with me). The kidnapped
passwords reside in the shadow file in the /etc directory, available
with your standard everyday run-of-the-mill-but-distinct-in-the-fact-
that-only-root-level-accounts-can-use-it-to-this-extent 'cat' command.
Well, if the passwords are encrypted, what good are they?
By themselves, nothing. A account with a Unix encrypted password will
get you no further than an account with no listed password at all. You
can't even deduce the amount of characters in the password if it's
encrypted. So what's the use?
The Unix method of encrypting files is available to the public. It is
also, to most mortals, irreversable. Essentially, this means you can
encrypt a string of characters, but not decrypt it. Even the unix
system itself doesn't decrypt the password when you log on...
When you log on, the Unix system takes whatever you enter at the
password prompt, encrypts it, and matches it to the entry in the
passwd file. Thus, the Unix system never decrypts the password... it
only compares it to a different encrypted string.
While this may not sound too particularly useful at first, it is.
There are programs that have been written to do the same thing on a
personal computer... you supply it a list of passwords and a list of
words to attempt to use as passwords (called dictionaries), and it
spends the night encrypting dictionaries and matching them to password
entries. By running a dictionary through a passwd file, on a typical
system, you can usually get 10-20 accounts. Good personal computer
examples of this program idea include Killer Cracker (the industry
standard, so to speak) and CrackerJack (faster than Killer Cracker).
Quick tips for CrackerJunkies with leech access at an H/P BBS:
A standard dictionary will not uncover passwords protected with an
appended digit or non-alphanumeric character. In order to get around
this, you need only grab a program that processes the dictionary file
to add that digit to each entry in the dictionary... although this
takes longer, and you'll need to do it multiple times, you can
typically get 10 more accounts just by adding a 1 to every entry.
Files and directories in Unix are characterized further by their
permissions. Permissions are a standard system of who gets access to a
specific function of that file or directory. Standard permissions
include read, write, and execute. You can get a list of permissions by
typing 'ls -l'. The first field in the listing contains the
permissions, grouped as follows:
owner group world
--------------------
rwx rwx rwx
(Not drawn to scale... in fact, it doesn't look anything like that).
Essentially, as long as the letter is there, you have access to that
facet of the file. If the letter is not there, you'll see a dash...
meaning you don't have access to that function. An example:
rwxr-x--x
In this case, the owner of the file can Read the file, Write to the
file, and eXecute the file; members of his group (a bunch of linked
accounts) can Read the file, CANNOT Write to the file, and can eXecute
the file; and the rest of the user population CANNOT Read or Write to
the file, but CAN eXecute the file.
rwx---rwx
is a WORLD-READABLE, WORLD-WRITABLE, WORLD-EXECUTABLE file. This
simply means that anyone can read, write, or execute the file.
Another permission sometimes set to a file is the SUID bit. An SUID
file contains a smallcase s in the user executable section of the
permissions list...
rws--x--x
When you execute an SUID file, your user ID becomes that of the owner
of the file. While this may not look to important at first, by now you
should know that no really important super elite hacker concept does.
Take a look at this:
rwsr-x--x
Synopsis? It's a world executable SUID file. In essence, anyone can
execute the file, and in doing so, become the owner of the file for
the duration of the time that file is operating. However, this doesn't
...
ms_lo