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The Linux-PAM Application Developers' Guide
Andrew G. Morgan
<morgan@kernel.org>
Thorsten Kukuk
<kukuk@thkukuk.de>
Version 1.1.2, 31. August 2010
Abstract
This manual documents what an application developer needs to know about
the Linux-PAM library. It describes how an application might use the
Linux-PAM library to authenticate users. In addition it contains a
description of the functions to be found in libpam_misc library, that can
be used in general applications. Finally, it contains some comments on PAM
related security issues for the application developer.
--------------------------------------------------------------------------
1. Introduction
1.1. Description
1.2. Synopsis
2. Overview
3. The public interface to Linux-PAM
3.1. What can be expected by the application
3.1.1. Initialization of PAM transaction
3.1.2. Termination of PAM transaction
3.1.3. Setting PAM items
3.1.4. Getting PAM items
3.1.5. Strings describing PAM error codes
3.1.6. Request a delay on failure
3.1.7. Authenticating the user
3.1.8. Setting user credentials
3.1.9. Account validation management
3.1.10. Updating authentication tokens
3.1.11. Start PAM session management
3.1.12. terminating PAM session management
3.1.13. Set or change PAM environment variable
3.1.14. Get a PAM environment variable
3.1.15. Getting the PAM environment
3.2. What is expected of an application
3.2.1. The conversation function
3.3. Programming notes
4. Security issues of Linux-PAM
4.1. Care about standard library calls
4.2. Choice of a service name
4.3. The conversation function
4.4. The identity of the user
4.5. Sufficient resources
5. A library of miscellaneous helper functions
5.1. Functions supplied
5.1.1. Text based conversation function
5.1.2. Transcribing an environment to that of
PAM
5.1.3. Liberating a locally saved environment
5.1.4. BSD like PAM environment variable setting
6. Porting legacy applications
7. Glossary of PAM related terms
8. An example application
9. Files
10. See also
11. Author/acknowledgments
12. Copyright information for this document
Chapter 1. Introduction
1.1. Description
Linux-PAM (Pluggable Authentication Modules for Linux) is a library that
enables the local system administrator to choose how individual
applications authenticate users. For an overview of the Linux-PAM library
see the Linux-PAM System Administrators' Guide.
It is the purpose of the Linux-PAM project to liberate the development of
privilege granting software from the development of secure and appropriate
authentication schemes. This is accomplished by providing a documented
library of functions that an application may use for all forms of user
authentication management. This library dynamically loads locally
configured authentication modules that actually perform the authentication
tasks.
From the perspective of an application developer the information contained
in the local configuration of the PAM library should not be important.
Indeed it is intended that an application treat the functions documented
here as a 'black box' that will deal with all aspects of user
authentication. 'All aspects' includes user verification, account
management, session initialization/termination and also the resetting of
passwords (authentication tokens).
1.2. Synopsis
For general applications that wish to use the services provided by
Linux-PAM the following is a summary of the relevant linking information:
#include <security/pam_appl.h>
cc -o application .... -lpam
In addition to libpam, there is a library of miscellaneous functions that
make the job of writing PAM-aware applications easier (this library is not
covered in the DCE-RFC for PAM and is specific to the Linux-PAM
distribution):
#include <security/pam_appl.h>
#include <security/pam_misc.h>
cc -o application .... -lpam -lpam_misc
Chapter 2. Overview
Most service-giving applications are restricted. In other words, their
service is not available to all and every prospective client. Instead, the
applying client must jump through a number of hoops to convince the
serving application that they are authorized to obtain service.
The process of authenticating a client is what PAM is designed to manage.
In addition to authentication, PAM provides account management, credential
management, session management and authentication-token (password
changing) management services. It is important to realize when writing a
PAM based application that these services are provided in a manner that is
transparent to the application. That is to say, when the application is
written, no assumptions can be made about how the client will be
authenticated.
The process of authentication is performed by the PAM library via a call
to pam_authenticate(). The return value of this function will indicate
whether a named client (the user) has been authenticated. If the PAM
library needs to prompt the user for any information, such as their name
or a password then it will do so. If the PAM library is configured to
authenticate the user using some silent protocol, it will do this too.
(This latter case might be via some hardware interface for example.)
It is important to note that the application must leave all decisions
about when to prompt the user at the discretion of the PAM library.
The PAM library, however, must work equally well for different styles of
application. Some applications, like the familiar login and passwd are
terminal based applications, exchanges of information with the client in
these cases is as plain text messages. Graphically based applications,
however, have a more sophisticated interface. They generally interact with
the user via specially constructed dialogue boxes. Additionally, network
based services require that text messages exchanged with the client are
specially formatted for automated processing: one such example is ftpd
which prefixes each exchanged message with a numeric identifier.
