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Linux Shadow Password HOWTO
Michael H. Jackson, mhjack@tscnet.com
v1.3, 3 April 1996
This document aims to describe how to obtain, install, and configure
the Linux password Shadow Suite. It also discusses obtaining, and
re]installing other software and network daemons that require access
to user passwords. This other software is not actually part of the
Shadow Suite, but these programs will need to be recompiled to support
the Shadow Suite. This document also contains a programming example
for adding shadow support to a program. Answers to some of the more
frequently asked questions are included near the end of this document.
______________________________________________________________________
Table of Contents
1. Introduction.
1.1 Changes from the previous release.
1.2 New versions of this document.
1.3 Feedback.
2. Why shadow your passwd file?
2.1 Why you might NOT want to shadow your passwd file.
2.2 Format of the /etc/passwd file
2.3 Format of the shadow file
2.4 Review of crypt(3).
3. Getting the Shadow Suite.
3.1 History of the Shadow Suite for Linux
3.2 Where to get the Shadow Suite.
3.3 What is included with the Shadow Suite.
4. Compiling the programs.
4.1 Unpacking the archive.
4.2 Configuring with the config.h file
4.3 Making backup copies of your original programs.
4.4 Running make
5. Installing
5.1 Have a boot disk handy in case you break anything.
5.2 Removing duplicate man pages
5.3 Running make install
5.4 Running pwconv
5.5 Renaming npasswd and nshadow
6. Other programs you may need to upgrade or patch
6.1 Slackware adduser program
6.2 The wu_ftpd Server
6.3 Standard ftpd
6.4 pop3d (Post Office Protocol 3)
6.5 xlock
6.6 xdm
6.7 sudo
6.8 imapd (E-Mail
6.9 pppd (Point-to-Point Protocol Server)
7. Putting the Shadow Suite to use.
7.1 Adding, Modifying, and deleting users
7.1.1 useradd
7.1.2 usermod
7.1.3 userdel
7.2 The passwd command and passwd aging.
7.3 The login.defs file.
7.4 Group passwords.
7.5 Consistency checking programs
7.5.1 pwck
7.5.2 grpck
7.6 Dial-up passwords.
8. Adding shadow support to a C program
8.1 Header files
8.2 libshadow.a library
8.3 Shadow Structure
8.4 Shadow Functions
8.5 Example
9. Frequently Asked Questions.
10. Copyright Message.
11. Miscellaneous and Acknowledgments.
______________________________________________________________________
1. Introduction.
This is the Linux Shadow-Password-HOWTO. This document describes why
and how to add shadow password support on a Linux system. Some
examples of how to use some of the Shadow Suite's features is also
included.
When installing the Shadow Suite and when using many of the utility
programs, you must be logged in as root. When installing the Shadow
Suite you will be making changes to system software, and it is highly
recommended that you make backup copies of programs as indicated. I
also recommend that you read and understand all the instructions
before you begin.
1.1. Changes from the previous release.
Additions:
Added a sub-section on why you might not want to install shadow
Added a sub-section on updating the xdm program
Added a section on how to put Shadow Suite features to work
Added a section containing frequently asked questions
Corrections/Updates:
Corrected html references on Sunsite
Corrected section on wu-ftp to reflect adding -lshadow to the Makefile
Corrected minor spelling and verbiage errors
Changed section on wu-ftpd to support ELF
Updated to reflect security problems in various login programs
Updated to recommend the Linux Shadow Suite by Marek Michalkiewicz
1.2. New versions of this document.
The latest released version of this document can always be retrieved
by anonymous FTP from:
sunsite.unc.edu
/pub/Linux/docs/HOWTO/Shadow-Password-HOWTO
or:
/pub/Linux/docs/HOWTO/other-formats/Shadow-Password-HOWTO{-html.tar,ps,dvi}.gz
or via the World Wide Web from the Linux Documentation Project Web
Server <http://sunsite.unc.edu/mdw/linux.html>, at page: Shadow-
Password-HOWTO <http://sunsite.unc.edu/linux/HOWTO/Shadow-Password-
HOWTO.html> or directly from me, <mhjack@tscnet.com>. It will also be
posted to the newsgroup: comp.os.linux.answers
This document is now packaged with the Shadow-YYDDMM packages.
1.3. Feedback.
Please send any comments, updates, or suggestions to me: Michael H.
Jackson <mhjack@tscnet.com> The sooner I get feedback, the sooner I
can update and correct this document. If you find any problems with
it, please mail me directly as I very rarely stay up-to-date on the
newsgroups.
2. Why shadow your passwd file?
By default, most current Linux distributions do not contain the Shadow
Suite installed. This includes Slackware 2.3, Slackware 3.0, and
other popular distributions. One of the reasons for this is that the
copyright notices in the original Shadow Suite were not clear on
redistribution if a fee was charged. Linux uses a GNU Copyright
(sometimes refereed to as a Copyleft) that allows people to package it
into a convenient package (like a CD-ROM distribution) and charge a
fee for it.
The current maintainer of the Shadow Suite, Marek Michalkiewicz
<marekm@i17linuxb.ists.pwr.wroc.pl> received the source code from the
original author under a BSD style copyright that allowed
redistribution. Now that the copyright issues are resolved, it is
expected that future distributions will contain password shadowing by
default. Until then, you will need to install it yourself.
If you installed your distribution from a CD-ROM, you may find that,
even though the distribution did not have the Shadow Suite installed,
some of the files you need to install the Shadow Suite may be on the
CD-ROM.
However, Shadow Suite versions 3.3.1, 3.3.1-2, and shadow-mk all have
security problems with their login program and several other suid root
programs that came with them, and should no longer be used.
All of the necessary files may be obtained via anonymous FTP or
through the World Wide Web.
On a Linux system without the Shadow Suite installed, user information
including passwords is stored in the /etc/passwd file. The password
is stored in an encrypted format. If you ask a cryptography expert,
however, he or she will tell you that the password is actually in an
encoded rather than encrypted format because when using crypt(3), the
text is set to null and the password is the key. Therefore, from here
on, I will use the term encoded in this document.
The algorithm used to encode the password field is technically
referred to as a one way hash function. This is an algorithm that is
easy to compute in one direction, but very difficult to calculate in
the reverse direction. More about the actual algorithm used can be
found in section 2.4 or your crypt(3) manual page.
When a user picks or is assigned a password, it is encoded with a
randomly generated value called the salt. This means that any
particular password could be stored in 4096 different ways. The salt
value is then stored with the encoded password.
When a user logs in and supplies a password, the salt is first
retrieved from the stored encoded password. Then the supplied
password is encoded with the salt value, and then compared with the
encoded password. If there is a match, then the user is
authenticated.
It is computationally difficult (but not impossible) to take a
randomly encoded password and recover the original password. However,
on any system with more than just a few users, at least some of the
passwords will be common words (or simple variations of common words).
System crackers know all this, and will simply encrypt a dictionary of
words and common passwords using all possible 4096 salt values. Then
they will compare the encoded passwords in your /etc/passwd file with
their database. Once they have found a match, they have the password
for another account. This is referred to as a dictionary attack, and
is one of the most common methods for gaining or expanding
unauthorized access to a system.
If you think about it, an 8 character password encodes to 4096 * 13
character strings. So a dictionary of say 400,000 common words,
names, passwords, and simple variations would easily fit on a 4GB hard
drive. The attacker need only sort them, and then check for matches.
