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+[ Note: the installation instructions in this document are somewhat
+  out of date - the package now uses GNU autoconf and is configured
+  just like most GNU packages: run ./configure then make.  --marekm ]
+
+  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
+  reinstalling 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.
+
+  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:
+
+  ·  A configuration file to set login defaults (/etc/login.defs)
+
+  ·  Utilities for adding, modifying, and deleting user accounts and
+     groups
+
+  ·  Password aging and expiration
+
+  ·  Account expiration and locking
+
+  ·  Shadowed group passwords (optional)
+
+  ·  Double length passwords (16 character passwords) NOT RECOMMENDED
+
+  ·  Better control over user's password selection
+
+  ·  Dial-up passwords
+
+  ·  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:
+
+  ·  The machine does not contain user accounts.
+
+  ·  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)
+
+  ·  Your machine is being used by terminal servers to verify users via
+     NFS (Network File System), NIS, or some other method.
+
+  ·  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 bits 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 Julianne F. Haugh
+
+  There are several versions that have been used on Linux systems:
+
+  ·  shadow-3.3.1 is the original.
+
+  ·  shadow-3.3.1-2 is Linux specific patch made by Florian La Roche
+     <flla@stud.uni-sb.de> and contains some further enhancements.
+
+  ·  shadow-mk was specifically packaged for Linux.
+
+  The shadow-mk package contains the shadow-3.3.1 package distributed by
+  Julianne F. Haugh 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:
+
+  ·  ftp://ftp.icm.edu.pl/pub/Linux/shadow/shadow-current.tar.gz
+
+  ·  ftp://iguana.hut.fi/pub/linux/shadow/shadow-current.tar.gz
+
+  ·  ftp://ftp.cin.net/usr/ggallag/shadow/shadow-current.tar.gz
+
+  ·  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:
+
+  ·  /bin/su
+
+  ·  /bin/login
+
+  ·  /usr/bin/passwd
+
+  ·  /usr/bin/newgrp
+
+  ·  /usr/bin/chfn
+
+  ·  /usr/bin/chsh
+
+  ·  /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:
+
+  ·  /usr/man/man1/chfn.1.gz
+
+  ·  /usr/man/man1/chsh.1.gz
+
+  ·  /usr/man/man1/id.1.gz
+
+  ·  /usr/man/man1/login.1.gz
+
+  ·  /usr/man/man1/passwd.1.gz
+
+  ·  /usr/man/man1/su.1.gz
+
+  ·  /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.  imapd (E-Mail pine package)
+
+  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:
+
+  ·  I want the default group to be 100
+
+  ·  I want passwords to expire every 60 days
+
+  ·  I don't want to lock an account because the password is expired
+
+  ·  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:
+
+  ·  Lock and unlock accounts (-l and -u)
+
+  ·  Set the maximum number of days that a password remains valid (-x)
+
+  ·  Set the minimum days between password changes (-n)
+
+  ·  Sets the number of days of warning that a password is about to
+     expire (-w)
+
+  ·  Sets the number of days after the password expires before the
+     account is locked (-i)
+
+  ·  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 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:
+
+  ·  It contains flags that can be turned on or off that determine the
+     amount of logging that takes place.
+
+  ·  It contains pointers to other configuration files.
+
+  ·  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:
+
+  ·  the correct number of fields
+
+  ·  unique user name
+
+  ·  valid user and group identifier
+
+  ·  valid primary group
+
+  ·  valid home directory
+
+  ·  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:
+
+  ·  the correct number of fields
+
+  ·  unique group name
+
+  ·  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 supplying 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>
+