The presentation of simple requests to a client is thus something very
dependent on the protocol that the serving application will use. In spite
of the fact that PAM demands that it drives the whole authentication
process, it is not possible to leave such protocol subtleties up to the
PAM library. To overcome this potential problem, the application provides
the PAM library with a conversation function. This function is called from
within the PAM library and enables the PAM to directly interact with the
client. The sorts of things that this conversation function must be able
to do are prompt the user with text and/or obtain textual input from the
user for processing by the PAM library. The details of this function are
provided in a later section.
For example, the conversation function may be called by the PAM library
with a request to prompt the user for a password. Its job is to reformat
the prompt request into a form that the client will understand. In the
case of ftpd, this might involve prefixing the string with the number 331
and sending the request over the network to a connected client. The
conversation function will then obtain any reply and, after extracting the
typed password, will return this string of text to the PAM library.
Similar concerns need to be addressed in the case of an X-based graphical
server.
There are a number of issues that need to be addressed when one is porting
an existing application to become PAM compliant. A section below has been
devoted to this: Porting legacy applications.
Besides authentication, PAM provides other forms of management. Session
management is provided with calls to pam_open_session() and
pam_close_session(). What these functions actually do is up to the local
administrator. But typically, they could be used to log entry and exit
from the system or for mounting and unmounting the user's home directory.
If an application provides continuous service for a period of time, it
should probably call these functions, first open after the user is
authenticated and then close when the service is terminated.
Account management is another area that an application developer should
include with a call to pam_acct_mgmt(). This call will perform checks on
the good health of the user's account (has it expired etc.). One of the
things this function may check is whether the user's authentication token
has expired - in such a case the application may choose to attempt to
update it with a call to pam_chauthtok(), although some applications are
not suited to this task (ftp for example) and in this case the application
should deny access to the user.
PAM is also capable of setting and deleting the users credentials with the
call pam_setcred(). This function should always be called after the user
is authenticated and before service is offered to the user. By convention,
this should be the last call to the PAM library before the PAM session is
opened. What exactly a credential is, is not well defined. However, some
examples are given in the glossary below.
Chapter 3. The public interface to Linux-PAM
Firstly, the relevant include file for the Linux-PAM library is
<security/pam_appl.h>. It contains the definitions for a number of
functions. After listing these functions, we collect some guiding remarks
for programmers.
3.1. What can be expected by the application
3.1.1. Initialization of PAM transaction
#include <security/pam_appl.h>
int pam_start( service_name,
user,
pam_conversation,
pamh);
const char *service_name;
const char *user;
const struct pam_conv *pam_conversation;
pam_handle_t **pamh;
3.1.1.1. DESCRIPTION
The pam_start function creates the PAM context and initiates the PAM
transaction. It is the first of the PAM functions that needs to be called
by an application. The transaction state is contained entirely within the
structure identified by this handle, so it is possible to have multiple
transactions in parallel. But it is not possible to use the same handle
for different transactions, a new one is needed for every new context.
The service_name argument specifies the name of the service to apply and
will be stored as PAM_SERVICE item in the new context. The policy for the
service will be read from the file /etc/pam.d/service_name or, if that
file does not exist, from /etc/pam.conf.
The user argument can specify the name of the target user and will be
stored as PAM_USER item. If the argument is NULL, the module has to ask
for this item if necessary.
The pam_conversation argument points to a struct pam_conv describing the
conversation function to use. An application must provide this for direct
communication between a loaded module and the application.
Following a successful return (PAM_SUCCESS) the contents of pamh is a
handle that contains the PAM context for successive calls to the PAM
functions. In an error case is the content of pamh undefined.
The pam_handle_t is a blind structure and the application should not
attempt to probe it directly for information. Instead the PAM library
provides the functions pam_set_item(3) and pam_get_item(3). The PAM handle
cannot be used for mulitiple authentications at the same time as long as
pam_end was not called on it before.
3.1.1.2. RETURN VALUES
PAM_ABORT
General failure.
PAM_BUF_ERR
Memory buffer error.
PAM_SUCCESS
Transaction was successful created.
PAM_SYSTEM_ERR
System error, for example a NULL pointer was submitted instead of
a pointer to data.
3.1.2. Termination of PAM transaction
#include <security/pam_appl.h>
int pam_end( pamh,
pam_status);
pam_handle_t *pamh;
int pam_status;
3.1.2.1. DESCRIPTION
The pam_end function terminates the PAM transaction and is the last
function an application should call in the PAM context. Upon return the
handle pamh is no longer valid and all memory associated with it will be
invalid.