Since a 4GB hard drive can be had for under $1000.00, this is well
within the means of most system crackers.
Also, if a cracker obtains your /etc/passwd file first, they only need
to encode the dictionary with the salt values actually contained in
your /etc/passwd file. This method is usable by your average teenager
with a couple of hundred spare Megabytes and a 486 class computer.
Even without lots of drive space, utilities like crack(1) can usually
break at least a couple of passwords on a system with enough users
(assuming the users of the system are allowed to pick their own
passwords).
The /etc/passwd file also contains information like user ID's and
group ID's that are used by many system programs. Therefore, the
/etc/passwd file must remain world readable. If you were to change
the /etc/passwd file so that nobody can read it, the first thing that
you would notice is that the ls -l command now displays user ID's
instead of names!
The Shadow Suite solves the problem by relocating the passwords to
another file (usually /etc/shadow). The /etc/shadow file is set so
that it cannot be read by just anyone. Only root will be able to read
and write to the /etc/shadow file. Some programs (like xlock) don't
need to be able to change passwords, they only need to be able to
verify them. These programs can either be run suid root or you can
set up a group shadow that is allowed read only access to the
/etc/shadow file. Then the program can be run sgid shadow.
By moving the passwords to the /etc/shadow file, we are effectively
keeping the attacker from having access to the encoded passwords with
which to perform a dictionary attack.
Additionally, the Shadow Suite adds lots of other nice features:
o A configuration file to set login defaults (/etc/login.defs)
o Utilities for adding, modifying, and deleting user accounts and
groups
o Password aging and expiration
o Account expiration and locking
o Shadowed group passwords (optional)
o Double length passwords (16 character passwords) NOT RECOMMENDED]
o Better control over user's password selection
o Dial-up passwords
o Secondary authentication programs [NOT RECOMMENDED]
Installing the Shadow Suite contributes toward a more secure system,
but there are many other things that can also be done to improve the
security of a Linux system, and there will eventually be a series of
Linux Security HOWTO's that will discuss other security measures and
related issues.
For current information on other Linux security issues, including
warnings on known vulnerabilities see the Linux Security home page.
<http://bach.cis.temple.edu/linux/linux-security/>
2.1. Why you might NOT want to shadow your passwd file.
There are a few circumstances and configurations in which installing
the Shadow Suite would NOT be a good idea:
o The machine does not contain user accounts.
o Your machine is running on a LAN and is using NIS (Network
Information Services) to get or supply user names and passwords to
other machines on the network. (This can actually be done, but is
beyond the scope of this document, and really won't increase
security much anyway)
o Your machine is being used by terminal servers to verify users via
NFS (Network File System), NIS, or some other method.
o Your machine runs other software that validates users, and there is
no shadow version available, and you don't have the source code.
2.2. Format of the /etc/passwd file
A non-shadowed /etc/passwd file has the following format:
username:passwd:UID:GID:full_name:directory:shell
Where:
username
The user (login) name
passwd
The encoded password
UID
Numerical user ID
GID
Numerical default group ID
full_name
The user's full name - Actually this field is called the GECOS
(General Electric Comprehensive Operating System) field and can
store information other than just the full name. The Shadow
commands and manual pages refer to this field as the comment
field.
directory
User's home directory (Full pathname)
shell
User's login shell (Full Pathname)
For example:
username:Npge08pfz4wuk:503:100:Full Name:/home/username:/bin/sh
Where Np is the salt and ge08pfz4wuk is the encoded password. The
encoded salt/password could just as easily have been kbeMVnZM0oL7I and
the two are exactly the same password. There are 4096 possible encod-
ings for the same password. (The example password in this case is
'password', a really bad password).
Once the shadow suite is installed, the /etc/passwd file would instead
contain:
username:x:503:100:Full Name:/home/username:/bin/sh
The x in the second field in this case is now just a place holder.
The format of the /etc/passwd file really didn't change, it just no
longer contains the encoded password. This means that any program
that reads the /etc/passwd file but does not actually need to verify
passwords will still operate correctly.
The passwords are now relocated to the shadow file (usually
/etc/shadow file).
2.3. Format of the shadow file
The /etc/shadow file contains the following information:
username:passwd:last:may:must:warn:expire:disable:reserved
Where:
username
The User Name
passwd
The Encoded password
last
Days since Jan 1, 1970 that password was last changed
may
Days before password may be changed
must
Days after which password must be changed
warn
Days before password is to expire that user is warned
expire
Days after password expires that account is disabled
disable
Days since Jan 1, 1970 that account is disabled
reserved
A reserved field
The previous example might then be:
username:Npge08pfz4wuk:9479:0:10000::::
2.4. Review of crypt(3).
From the crypt(3) manual page:
"crypt is the password encryption function. It is based on the Data
Encryption Standard algorithm with variations intended (among other
things) to discourage use of hardware implementations of a key search.
[The] key is a user's typed password. [The encoded string is all
NULLs]
[The] salt is a two-character string chosen from the set [a-zA-
Z0-9./]. This string is used to perturb the algorithm in one of 4096
different ways.
By taking the lowest 7 bit[s] of each character of the key, a 56-bit
key is obtained. This 56-bit key is used to encrypt repeatedly a
constant string (usually a string consisting of all zeros). The
returned value points to the encrypted password, a series of 13
printable ASCII characters (the first two characters represent the
salt itself). The return value points to static data whose content is
overwritten by each call.
Warning: The key space consists of 2**56 equal 7.2e16 possible values.
Exhaustive searches of this key space are possible using massively
parallel computers. Software, such as crack(1), is available which
will search the portion of this key space that is generally used by
humans for passwords. Hence, password selection should, at minimum,
avoid common words and names. The use of a passwd(1) program that
checks for crackable passwords during the selection process is
recommended.
The DES algorithm itself has a few quirks which make the use of the
crypt(3) interface a very poor choice for anything other than password
authentication. If you are planning on using the crypt(3) interface
for a cryptography project, don't do it: get a good book on encryption
and one of the widely available DES libraries."
Most Shadow Suites contain code for doubling the length of the
password to 16 characters. Experts in des recommend against this, as
the encoding is simply applied first to the left half and then to the
right half of the longer password. Because of the way crypt works,
this may make for a less secure encoded password then if double length
passwords were not used in the first place. Additionally, it is less
likely that a user will be able to remember a 16 character password.
There is development work under way that would allow the
authentication algorithm to be replaced with something more secure and
with support for longer passwords (specifically the MD5 algorithm) and
retain compatibility with the crypt method.
If you are looking for a good book on encryption, I recommend:
"Applied Cryptography: Protocols, Algorithms, and Source Code in C"
by Bruce Schneier <schneier@chinet.com>
ISBN: 0-471-59756-2
3. Getting the Shadow Suite.
3.1. History of the Shadow Suite for Linux
DO NOT USE THE PACKAGES IN THIS SECTION, THEY HAVE SECURITY PROBLEMS
The original Shadow Suite was written by John F. Haugh II.
There are several versions that have been used on Linux systems:
o shadow-3.3.1 is the original.
o shadow-3.3.1-2 is Linux specific patch made by Florian La Roche
<flla@stud.uni-sb.de> and contains some further enhancements.
o shadow-mk was specifically packaged for Linux.
The shadow-mk package contains the shadow-3.3.1 package distributed by
John F. Haugh II with the shadow-3.3.1-2 patch installed, a few fixes
made by Mohan Kokal <magnus@texas.net> that make installation a lot
easier, a patch by Joseph R.M. Zbiciak for login1.c (login.secure)
that eliminates the -f, -h security holes in /bin/login, and some
other miscellaneous patches.
The shadow.mk package was the previously recommended package, but
should be replaced due to a security problem with the login program.
There are security problems with Shadow versions 3.3.1, 3.3.1-2, and
shadow-mk involving the login program. This login bug involves not
checking the length of a login name. This causes the buffer to
overflow causing crashes or worse. It has been rumored that this
buffer overflow can allow someone with an account on the system to use
this bug and the shared libraries to gain root access. I won't
discuss exactly how this is possible because there are a lot of Linux
systems that are affected, but systems with these Shadow Suites
installed, and most pre-ELF distributions without the Shadow Suite are
vulnerable!
For more information on this and other Linux security issues, see the
Linux Security home page (Shared Libraries and login Program
Vulnerability) <http://bach.cis.temple.edu/linux/linux-security/Linux-
Security-FAQ/Linux-telnetd.html>
3.2. Where to get the Shadow Suite.
The only recommended Shadow Suite is still in BETA testing, however
the latest versions are safe in a production environment and don't
contain a vulnerable login program.
The package uses the following naming convention:
shadow-YYMMDD.tar.gz
where YYMMDD is the issue date of the Suite.
This version will eventually be Version 3.3.3 when it is released from
Beta testing, and is maintained by Marek Michalkiewicz
<marekm@i17linuxb.ists.pwr.wroc.pl>. It's available as: shadow-
current.tar.gz
<ftp://i17linuxb.ists.pwr.wroc.pl/pub/linux/shadow/shadow-
current.tar.gz>.
The following mirror sites have also been established:
o ftp://ftp.icm.edu.pl/pub/Linux/shadow/shadow-current.tar.gz
o ftp://iguana.hut.fi/pub/linux/shadow/shadow-current.tar.gz
o ftp://ftp.cin.net/usr/ggallag/shadow/shadow-current.tar.gz
o ftp://ftp.netural.com/pub/linux/shadow/shadow-current.tar.gz
You should use the currently available version.
You should NOT use a version older than shadow-960129 as they also
have the login security problem discussed above.
When this document refers to the Shadow Suite I am referring to the
this package. It is assumed that this is the package that you are
using.
For reference, I used shadow-960129 to make these installation
instructions.
If you were previously using shadow-mk, you should upgrade to this
version and rebuild everything that you originally compiled.
3.3. What is included with the Shadow Suite.
The Shadow Suite contains replacement programs for:
su, login, passwd, newgrp, chfn, chsh, and id
The package also contains the new programs:
chage, newusers, dpasswd, gpasswd, useradd, userdel, usermod,
groupadd, groupdel, groupmod, groups, pwck, grpck, lastlog, pwconv,
and pwunconv
Additionally, the library: libshadow.a is included for writing and/or
compiling programs that need to access user passwords.
Also, manual pages for the programs are also included.
There is also a configuration file for the login program which will be
installed as /etc/login.defs.
4. Compiling the programs.
4.1. Unpacking the archive.
The first step after retrieving the package is unpacking it. The
package is in the tar (tape archive) format and compressed using gzip,
so first move it to /usr/src, then type:
tar -xzvf shadow-current.tar.gz
This will unpack it into the directory: /usr/src/shadow-YYMMDD
4.2. Configuring with the config.h file
The first thing that you need to do is to copy over the Makefile and
the config.h file:
cd /usr/src/shadow-YYMMDD
cp Makefile.linux Makefile
cp config.h.linux config.h
You should then take a look at the config.h file. This file contains
definitions for some of the configuration options. If you are using
the recommended package, I recommend that you disable group shadow
support for your first time around.
By default shadowed group passwords are enabled. To disable these
edit the config.h file, and change the #define SHADOWGRP to #undef
SHADOWGRP. I recommend that you disable them to start with, and then
if you really want group passwords and group administrators that you
enable it later and recompile. If you leave it enabled, you must
create the file /etc/gshadow.
Enabling the long passwords option is NOT recommended as discussed
above.
Do NOT change the setting: #undef AUTOSHADOW
The AUTOSHADOW option was originally designed so that programs that
were shadow ignorant would still function. This sounds good in
theory, but does not work correctly. If you enable this option, and
the program runs as root, it may call getpwnam() as root, and later
write the modified entry back to the /etc/passwd file (with the no-
longer-shadowed password). Such programs include chfn and chsh. (You
can't get around this by swapping real and effective uid before
calling getpwnam() because root may use chfn and chsh too.)
The same warning is also valid if you are building libc, it has a
SHADOW_COMPAT option which does the same thing. It should NOT be
used! If you start getting encoded passwords back in your /etc/passwd
file, this is the problem.
If you are using a libc version prior to 4.6.27, you will need to make
a couple more changes to config.h and the Makefile. To config.h edit
and change:
#define HAVE_BASENAME
to:
#undef HAVE_BASENAME
And then in the Makefile, change:
SOBJS = smain.o env.o entry.o susetup.o shell.o \
sub.o mail.o motd.o sulog.o age.o tz.o hushed.o
SSRCS = smain.c env.c entry.c setup.c shell.c \
pwent.c sub.c mail.c motd.c sulog.c shadow.c age.c pwpack.c rad64.c \
tz.c hushed.c
SOBJS = smain.o env.o entry.o susetup.o shell.o \
sub.o mail.o motd.o sulog.o age.o tz.o hushed.o basename.o
SSRCS = smain.c env.c entry.c setup.c shell.c \
pwent.c sub.c mail.c motd.c sulog.c shadow.c age.c pwpack.c rad64.c \
tz.c hushed.c basename.c
These changes add the code contained in basename.c which is contained
in libc 4.6.27 and later.
4.3. Making backup copies of your original programs.
It would also be a good idea to track down and make backup copies of
the programs that the shadow suite will replace. On a Slackware 3.0
system these are:
o /bin/su
o /bin/login
o /usr/bin/passwd
o /usr/bin/newgrp
o /usr/bin/chfn
o /usr/bin/chsh
o /usr/bin/id
The BETA package has a save target in the Makefile, but it's commented
out because different distributions place the programs in different
places.
You should also make a backup copy of your /etc/passwd file, but be
careful to name it something else if you place it in the same
directory so you don't overwrite the passwd command.
4.4. Running make
You need to be logged as root to do most of the installation.
Run make to compile the executables in the package:
make all
You may see the warning: rcsid defined but not used. This is fine, it
just happens because the author is using a version control package.
5. Installing
5.1. Have a boot disk handy in case you break anything.
If something goes terribly wrong, it would be handy to have a boot
disk. If you have a boot/root combination from your installation,
that will work, otherwise see the Bootdisk-HOWTO
<http://sunsite.unc.edu/mdw/HOWTO/Bootdisk-HOWTO.html>, which
describes how to make a bootable disk.
5.2. Removing duplicate man pages
You should also move the manual pages that are about to be replaced.
Even if you are brave enough install the Shadow Suite without making
backups, you will still want to remove the old manual pages. The new
manual pages won't normally overwrite the old ones because the old
ones are probably compressed.
You can use a combination of: man -aW command and locate command to
locate the manual pages that need to be (re)moved. It's generally
easier to figure out which are the older pages before you run make
install.