The pam_status argument should be set to the value returned to the
application by the last PAM library call.
The value taken by pam_status is used as an argument to the module
specific callback function, cleanup() (See pam_set_data(3) and
pam_get_data(3)). In this way the module can be given notification of the
pass/fail nature of the tear-down process, and perform any last minute
tasks that are appropriate to the module before it is unlinked. This
argument can be logically OR'd with PAM_DATA_SILENT to indicate to
indicate that the module should not treat the call too seriously. It is
generally used to indicate that the current closing of the library is in a
fork(2)ed process, and that the parent will take care of cleaning up
things that exist outside of the current process space (files etc.).
This function free's all memory for items associated with the
pam_set_item(3) and pam_get_item(3) functions. Pointers associated with
such objects are not valid anymore after pam_end was called.
3.1.2.2. RETURN VALUES
PAM_SUCCESS
Transaction was successful terminated.
PAM_SYSTEM_ERR
System error, for example a NULL pointer was submitted as PAM
handle or the function was called by a module.
3.1.3. Setting PAM items
#include <security/pam_modules.h>
int pam_set_item( pamh,
item_type,
item);
pam_handle_t *pamh;
int item_type;
const void *item;
3.1.3.1. DESCRIPTION
The pam_set_item function allows applications and PAM service modules to
access and to update PAM informations of item_type. For this a copy of the
object pointed to by the item argument is created. The following
item_types are supported:
PAM_SERVICE
The service name (which identifies that PAM stack that the PAM
functions will use to authenticate the program).
PAM_USER
The username of the entity under whose identity service will be
given. That is, following authentication, PAM_USER identifies the
local entity that gets to use the service. Note, this value can be
mapped from something (eg., "anonymous") to something else (eg.
"guest119") by any module in the PAM stack. As such an application
should consult the value of PAM_USER after each call to a PAM
function.
PAM_USER_PROMPT
The string used when prompting for a user's name. The default
value for this string is a localized version of "login: ".
PAM_TTY
The terminal name: prefixed by /dev/ if it is a device file; for
graphical, X-based, applications the value for this item should be
the $DISPLAY variable.
PAM_RUSER
The requesting user name: local name for a locally requesting user
or a remote user name for a remote requesting user.
Generally an application or module will attempt to supply the
value that is most strongly authenticated (a local account before
a remote one. The level of trust in this value is embodied in the
actual authentication stack associated with the application, so it
is ultimately at the discretion of the system administrator.
PAM_RUSER@PAM_RHOST should always identify the requesting user. In
some cases, PAM_RUSER may be NULL. In such situations, it is
unclear who the requesting entity is.
PAM_RHOST
The requesting hostname (the hostname of the machine from which
the PAM_RUSER entity is requesting service). That is
PAM_RUSER@PAM_RHOST does identify the requesting user. In some
applications, PAM_RHOST may be NULL. In such situations, it is
unclear where the authentication request is originating from.
PAM_AUTHTOK
The authentication token (often a password). This token should be
ignored by all module functions besides pam_sm_authenticate(3) and
pam_sm_chauthtok(3). In the former function it is used to pass the
most recent authentication token from one stacked module to
another. In the latter function the token is used for another
purpose. It contains the currently active authentication token.
PAM_OLDAUTHTOK
The old authentication token. This token should be ignored by all
module functions except pam_sm_chauthtok(3).
PAM_CONV
The pam_conv structure. See pam_conv(3).
The following additional items are specific to Linux-PAM and should not be
used in portable applications:
PAM_FAIL_DELAY
A function pointer to redirect centrally managed failure delays.
See pam_fail_delay(3).
PAM_XDISPLAY
The name of the X display. For graphical, X-based applications the
value for this item should be the $DISPLAY variable. This value
may be used independently of PAM_TTY for passing the name of the
display.
PAM_XAUTHDATA
A pointer to a structure containing the X authentication data
required to make a connection to the display specified by
PAM_XDISPLAY, if such information is necessary. See
pam_xauth_data(3).
PAM_AUTHTOK_TYPE
The default action is for the module to use the following prompts
when requesting passwords: "New UNIX password: " and "Retype UNIX
password: ". The example word UNIX can be replaced with this item,
by default it is empty. This item is used by pam_get_authtok(3).
For all item_types, other than PAM_CONV and PAM_FAIL_DELAY, item is a
pointer to a <NUL> terminated character string. In the case of PAM_CONV,
item points to an initialized pam_conv structure. In the case of
PAM_FAIL_DELAY, item is a function pointer: void (*delay_fn)(int retval,
unsigned usec_delay, void *appdata_ptr)
Both, PAM_AUTHTOK and PAM_OLDAUTHTOK, will be reseted before returning to
the application. Which means an application is not able to access the
authentication tokens.