If you are using the Slackware 3.0 distribution, then the manual pages
you want to remove are:
o /usr/man/man1/chfn.1.gz
o /usr/man/man1/chsh.1.gz
o /usr/man/man1/id.1.gz
o /usr/man/man1/login.1.gz
o /usr/man/man1/passwd.1.gz
o /usr/man/man1/su.1.gz
o /usr/man/man5/passwd.5.gz
There may also be man pages of the same name in the /var/man/cat[1-9]
subdirectories that should also be deleted.
5.3. Running make install
You are now ready to type: (do this as root)
make install
This will install the new and replacement programs and fix-up the file
permissions. It will also install the man pages.
This also takes care of installing the Shadow Suite include files in
the correct places in /usr/include/shadow.
Using the BETA package you must manually copy the file login.defs to
the /etc subdirectory and make sure that only root can make changes to
it.
cp login.defs /etc
chmod 700 /etc/login.defs
This file is the configuration file for the login program. You should
review and make changes to this file for your particular system. This
is where you decide which tty's root can login from, and set other
security policy settings (like password expiration defaults).
5.4. Running pwconv
The next step is to run pwconv. This must also be done as root, and
is best done from the /etc subdirectory:
cd /etc
/usr/sbin/pwconv
pwconv takes your /etc/passwd file and strips out the fields to create
two files: /etc/npasswd and /etc/nshadow.
A pwunconv program is also provided if you need to make a normal
/etc/passwd file out of an /etc/passwd and /etc/shadow combination.
5.5. Renaming npasswd and nshadow
Now that you have run pwconv you have created the files /etc/npasswd
and /etc/nshadow. These need to be copied over to /etc/passwd and
/etc/shadow. We also want to make a backup copy of the original
/etc/passwd file, and make sure only root can read it. We'll put the
backup in root's home directory:
cd /etc
cp passwd ~passwd
chmod 600 ~passwd
mv npasswd passwd
mv nshadow shadow
You should also ensure that the file ownerships and permissions are
correct. If you are going to be using X-Windows, the xlock and xdm
programs need to be able to read the shadow file (but not write it).
There are two ways that this can be done. You can set xlock to suid
root (xdm is usually run as root anyway). Or you can make the shadow
file owned by root with a group of shadow, but before you do this,
make sure that you have a shadow group (look in /etc/group). None of
the users on the system should actually be in the shadow group.
chown root.root passwd
chown root.shadow shadow
chmod 0644 passwd
chmod 0640 shadow
Your system now has the password file shadowed. You should now pop
over to another virtual terminal and verify that you can login.
Really, do this now!
If you can't, then something is wrong! To get back to a non-shadowed
state, do the following the following:
cd /etc
cp ~passwd passwd
chmod 644 passwd
You would then restore the files that you saved earlier to their
proper locations.
6. Other programs you may need to upgrade or patch
Even though the shadow suite contains replacement programs for most
programs that need to access passwords, there are a few additional
programs on most systems that require access to passwords.
If you are running a Debian Distribution (or even if you are not), you
can obtain Debian sources for the programs that need to be rebuild
from: ftp://ftp.debian.org/debian/stable/source/
The remainder of this section discusses how to upgrade adduser,
wu_ftpd, ftpd, pop3d, xlock, xdm and sudo so that they support the
shadow suite.
See the section ``Adding Shadow Support to a C program'' for a
discussion on how to put shadow support into any other program that
needs it (although the program must then be run SUID root or SGID
shadow to be able to actually access the shadow file).
6.1. Slackware adduser program
Slackware distributions (and possibly some others) contain a
interactive program for adding users called /sbin/adduser. A shadow
version of this program can be obtained from
ftp://sunsite.unc.edu/pub/Linux/
system/Admin/accounts/adduser.shadow-1.4.tar.gz.
I would encourage you to use the programs that are supplied with the
Shadow Suite (useradd, usermod, and userdel) instead of the slackware
adduser program. They take a little time to learn how to use, but
it's well worth the effort because you have much more control and they
perform proper file locking on the /etc/passwd and /etc/shadow file
(adduser doesn't).
See the section on ``Putting the Shadow Suite to use'' for more
information.
But if you gotta have it, here is what you do:
tar -xzvf adduser.shadow-1.4.tar.gz
cd adduser
make clean
make adduser
chmod 700 adduser
cp adduser /sbin
6.2. The wu_ftpd Server
Most Linux systems some with the wu_ftpd server. If your distribution
does not come with shadow installed, then your wu_ftpd will not be
compiled for shadow. wu_ftpd is launched from inetd/tcpd as a root
process. If you are running an old wu_ftpd daemon, you will want to
upgrade it anyway because older ones had a bug that would allow the
root account to be compromised (For more info see the Linux security
home page <http://bach.cis.temple.edu/linux/linux-security/Linux-
Security-FAQ/Linux-wu.ftpd-2.4-Update.html>).
Fortunately, you only need to get the source code and recompile it
with shadow enabled.
If you are not running an ELF system, The wu_ftp server can be found
on Sunsite as wu-ftp-2.4-fixed.tar.gz
<ftp://sunsite.unc.edu/pub/Linux/system/Network/file-transfer/wu-
ftpd-2.4-fixed.tar.gz>
Once you retrieve the server, put it in /usr/src, then type:
cd /usr/src
tar -xzvf wu-ftpd-2.4-fixed.tar.gz
cd wu-ftpd-2.4-fixed
cp ./src/config/config.lnx.shadow ./src/config/config.lnx
Then edit ./src/makefiles/Makefile.lnx, and change the line:
LIBES = -lbsd -support
to:
LIBES = -lbsd -support -lshadow
Now you are ready to run the build script and install:
cd /usr/src/wu-ftpd-2.4-fixed
/usr/src/wu-ftp-2.4.fixed/build lnx
cp /usr/sbin/wu.ftpd /usr/sbin/wu.ftpd.old
cp ./bin/ftpd /usr/sbin/wu.ftpd
This uses the Linux shadow configuration file, compiles and installs
the server.
On my Slackware 2.3 system I also had to do the following before
running build:
cd /usr/include/netinet
ln -s in_systm.h in_system.h
cd -
Problems have been reported compiling this package under ELF systems,
but the Beta version of the next release works fine. It can be found
as wu-ftp-2.4.2-beta-10.tar.gz
<ftp://tscnet.com/pub/linux/network/ftp/wu-ftpd-2.4.2-beta-10.tar.gz>
Once you retrieve the server, put it in /usr/src, then type:
cd /usr/src
tar -xzvf wu-ftpd-2.4.2-beta-9.tar.gz
cd wu-ftpd-beta-9
cd ./src/config
Then edit config.lnx, and change:
#undef SHADOW.PASSWORD
to:
#define SHADOW.PASSWORD
Then,
cd ../Makefiles
and edit the file Makefile.lnx and change:
LIBES = -lsupport -lbsd # -lshadow
to:
LIBES = -lsupport -lbsd -lshadow
Then build and install:
cd ..
build lnx
cp /usr/sbin/wu.ftpd /usr/sbin/wu.ftpd.old
cp ./bin/ftpd /usr/sbin/wu.ftpd
Note that you should check your /etc/inetd.conf file to make sure that
this is where your wu.ftpd server really lives. It has been reported
that some distributions place the server daemons in different places,
and then wu.ftpd in particular may be named something else.
6.3. Standard ftpd
If you are running the standard ftpd server, I would recommend that
you upgrade to the wu_ftpd server. Aside from the known bug discussed
above, it's generally thought to be more secure.