3.1.3.2. RETURN VALUES
PAM_BAD_ITEM
The application attempted to set an undefined or inaccessible
item.
PAM_BUF_ERR
Memory buffer error.
PAM_SUCCESS
Data was successful updated.
PAM_SYSTEM_ERR
The pam_handle_t passed as first argument was invalid.
3.1.4. Getting PAM items
#include <security/pam_modules.h>
int pam_get_item( pamh,
item_type,
item);
const pam_handle_t *pamh;
int item_type;
const void **item;
3.1.4.1. DESCRIPTION
The pam_get_item function allows applications and PAM service modules to
access and retrieve PAM informations of item_type. Upon successful return,
item contains a pointer to the value of the corresponding item. Note, this
is a pointer to the actual data and should not be free()'ed or
over-written! The following values are supported for item_type:
PAM_SERVICE
The service name (which identifies that PAM stack that the PAM
functions will use to authenticate the program).
PAM_USER
The username of the entity under whose identity service will be
given. That is, following authentication, PAM_USER identifies the
local entity that gets to use the service. Note, this value can be
mapped from something (eg., "anonymous") to something else (eg.
"guest119") by any module in the PAM stack. As such an application
should consult the value of PAM_USER after each call to a PAM
function.
PAM_USER_PROMPT
The string used when prompting for a user's name. The default
value for this string is a localized version of "login: ".
PAM_TTY
The terminal name: prefixed by /dev/ if it is a device file; for
graphical, X-based, applications the value for this item should be
the $DISPLAY variable.
PAM_RUSER
The requesting user name: local name for a locally requesting user
or a remote user name for a remote requesting user.
Generally an application or module will attempt to supply the
value that is most strongly authenticated (a local account before
a remote one. The level of trust in this value is embodied in the
actual authentication stack associated with the application, so it
is ultimately at the discretion of the system administrator.
PAM_RUSER@PAM_RHOST should always identify the requesting user. In
some cases, PAM_RUSER may be NULL. In such situations, it is
unclear who the requesting entity is.
PAM_RHOST
The requesting hostname (the hostname of the machine from which
the PAM_RUSER entity is requesting service). That is
PAM_RUSER@PAM_RHOST does identify the requesting user. In some
applications, PAM_RHOST may be NULL. In such situations, it is
unclear where the authentication request is originating from.
PAM_AUTHTOK
The authentication token (often a password). This token should be
ignored by all module functions besides pam_sm_authenticate(3) and
pam_sm_chauthtok(3). In the former function it is used to pass the
most recent authentication token from one stacked module to
another. In the latter function the token is used for another
purpose. It contains the currently active authentication token.
PAM_OLDAUTHTOK
The old authentication token. This token should be ignored by all
module functions except pam_sm_chauthtok(3).
PAM_CONV
The pam_conv structure. See pam_conv(3).
The following additional items are specific to Linux-PAM and should not be
used in portable applications:
PAM_FAIL_DELAY
A function pointer to redirect centrally managed failure delays.
See pam_fail_delay(3).
PAM_XDISPLAY
The name of the X display. For graphical, X-based applications the
value for this item should be the $DISPLAY variable. This value
may be used independently of PAM_TTY for passing the name of the
display.
PAM_XAUTHDATA
A pointer to a structure containing the X authentication data
required to make a connection to the display specified by
PAM_XDISPLAY, if such information is necessary. See
pam_xauth_data(3).
PAM_AUTHTOK_TYPE
The default action is for the module to use the following prompts
when requesting passwords: "New UNIX password: " and "Retype UNIX
password: ". The example word UNIX can be replaced with this item,
by default it is empty. This item is used by pam_get_authtok(3).
If a service module wishes to obtain the name of the user, it should not
use this function, but instead perform a call to pam_get_user(3).
Only a service module is privileged to read the authentication tokens,
PAM_AUTHTOK and PAM_OLDAUTHTOK.
3.1.4.2. RETURN VALUES
PAM_BAD_ITEM
The application attempted to set an undefined or inaccessible
item.
PAM_BUF_ERR
Memory buffer error.
PAM_PERM_DENIED
The value of item was NULL.
PAM_SUCCESS
Data was successful updated.
PAM_SYSTEM_ERR
The pam_handle_t passed as first argument was invalid.