If you insist on the standard one, or you need NIS support, Sunsite
has ftpd-shadow-nis.tgz
<ftp://sunsite.unc.edu/pub/Linux/system/Network/file-transfer/ftpd-
shadow-nis.tgz>
6.4. pop3d (Post Office Protocol 3)
If you need to support the third Post Office Protocol (POP3), you will
need to recompile a pop3d program. pop3d is normally run by
inetd/tcpd as root.
There are two versions available from Sunsite:
pop3d-1.00.4.linux.shadow.tar.gz
<ftp://sunsite.unc.edu/pub/Linux/system/Mail/pop/pop3d-1.00.4.linux.shadow.tar.gz>
and pop3d+shadow+elf.tar.gz
<ftp://sunsite.unc.edu/pub/Linux/system/Mail/pop/pop3d+shadow+elf.tar.gz>
Both of these are fairly straight forward to install.
6.5. xlock
If you install the shadow suite, and then run X Windows System and
lock the screen without upgrading your xlock, you will have to use
CNTL-ALT-Fx to switch to another tty, login, and kill the xlock
process (or use CNTL-ALT-BS to kill the X server). Fortunately it's
fairly easy to upgrade your xlock program.
If you are running XFree86 Versions 3.x.x, you are probably using
xlockmore (which is a great screen-saver in addition to a lock). This
package supports shadow with a recompile. If you have an older xlock,
I recommend that you upgrade to this one.
xlockmore-3.5.tgz is available at:
<ftp://sunsite.unc.edu/pub/Linux/X11/xutils/screensavers/xlockmore-3.7.tgz>
Basically, this is what you need to do:
Get the xlockmore-3.7.tgz file and put it in /usr/src unpack it:
tar -xzvf xlockmore-3.7.tgz
Edit the file: /usr/X11R6/lib/X11/config/linux.cf, and change the
line:
#define HasShadowPasswd NO
to
#define HasShadowPasswd YES
Then build the executables:
cd /usr/src/xlockmore
xmkmf
make depend
make
Then move everything into place and update file ownerships and
permissions:
cp xlock /usr/X11R6/bin/
cp XLock /var/X11R6/lib/app-defaults/
chown root.shadow /usr/X11R6/bin/xlock
chmod 2755 /usr/X11R6/bin/xlock
chown root.shadow /etc/shadow
chmod 640 /etc/shadow
Your xlock will now work correctly.
6.6. xdm
xdm is a program that presents a login screen for X-Windows. Some
systems start xdm when the system is told to goto a specified run
level (see /etc/inittab.
With the Shadow Suite install, xdm will need to be updated.
Fortunately it's fairly easy to upgrade your xdm program.
xdm.tar.gz is available at:
<ftp://sunsite.unc.edu/pub/Linux/X11/xutils/xdm.tar.gz>
Get the xdm.tar.gz file and put it in /usr/src, then to unpack it:
tar -xzvf xdm.tar.gz
Edit the file: /usr/X11R6/lib/X11/config/linux.cf, and change the
line:
#define HasShadowPasswd NO
to
#define HasShadowPasswd YES
Then build the executables:
cd /usr/src/xdm
xmkmf
make depend
make
Then move everything into place:
cp xdm /usr/X11R6/bin/
xdm is run as root so you don't need to change it file permissions.
6.7. sudo
The program sudo allows a system administrator to let users run
programs that would normally require root access. This is handy
because it lets the administrator limit access to the root account
itself while still allowing users to do things like mounting drives.
sudo needs to read passwords because it verifies the users password
when it's invoked. sudo already runs SUID root, so accessing the
/etc/shadow file is not a problem.
sudo for the shadow suite, is available as at:
<ftp://sunsite.unc.edu/pub/Linux/system/Admin/sudo-1.2-shadow.tgz>
Warning: When you install sudo your /etc/sudoers file will be replaced
with a default one, so you need to make a backup of it if you have
added anything to the default one. (you could also edit the Makefile
and remove the line that copies the default file to /etc).
The package is already setup for shadow, so all that's required is to
recompile the package (put it in /usr/src):
cd /usr/src
tar -xzvf sudo-1.2-shadow.tgz
cd sudo-1.2-shadow
make all
make install
6.8. roman }
imapd is an e-mail server similar to pop3d . imapd comes with the
Pine E-mail package. The documentation that comes with the package
states that the default for Linux systems is to include support for
shadow. However, I have found that this is not true. Furthermore,
the build script / Makefile combination on this package is makes it
very difficult to add the libshadow.a library at compile time, so I
was unable to add shadow support for imapd .
If anyone has this figured out, please E-mail me, and I'll include the
solution here.
6.9. pppd (Point-to-Point Protocol Server)
The pppd server can be setup to use several types of authentication:
Password Authentication Protocol (PAP) and Cryptographic Handshake
Authentication Protocol (CHAP). The pppd server usually reads the
password strings that it uses from /etc/ppp/chap-secrets and/or
/etc/ppp/pap-secrets. If you are using this default behavior of pppd,
it is not necessary to reinstall pppd.
pppd also allows you to use the login parameter (either on the command
line, or in the configuration or options file). If the login option
is given, then pppd will use the /etc/passwd file for the username and
passwords for the PAP. This, of course, will no longer work now that
our password file is shadowed. For pppd-1.2.1d this requires adding
code for shadow support.
The example given in the next section is adding shadow support to
pppd-1.2.1d (an older version of pppd).
pppd-2.2.0 already contains shadow support.
7. Putting the Shadow Suite to use.
This section discusses some of the things that you will want to know
now that you have the Shadow Suite installed on your system. More
information is contained in the manual pages for each command.
7.1. Adding, Modifying, and deleting users
The Shadow Suite added the following command line oriented commands
for adding, modifying, and deleting users. You may also have
installed the adduser program.
7.1.1. useradd
The useradd command can be used to add users to the system. You also
invoke this command to change the default settings.
The first thing that you should do is to examine the default settings
and make changes specific to your system:
useradd -D
______________________________________________________________________
GROUP=1
HOME=/home
INACTIVE=0
EXPIRE=0
SHELL=
SKEL=/etc/skel
______________________________________________________________________
The defaults are probably not what you want, so if you started adding
users now you would have to specify all the information for each user.
However, we can and should change the default values.
On my system:
o I want the default group to be 100
o I want passwords to expire every 60 days
o I don't want to lock an account because the password is expired
o I want to default shell to be /bin/bash
To make these changes I would use:
useradd -D -g100 -e60 -f0 -s/bin/bash
Now running useradd -D will give:
______________________________________________________________________
GROUP=100
HOME=/home
INACTIVE=0
EXPIRE=60
SHELL=/bin/bash
SKEL=/etc/skel
______________________________________________________________________
Just in case you wanted to know, these defaults are stored in the file
/etc/default/useradd.
Now you can use useradd to add users to the system. For example, to
add the user fred, using the defaults, you would use the following:
useradd -m -c "Fred Flintstone" fred
This will create the following entry in the /etc/passwd file:
fred:*:505:100:Fred Flintstone:/home/fred:/bin/bash
And the following entry in the /etc/shadow file:
fred:!:0:0:60:0:0:0:0
fred's home directory will be created and the contents of /etc/skel
will be copied there because of the -m switch.