3.1.5. Strings describing PAM error codes
#include <security/pam_appl.h>
const char *pam_strerror( pamh,
errnum);
pam_handle_t *pamh;
int errnum;
3.1.5.1. DESCRIPTION
The pam_strerror function returns a pointer to a string describing the
error code passed in the argument errnum, possibly using the LC_MESSAGES
part of the current locale to select the appropriate language. This string
must not be modified by the application. No library function will modify
this string.
3.1.5.2. RETURN VALUES
This function returns always a pointer to a string.
3.1.6. Request a delay on failure
#include <security/pam_appl.h>
int pam_fail_delay( pamh,
usec);
pam_handle_t *pamh;
unsigned int usec;
3.1.6.1. DESCRIPTION
The pam_fail_delay function provides a mechanism by which an application
or module can suggest a minimum delay of usec micro-seconds. The function
keeps a record of the longest time requested with this function. Should
pam_authenticate(3) fail, the failing return to the application is delayed
by an amount of time randomly distributed (by up to 50%) about this
longest value.
Independent of success, the delay time is reset to its zero default value
when the PAM service module returns control to the application. The delay
occurs after all authentication modules have been called, but before
control is returned to the service application.
When using this function the programmer should check if it is available
with:
#ifdef HAVE_PAM_FAIL_DELAY
....
#endif /* HAVE_PAM_FAIL_DELAY */
For applications written with a single thread that are event driven in
nature, generating this delay may be undesirable. Instead, the application
may want to register the delay in some other way. For example, in a single
threaded server that serves multiple authentication requests from a single
event loop, the application might want to simply mark a given connection
as blocked until an application timer expires. For this reason the delay
function can be changed with the PAM_FAIL_DELAY item. It can be queried
and set with pam_get_item(3) and pam_set_item (3) respectively. The value
used to set it should be a function pointer of the following prototype:
void (*delay_fn)(int retval, unsigned usec_delay, void *appdata_ptr);
The arguments being the retval return code of the module stack, the
usec_delay micro-second delay that libpam is requesting and the
appdata_ptr that the application has associated with the current pamh.
This last value was set by the application when it called pam_start(3) or
explicitly with pam_set_item(3). Note, if PAM_FAIL_DELAY item is unset (or
set to NULL), then no delay will be performed.
3.1.6.2. RETURN VALUES
PAM_SUCCESS
Delay was successful adjusted.
PAM_SYSTEM_ERR
A NULL pointer was submitted as PAM handle.
3.1.7. Authenticating the user
#include <security/pam_appl.h>
int pam_authenticate( pamh,
flags);
pam_handle_t *pamh;
int flags;
3.1.7.1. DESCRIPTION
The pam_authenticate function is used to authenticate the user. The user
is required to provide an authentication token depending upon the
authentication service, usually this is a password, but could also be a
finger print.
The PAM service module may request that the user enter their username vio
the the conversation mechanism (see pam_start(3) and pam_conv(3)). The
name of the authenticated user will be present in the PAM item PAM_USER.
This item may be recovered with a call to pam_get_item(3).
The pamh argument is an authentication handle obtained by a prior call to
pam_start(). The flags argument is the binary or of zero or more of the
following values:
PAM_SILENT
Do not emit any messages.
PAM_DISALLOW_NULL_AUTHTOK
The PAM module service should return PAM_AUTH_ERR if the user does
not have a registered authentication token.
3.1.7.2. RETURN VALUES
PAM_ABORT
The application should exit immediately after calling pam_end(3)
first.
PAM_AUTH_ERR
The user was not authenticated.
PAM_CRED_INSUFFICIENT
For some reason the application does not have sufficient
credentials to authenticate the user.
PAM_AUTHINFO_UNVAIL
The modules were not able to access the authentication
information. This might be due to a network or hardware failure
etc.
PAM_MAXTRIES
One or more of the authentication modules has reached its limit of
tries authenticating the user. Do not try again.
PAM_SUCCESS
The user was successfully authenticated.
PAM_USER_UNKNOWN
User unknown to authentication service.
3.1.8. Setting user credentials
#include <security/pam_appl.h>
int pam_setcred( pamh,
flags);
pam_handle_t *pamh;
int flags;
3.1.8.1. DESCRIPTION
The pam_setcred function is used to establish, maintain and delete the
credentials of a user. It should be called to set the credentials after a
user has been authenticated and before a session is opened for the user
(with pam_open_session(3)). The credentials should be deleted after the
session has been closed (with pam_close_session(3)).
A credential is something that the user possesses. It is some property,
such as a Kerberos ticket, or a supplementary group membership that make
up the uniqueness of a given user. On a Linux system the user's UID and
GID's are credentials too. However, it has been decided that these
properties (along with the default supplementary groups of which the user
is a member) are credentials that should be set directly by the
application and not by PAM. Such credentials should be established, by the
application, prior to a call to this function. For example, initgroups(2)
(or equivalent) should have been performed.