Also, since we did not specify a UID, the next available one was used.
fred's account is created, but fred still won't be able to login until
we unlock the account. We do this by changing the password.
passwd fred
______________________________________________________________________
Changing password for fred
Enter the new password (minimum of 5 characters)
Please use a combination of upper and lower case letters and numbers.
New Password: *******
Re-enter new password: *******
______________________________________________________________________
Now the /etc/shadow will contain:
fred:J0C.WDR1amIt6:9559:0:60:0:0:0:0
And fred will now be able to login and use the system. The nice thing
about useradd and the other programs that come with the Shadow Suite
is that they make changes to the /etc/passwd and /etc/shadow files
atomically. So if you are adding a user, and another user is changing
their password at the same time, both operations will be performed
correctly.
You should use the supplied commands rather than directly editing
/etc/passwd and /etc/shadow. If you were editing the /etc/shadow
file, and a user were to change his password while you are editing,
and then you were to save the file you were editing, the user's
password change would be lost.
Here is a small interactive script that adds users using useradd and
passwd:
______________________________________________________________________
#!/bin/bash
#
# /sbin/newuser - A script to add users to the system using the Shadow
# Suite's useradd and passwd commands.
#
# Written my Mike Jackson <mhjack@tscnet.com> as an example for the Linux
# Shadow Password Howto. Permission to use and modify is expressly granted.
#
# This could be modified to show the defaults and allow modification similar
# to the Slackware Adduser program. It could also be modified to disallow
# stupid entries. (i.e. better error checking).
#
##
# Defaults for the useradd command
##
GROUP=100 # Default Group
HOME=/home # Home directory location (/home/username)
SKEL=/etc/skel # Skeleton Directory
INACTIVE=0 # Days after password expires to disable account (0=never)
EXPIRE=60 # Days that a passwords lasts
SHELL=/bin/bash # Default Shell (full path)
##
# Defaults for the passwd command
##
PASSMIN=0 # Days between password changes
PASSWARN=14 # Days before password expires that a warning is given
##
# Ensure that root is running the script.
##
WHOAMI=`/usr/bin/whoami`
if [ $WHOAMI != "root" ]; then
echo "You must be root to add news users!"
exit 1
fi
##
# Ask for username and fullname.
##
echo ""
echo -n "Username: "
read USERNAME
echo -n "Full name: "
read FULLNAME
#
echo "Adding user: $USERNAME."
#
# Note that the "" around $FULLNAME is required because this field is
# almost always going to contain at least on space, and without the "'s
# the useradd command would think that you we moving on to the next
# parameter when it reached the SPACE character.
#
/usr/sbin/useradd -c"$FULLNAME" -d$HOME/$USERNAME -e$EXPIRE \
-f$INACTIVE -g$GROUP -m -k$SKEL -s$SHELL $USERNAME
##
# Set password defaults
##
/bin/passwd -n $PASSMIN -w $PASSWARN $USERNAME >/dev/null 2>&1
##
# Let the passwd command actually ask for password (twice)
##
/bin/passwd $USERNAME
##
# Show what was done.
##
echo ""
echo "Entry from /etc/passwd:"
echo -n " "
grep "$USERNAME:" /etc/passwd
echo "Entry from /etc/shadow:"
echo -n " "
grep "$USERNAME:" /etc/shadow
echo "Summary output of the passwd command:"
echo -n " "
passwd -S $USERNAME
echo ""
______________________________________________________________________
Using a script to add new users is really much more preferable than
editing the /etc/passwd or /etc/shadow files directly or using a
program like the Slackware adduser program. Feel free to use and
modify this script for your particular system.
For more information on the useradd see the online manual page.
7.1.2. usermod
The usermod program is used to modify the information on a user. The
switches are similar to the useradd program.
Let's say that you want to change fred's shell, you would do the
following:
usermod -s /bin/tcsh fred
Now fred's /etc/passwd file entry would be change to this:
fred:*:505:100:Fred Flintstone:/home/fred:/bin/tcsh
Let's make fred's account expire on 09/15/97:
usermod -e 09/15/97 fred
Now fred's entry in /etc/shadow becomes:
fred:J0C.WDR1amIt6:9559:0:60:0:0:10119:0
For more information on the usermod command see the online manual
page.
7.1.3. userdel
userdel does just what you would expect, it deletes the user's
account. You simply use:
userdel -r username
The -r causes all files in the user's home directory to be removed
along with the home directory itself. Files located in other file
system will have to be searched for and deleted manually.
If you want to simply lock the account rather than delete it, use the
passwd command instead.
7.2. The passwd command and passwd aging.
The passwd command has the obvious use of changing passwords.
Additionally, it is used by the root user to:
o Lock and unlock accounts (-l and -u)
o Set the maximum number of days that a password remains valid (-x)
o Set the minimum days between password changes (-n)
o Sets the number of days of warning that a password is about to
expire (-w)
o Sets the number of days after the password expires before the
account is locked (-i)
o Allow viewing of account information in a clearer format (-S)
For example, let look again at fred
passwd -S fred
fred P 03/04/96 0 60 0 0
This means that fred's password is valid, it was last changed on
03/04/96, it can be changed at any time, it expires after 60 days,
fred will not be warned, and and the account won't be disabled when
the password expires.
This simply means that if fred logs in after the password expires, he
will be prompted for a new password at login.
If we decide that we want to warn fred 14 days before his password
expires and make his account inactive 14 days after he lets it expire,
we would need to do the following:
passwd -w14 -i14 fred
Now fred is changed to:
fred P 03/04/96 0 60 14 14
For more information on the passwd command see the online manual page.
7.3. The login.defs file.
The file /etc/login is the configuration file for the login program
and also for the Shadow Suite as a whole.
/etc/login contains settings from what the prompts will look like to
what the default expiration will be when a user changes his password.
The /etc/login.defs file is quite well documented just by the comments
that are contained within it. However, there are a few things to
note:
o It contains flags that can be turned on or off that determine the
amount of logging that takes place.
o It contains pointers to other configuration files.
o It contains defaults assignments for things like password aging.
From the above list you can see that this is a rather important file,
and you should make sure that it is present, and that the settings are
what you desire for your system.
7.4. Group passwords.
The /etc/groups file may contain passwords that permit a user to
become a member of a particular group. This function is enabled if
you define the constant SHADOWGRP in the /usr/src/shadow-
YYMMDD/config.h file.
If you define this constant and then compile, you must create an
/etc/gshadow file to hold the group passwords and the group
administrator information.
When you created the /etc/shadow, you used a program called pwconv,
there no equivalent program to create the /etc/gshadow file, but it
really doesn't matter, it takes care of itself.
To create the initial /etc/gshadow file do the following:
touch /etc/gshadow
chown root.root /etc/gshadow
chmod 700 /etc/gshadow
Once you create new groups, they will be added to the /etc/group and
the /etc/gshadow files. If you modify a group by adding or removing
users or changing the group password, the /etc/gshadow file will be
changed.
The programs groups, groupadd, groupmod, and groupdel are provided as
part of the Shadow Suite to modify groups.
The format of the /etc/group file is as follows:
groupname:!:GID:member,member,...
Where:
groupname
The name of the group
! The field that normally holds the password, but that is now
relocated to the /etc/gshadow file.
GID
The numerical group ID number
member
List of group members
The format of the /etc/gshadow file is as follows:
groupname:password:admin,admin,...:member,member,...
Where:
groupname
The name of the group
password
The encoded group password.
admin
List of group administrators
member
List of group members
The command gpasswd is used only for adding or removing administrators
and members to or from a group. root or someone in the list of
administrators may add or remove group members.
The groups password can be changed using the passwd command by root or
anyone listed as an administrator for the group.
Despite the fact that there is not currently a manual page for
gpasswd, typing gpasswd without any parameters gives a listing of
options. It's fairly easy to grasp how it all works once you
understand the file formats and the concepts.
7.5. Consistency checking programs
7.5.1. pwck
The program pwck is provided to provide a consistency check on the
/etc/passwd and /etc/shadow files. It will check each username and
verify that it has the following:
o the correct number of fields
o unique user name
o valid user and group identifier
o valid primary group
o valid home directory
o valid login shell
It will also warn of any account that has no password.
It's a good idea to run pwck after installing the Shadow Suite. It's
also a good idea to run it periodically, perhaps weekly or monthly.
If you use the -r option, you can use cron to run it on a regular
basis and have the report mailed to you.
7.5.2. grpck
grpck is the consistency checking program for the /etc/group and
/etc/gshadow files. It performs the following checks:
o the correct number of fields
o unique group name
o valid list of members and administrators
It also has the -r option for automated reports.
7.6. Dial-up passwords.
Dial-up passwords are another optional line of defense for systems
that allow dial-in access. If you have a system that allows many
people to connect locally or via a network, but you want to limit who
can dial in and connect, then dial-up passwords are for you. To
enable dial-up passwords, you must edit the file /etc/login.defs and
ensure that DIALUPS_CHECK_ENAB is set to yes.
Two files contain the dial-up information, /etc/dialups which contains
the ttys (one per line, with the leading "/dev/" removed). If a tty
is listed then dial-up checks are performed.
The second file is the /etc/d_passwd file. This file contains the
fully qualified path name of a shell, followed by an optional
password.
If a user logs into a line that is listed in /etc/dialups, and his
shell is listed in the file /etc/d_passwd he will be allowed access
only by suppling the correct password.
Another useful purpose for using dial-up passwords might be to setup a
line that only allows a certain type of connect (perhaps a PPP or UUCP
connection). If a user tries to get another type of connection (i.e.
a list of shells), he must know a password to use the line.
Before you can use the dial-up feature, you must create the files.
The command dpasswd is provided to assign passwords to the shells in
the /etc/d_passwd file. See the manual page for more information.
8. Adding shadow support to a C program
Adding shadow support to a program is actually fairly straightforward.
The only problem is that the program must be run by root (or SUID
root) in order for the the program to be able to access the
/etc/shadow file.
This presents one big problem: very careful programming practices must
be followed when creating SUID programs. For instance, if a program
has a shell escape, this must not occur as root if the program is SUID
root.
For adding shadow support to a program so that it can check passwords,
but otherwise does need to run as root, it's a lot safer to run the
program SUID shadow instead. The xlock program is an example of this.
In the example given below, pppd-1.2.1d already runs SUID as root, so
adding shadow support should not make the program any more vulnerable.
8.1. Header files
The header files should reside in /usr/include/shadow. There should
also be a /usr/include/shadow.h, but it will be a symbolic link to
/usr/include/shadow/shadow.h.
To add shadow support to a program, you need to include the header
files:
#include <shadow/shadow.h>
#include <shadow/pwauth.h>
It might be a good idea to use compiler directives to conditionally
compile the shadow code (I do in the example below).
8.2. libshadow.a library
When you installed the Shadow Suite the libshadow.a file was created
and installed in /usr/lib.
When compiling shadow support into a program, the linker needs to be
told to include the libshadow.a library into the link.
This is done by:
gcc program.c -o program -lshadow
However, as we will see in the example below, most large programs use
a Makefile, and usually have a variable called LIBS=... that we will
modify.
8.3. Shadow Structure
The libshadow.a library uses a structure called spwd for the
information it retrieves from the /etc/shadow file. This is the
definition of the spwd structure from the /usr/include/shadow/shadow.h
header file:
______________________________________________________________________
struct spwd
{
char *sp_namp; /* login name */
char *sp_pwdp; /* encrypted password */
sptime sp_lstchg; /* date of last change */
sptime sp_min; /* minimum number of days between changes */
sptime sp_max; /* maximum number of days between changes */
sptime sp_warn; /* number of days of warning before password
expires */
sptime sp_inact; /* number of days after password expires
until the account becomes unusable. */
sptime sp_expire; /* days since 1/1/70 until account expires
*/
unsigned long sp_flag; /* reserved for future use */
};
______________________________________________________________________
The Shadow Suite can put things into the sp_pwdp field besides just
the encoded passwd. The password field could contain:
username:Npge08pfz4wuk;@/sbin/extra:9479:0:10000::::
This means that in addition to the password, the program /sbin/extra
should be called for further authentication. The program called will
get passed the username and a switch that indicates why it's being
called. See the file /usr/include/shadow/pwauth.h and the source code
for pwauth.c for more information.
What this means is that we should use the function pwauth to perform
the actual authentication, as it will take care of the secondary
authentication as well. The example below does this.
The author of the Shadow Suite indicates that since most programs in
existence don't do this, and that it may be removed or changed in
future versions of the Shadow Suite.
8.4. Shadow Functions
The shadow.h file also contains the function prototypes for the
functions contained in the libshadow.a library:
______________________________________________________________________
extern void setspent __P ((void));
extern void endspent __P ((void));
extern struct spwd *sgetspent __P ((__const char *__string));
extern struct spwd *fgetspent __P ((FILE *__fp));
extern struct spwd *getspent __P ((void));
extern struct spwd *getspnam __P ((__const char *__name));
extern int putspent __P ((__const struct spwd *__sp, FILE *__fp));
______________________________________________________________________
The function that we are going to use in the example is: getspnam
which will retrieve for us a spwd structure for the supplied name.
8.5. Example
This is an example of adding shadow support to a program that needs
it, but does not have it by default.
This example uses the Point-to-Point Protocol Server (pppd-1.2.1d),
which has a mode in which it performs PAP authentication using user
names and passwords from the /etc/passwd file instead of the PAP or
CHAP files. You would not need to add this code to pppd-2.2.0 because
it's already there.
This feature of pppd probably isn't used very much, but if you
installed the Shadow Suite, it won't work anymore because the
passwords are no longer stored in /etc/passwd.
The code for authenticating users under pppd-1.2.1d is located in the
/usr/src/pppd-1.2.1d/pppd/auth.c file.
The following code needs to be added to the top of the file where all
the other #include directives are. We have surrounded the #includes
with conditional directives (i.e. only include if we are compiling for
shadow support).
______________________________________________________________________
#ifdef HAS_SHADOW
#include <shadow.h>
#include <shadow/pwauth.h>
#endif
______________________________________________________________________
The next thing to do is to modify the actual code. We are still
making changes to the auth.c file.
Function auth.c before modifications:
______________________________________________________________________
/*
* login - Check the user name and password against the system
* password database, and login the user if OK.
*
* returns:
* UPAP_AUTHNAK: Login failed.
* UPAP_AUTHACK: Login succeeded.
* In either case, msg points to an appropriate message.
*/
static int
login(user, passwd, msg, msglen)
char *user;
char *passwd;
char **msg;
int *msglen;
{
struct passwd *pw;
char *epasswd;
char *tty;
if ((pw = getpwnam(user)) == NULL) {
return (UPAP_AUTHNAK);
}
/*
* XXX If no passwd, let them login without one.