Valid flags, any one of which, may be logically OR'd with PAM_SILENT, are:
PAM_ESTABLISH_CRED
Initialize the credentials for the user.
PAM_DELETE_CRED
Delete the user's credentials.
PAM_REINITIALIZE_CRED
Fully reinitialize the user's credentials.
PAM_REFRESH_CRED
Extend the lifetime of the existing credentials.
3.1.8.2. RETURN VALUES
PAM_BUF_ERR
Memory buffer error.
PAM_CRED_ERR
Failed to set user credentials.
PAM_CRED_EXPIRED
User credentials are expired.
PAM_CRED_UNAVAIL
Failed to retrieve user credentials.
PAM_SUCCESS
Data was successful stored.
PAM_SYSTEM_ERR
A NULL pointer was submitted as PAM handle, the function was
called by a module or another system error occured.
PAM_USER_UNKNOWN
User is not known to an authentication module.
3.1.9. Account validation management
#include <security/pam_appl.h>
int pam_acct_mgmt( pamh,
flags);
pam_handle_t *pamh;
int flags;
3.1.9.1. DESCRIPTION
The pam_acct_mgmt function is used to determine if the users account is
valid. It checks for authentication token and account expiration and
verifies access restrictions. It is typically called after the user has
been authenticated.
The pamh argument is an authentication handle obtained by a prior call to
pam_start(). The flags argument is the binary or of zero or more of the
following values:
PAM_SILENT
Do not emit any messages.
PAM_DISALLOW_NULL_AUTHTOK
The PAM module service should return PAM_NEW_AUTHTOK_REQD if the
user has a null authentication token.
3.1.9.2. RETURN VALUES
PAM_ACCT_EXPIRED
User account has expired.
PAM_AUTH_ERR
Authentication failure.
PAM_NEW_AUTHTOK_REQD
The user account is valid but their authentication token is
expired. The correct response to this return-value is to require
that the user satisfies the pam_chauthtok() function before
obtaining service. It may not be possible for some applications to
do this. In such cases, the user should be denied access until
such time as they can update their password.
PAM_PERM_DENIED
Permission denied.
PAM_SUCCESS
The authentication token was successfully updated.
PAM_USER_UNKNOWN
User unknown to password service.
3.1.10. Updating authentication tokens
#include <security/pam_appl.h>
int pam_chauthtok( pamh,
flags);
pam_handle_t *pamh;
int flags;
3.1.10.1. DESCRIPTION
The pam_chauthtok function is used to change the authentication token for
a given user (as indicated by the state associated with the handle pamh).
The pamh argument is an authentication handle obtained by a prior call to
pam_start(). The flags argument is the binary or of zero or more of the
following values:
PAM_SILENT
Do not emit any messages.
PAM_CHANGE_EXPIRED_AUTHTOK
This argument indicates to the modules that the users
authentication token (password) should only be changed if it has
expired. If this argument is not passed, the application requires
that all authentication tokens are to be changed.
3.1.10.2. RETURN VALUES
PAM_AUTHTOK_ERR
A module was unable to obtain the new authentication token.
PAM_AUTHTOK_RECOVERY_ERR
A module was unable to obtain the old authentication token.
PAM_AUTHTOK_LOCK_BUSY
One or more of the modules was unable to change the authentication
token since it is currently locked.
PAM_AUTHTOK_DISABLE_AGING
Authentication token aging has been disabled for at least one of
the modules.
PAM_PERM_DENIED
Permission denied.
PAM_SUCCESS
The authentication token was successfully updated.
PAM_TRY_AGAIN
Not all of the modules were in a position to update the
authentication token(s). In such a case none of the user's
authentication tokens are updated.
PAM_USER_UNKNOWN
User unknown to password service.
3.1.11. Start PAM session management
#include <security/pam_appl.h>
int pam_open_session( pamh,
flags);
pam_handle_t *pamh;
int flags;
3.1.11.1. DESCRIPTION
The pam_open_session function sets up a user session for a previously
successful authenticated user. The session should later be terminated with
a call to pam_close_session(3).
It should be noted that the effective uid, geteuid(2). of the application
should be of sufficient privilege to perform such tasks as creating or
mounting the user's home directory for example.
The flags argument is the binary or of zero or more of the following
values:
PAM_SILENT
Do not emit any messages.
3.1.11.2. RETURN VALUES
PAM_ABORT
General failure.