*/
if (pw->pw_passwd == '\0') {
return (UPAP_AUTHACK);
}
epasswd = crypt(passwd, pw->pw_passwd);
if (strcmp(epasswd, pw->pw_passwd)) {
return (UPAP_AUTHNAK);
}
syslog(LOG_INFO, "user %s logged in", user);
/*
* Write a wtmp entry for this user.
*/
tty = strrchr(devname, '/');
if (tty == NULL)
tty = devname;
else
tty++;
logwtmp(tty, user, ""); /* Add wtmp login entry */
logged_in = TRUE;
return (UPAP_AUTHACK);
}
______________________________________________________________________
The user's password is placed into pw->pw_passwd, so all we really
need to do is add the function getspnam. This will put the password
into spwd->sp_pwdp.
We will add the function pwauth to perform the actual authentication.
This will automatically perform secondary authentication if the shadow
file is setup for it.
Function auth.c after modifications to support shadow:
______________________________________________________________________
/*
* login - Check the user name and password against the system
* password database, and login the user if OK.
*
* This function has been modified to support the Linux Shadow Password
* Suite if USE_SHADOW is defined.
*
* returns:
* UPAP_AUTHNAK: Login failed.
* UPAP_AUTHACK: Login succeeded.
* In either case, msg points to an appropriate message.
*/
static int
login(user, passwd, msg, msglen)
char *user;
char *passwd;
char **msg;
int *msglen;
{
struct passwd *pw;
char *epasswd;
char *tty;
#ifdef USE_SHADOW
struct spwd *spwd;
struct spwd *getspnam();
#endif
if ((pw = getpwnam(user)) == NULL) {
return (UPAP_AUTHNAK);
}
#ifdef USE_SHADOW
spwd = getspnam(user);
if (spwd)
pw->pw_passwd = spwd->sp-pwdp;
#endif
/*
* XXX If no passwd, let NOT them login without one.
*/
if (pw->pw_passwd == '\0') {
return (UPAP_AUTHNAK);
}
#ifdef HAS_SHADOW
if ((pw->pw_passwd && pw->pw_passwd[0] == '@'
&& pw_auth (pw->pw_passwd+1, pw->pw_name, PW_LOGIN, NULL))
|| !valid (passwd, pw)) {
return (UPAP_AUTHNAK);
}
#else
epasswd = crypt(passwd, pw->pw_passwd);
if (strcmp(epasswd, pw->pw_passwd)) {
return (UPAP_AUTHNAK);
}
#endif
syslog(LOG_INFO, "user %s logged in", user);
/*
* Write a wtmp entry for this user.
*/
tty = strrchr(devname, '/');
if (tty == NULL)
tty = devname;
else
tty++;
logwtmp(tty, user, ""); /* Add wtmp login entry */
logged_in = TRUE;
return (UPAP_AUTHACK);
}
______________________________________________________________________
Careful examination will reveal that we made another change as well.
The original version allowed access (returned UPAP_AUTHACK if there
was NO password in the /etc/passwd file. This is not good, because a
common use of this login feature is to use one account to allow access
to the PPP process and then check the username and password supplied
by PAP with the username in the /etc/passwd file and the password in
the /etc/shadow file.
So if we had set the original version up to run as the shell for a
user i.e. ppp, then anyone could get a ppp connection by setting
their PAP to user ppp and a password of null.
We fixed this also by returning UPAP_AUTHNAK instead of UPAP_AUTHACK
if the password field was empty.
Interestingly enough, pppd-2.2.0 has the same problem.
Next we need to modify the Makefile so that two things occur:
USE_SHADOW must be defined, and libshadow.a needs to be added to the
linking process.
Edit the Makefile, and add:
LIBS = -lshadow
Then we find the line:
COMPILE_FLAGS = -I.. -D_linux_=1 -DGIDSET_TYPE=gid_t
And change it to:
COMPILE_FLAGS = -I.. -D_linux_=1 -DGIDSET_TYPE=gid_t -DUSE_SHADOW
Now make and install.
9. Frequently Asked Questions.
Q: I used to control which tty's root could log into using the file
/etc/securettys, but it doesn't seem to work anymore, what's going on?
A: The file /etc/securettys does absolutely nothing now that the
Shadow Suite is installed. The tty's that root can use are now
located in the login configuration file /etc/login.defs. The entry in
this file may point to another file.
Q: I installed the Shadow Suite, but now I can't login, what did I
miss?
A: You probably installed the Shadow programs, but didn't run pwconv
or you forgot to copy /etc/npasswd to /etc/passwd and /etc/nshadow to
/etc/shadow. Also, you may need to copy login.defs to /etc.
Q: In the section on xlock, it said to change the group ownership of
the /etc/shadow file to shadow. I don't have a shadow group, what do
I do?
A: You can add one. Simply edit the /etc/group file, and insert a
line for the shadow group. You need to ensure that the group number
is not used by another group, and you need to insert it before the
nogroup entry. Or you can simply suid xlock to root.
Q: Is there a mailing list for the Linux Shadow Password Suite?
A: Yes, but it's for the development and beta testing of the next
Shadow Suite for Linux. You can get added to the list by mailing to:
shadow-list-request@neptune.cin.net with a subject of: subscribe. The
list is actually for discussions of the Linux shadow-YYMMSS series of
releases. You should join if you want to get involved in further
development or if you install the Suite on your system and want to get
information on newer releases.
Q: I installed the Shadow Suite, but when I use the userdel command, I
get "userdel: cannot open shadow group file", what did I do wrong?
A: You compiled the Shadow Suite with the SHADOWGRP option enabled,
but you don't have an /etc/gshadow file. You need to either edit the
config.h file and recompile, or create an /etc/group file. See the
section on shadow groups.
Q: I installed the Shadow Suite but now I'm getting encoded passwords
back in my /etc/passwd file, what's wrong?
A: You either enabled the AUTOSHADOW option in the Shadow config.h
file, or your libc was compiled with the SAHDOW_COMPAT option. You
need to determine which is the problem, and recompile.
10. Copyright Message.
The Linux Shadow Password HOWTO is Copyright (c) 1996 Michael H.
Jackson.
Permission is granted to make and distribute verbatim copies of this
document provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of this
document under the conditions for verbatim copies above, provided a
notice clearly stating that the document is a modified version is also
included in the modified document.
Permission is granted to copy and distribute translations of this
document into another language, under the conditions specified above
for modified versions.
Permission is granted to convert this document into another media
under the conditions specified above for modified versions provided
the requirement to acknowledge the source document is fulfilled by
inclusion of an obvious reference to the source document in the new
media. Where there is any doubt as to what defines 'obvious' the
copyright owner reserves the right to decide.
11. Miscellaneous and Acknowledgments.
The code examples for auth.c are taken from pppd-1.2.1d and
ppp-2.1.0e, Copyright (c) 1993 and The Australian National University
and Copyright (c) 1989 Carnegie Mellon University.
Thanks to Marek Michalkiewicz <marekm@i17linuxb.ists.pwr.wroc.pl> for
writing and maintaining the Shadow Suite for Linux, and for his review
and comments on this document.
Thanks to Ron Tidd <rtidd@tscnet.com> for his helpful review and
testing.
Thanks to everyone who has sent me feedback to help improve this
document.
Please, if you have any comments or suggestions then mail them to me.
regards
Michael H. Jackson <mhjack@tscnet.com>
} Miscellaneous and Acknowledgments.
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