PAM_BUF_ERR
Memory buffer error.
PAM_SESSION_ERR
Session failure.
PAM_SUCCESS
Session was successful created.
3.1.12. terminating PAM session management
#include <security/pam_appl.h>
int pam_close_session( pamh,
flags);
pam_handle_t *pamh;
int flags;
3.1.12.1. DESCRIPTION
The pam_close_session function is used to indicate that an authenticated
session has ended. The session should have been created with a call to
pam_open_session(3).
It should be noted that the effective uid, geteuid(2). of the application
should be of sufficient privilege to perform such tasks as unmounting the
user's home directory for example.
The flags argument is the binary or of zero or more of the following
values:
PAM_SILENT
Do not emit any messages.
3.1.12.2. RETURN VALUES
PAM_ABORT
General failure.
PAM_BUF_ERR
Memory buffer error.
PAM_SESSION_ERR
Session failure.
PAM_SUCCESS
Session was successful terminated.
3.1.13. Set or change PAM environment variable
#include <security/pam_appl.h>
int pam_putenv( pamh,
name_value);
pam_handle_t *pamh;
const char *name_value;
3.1.13.1. DESCRIPTION
The pam_putenv function is used to add or change the value of PAM
environment variables as associated with the pamh handle.
The pamh argument is an authentication handle obtained by a prior call to
pam_start(). The name_value argument is a single NUL terminated string of
one of the following forms:
NAME=value of variable
In this case the environment variable of the given NAME is set to
the indicated value: value of variable. If this variable is
already known, it is overwritten. Otherwise it is added to the PAM
environment.
NAME=
This function sets the variable to an empty value. It is listed
separately to indicate that this is the correct way to achieve
such a setting.
NAME
Without an '=' the pam_putenv() function will delete the
corresponding variable from the PAM environment.
pam_putenv() operates on a copy of name_value, which means in contrast to
putenv(3), the application is responsible to free the data.
3.1.13.2. RETURN VALUES
PAM_PERM_DENIED
Argument name_value given is a NULL pointer.
PAM_BAD_ITEM
Variable requested (for deletion) is not currently set.
PAM_ABORT
The pamh handle is corrupt.
PAM_BUF_ERR
Memory buffer error.
PAM_SUCCESS
The environment variable was successfully updated.
3.1.14. Get a PAM environment variable
#include <security/pam_appl.h>
const char *pam_getenv( pamh,
name);
pam_handle_t *pamh;
const char *name;
3.1.14.1. DESCRIPTION
The pam_getenv function searches the PAM environment list as associated
with the handle pamh for an item that matches the string pointed to by
name and returns a pointer to the value of the environment variable. The
application is not allowed to free the data.
3.1.14.2. RETURN VALUES
The pam_getenv function returns NULL on failure.
3.1.15. Getting the PAM environment
#include <security/pam_appl.h>
char **pam_getenvlist( pamh);
pam_handle_t *pamh;
3.1.15.1. DESCRIPTION
The pam_getenvlist function returns a complete copy of the PAM environment
as associated with the handle pamh. The PAM environment variables
represent the contents of the regular environment variables of the
authenticated user when service is granted.
The format of the memory is a malloc()'d array of char pointers, the last
element of which is set to NULL. Each of the non-NULL entries in this
array point to a NUL terminated and malloc()'d char string of the form:
"name=value".
It should be noted that this memory will never be free()'d by libpam. Once
obtained by a call to pam_getenvlist, it is the responsibility of the
calling application to free() this memory.
It is by design, and not a coincidence, that the format and contents of
the returned array matches that required for the third argument of the
execle(3) function call.
3.1.15.2. RETURN VALUES
The pam_getenvlist function returns NULL on failure.
3.2. What is expected of an application
3.2.1. The conversation function
#include <security/pam_appl.h>
struct pam_message {
int msg_style;
const char *msg;
};
struct pam_response {
char *resp;
int resp_retcode;
};
struct pam_conv {
int (*conv)(int num_msg, const struct pam_message **msg,
struct pam_response **resp, void *appdata_ptr);
void *appdata_ptr;
};
3.2.1.1. DESCRIPTION
The PAM library uses an application-defined callback to allow a direct
communication between a loaded module and the application. This callback
is specified by the struct pam_conv passed to pam_start(3) at the start of
the transaction.
When a module calls the referenced conv() function, the argument
appdata_ptr is set to the second element of this structure.
The other arguments of a call to conv() concern the information exchanged
by module and application. That is to say, num_msg holds the length of the
array of pointers, msg. After a successful return, the pointer resp points
to an array of pam_response structures, holding the application supplied
text. The resp_retcode member of this struct is unused and should be set
to zero. It is the caller's responsibility to release both, this array and
the responses themselves, using free(3). Note, *resp is a struct
pam_response array and not an array of pointers.
The number of responses is always equal to the num_msg conversation
function argument. This does require that the response array is free(3)'d
after every call to the conversation function. The index of the responses
corresponds directly to the prompt index in the pam_message array.
On failure, the conversation function should release any resources it has
allocated, and return one of the predefined PAM error codes.
Each message can have one of four types, specified by the msg_style member
of struct pam_message:
PAM_PROMPT_ECHO_OFF
Obtain a string without echoing any text.
PAM_PROMPT_ECHO_ON
Obtain a string whilst echoing text.
PAM_ERROR_MSG
Display an error message.
PAM_TEXT_INFO
Display some text.
The point of having an array of messages is that it becomes possible to
pass a number of things to the application in a single call from the
module. It can also be convenient for the application that related things
come at once: a windows based application can then present a single form
with many messages/prompts on at once.
In passing, it is worth noting that there is a descrepency between the way
Linux-PAM handles the const struct pam_message **msg conversation function
argument from the way that Solaris' PAM (and derivitives, known to include
HP/UX, are there others?) does. Linux-PAM interprets the msg argument as
entirely equivalent to the following prototype const struct pam_message
*msg[] (which, in spirit, is consistent with the commonly used prototypes
for argv argument to the familiar main() function: char **argv; and char
*argv[]). Said another way Linux-PAM interprets the msg argument as a
pointer to an array of num_msg read only 'struct pam_message' pointers.
Solaris' PAM implementation interprets this argument as a pointer to a
pointer to an array of num_msg pam_message structures. Fortunately,
perhaps, for most module/application developers when num_msg has a value
of one these two definitions are entirely equivalent. Unfortunately,
casually raising this number to two has led to unanticipated compatibility
problems.
For what its worth the two known module writer work-arounds for trying to
maintain source level compatibility with both PAM implementations are:
* never call the conversation function with num_msg greater than one.
* set up msg as doubly referenced so both types of conversation function
can find the messages. That is, make
msg[n] = & (( *msg )[n])
3.2.1.2. RETURN VALUES
PAM_BUF_ERR
Memory buffer error.
PAM_CONV_ERR
Conversation failure. The application should not set *resp.
PAM_SUCCESS
Success.
3.3. Programming notes
Note, all of the authentication service function calls accept the token
PAM_SILENT, which instructs the modules to not send messages to the
application. This token can be logically OR'd with any one of the
permitted tokens specific to the individual function calls. PAM_SILENT
does not override the prompting of the user for passwords etc., it only
stops informative messages from being generated.
Chapter 4. Security issues of Linux-PAM
PAM, from the perspective of an application, is a convenient API for
authenticating users. PAM modules generally have no increased privilege
over that possessed by the application that is making use of it. For this
reason, the application must take ultimate responsibility for protecting
the environment in which PAM operates.
A poorly (or maliciously) written application can defeat any Linux-PAM
module's authentication mechanisms by simply ignoring it's return values.
It is the applications task and responsibility to grant privileges and
access to services. The Linux-PAM library simply assumes the
responsibility of authenticating the user; ascertaining that the user is
who they say they are. Care should be taken to anticipate all of the
documented behavior of the Linux-PAM library functions. A failure to do
this will most certainly lead to a future security breach.
4.1. Care about standard library calls
In general, writers of authorization-granting applications should assume
that each module is likely to call any or all 'libc' functions. For 'libc'
functions that return pointers to static/dynamically allocated structures
(ie. the library allocates the memory and the user is not expected to
'free()' it) any module call to this function is likely to corrupt a
pointer previously obtained by the application. The application programmer
should either re-call such a 'libc' function after a call to the Linux-PAM
library, or copy the structure contents to some safe area of memory before
passing control to the Linux-PAM library.
Two important function classes that fall into this category are
getpwnam(3) and syslog(3).
4.2. Choice of a service name
When picking the service-name that corresponds to the first entry in the
Linux-PAM configuration file, the application programmer should avoid the
temptation of choosing something related to argv[0]. It is a trivial
matter for any user to invoke any application on a system under a
different name and this should not be permitted to cause a security
breach.
In general, this is always the right advice if the program is setuid, or
otherwise more privileged than the user that invokes it. In some cases,
avoiding this advice is convenient, but as an author of such an
application, you should consider well the ways in which your program will
be installed and used. (Its often the case that programs are not intended
to be setuid, but end up being installed that way for convenience. If your
program falls into this category, don't fall into the trap of making this
mistake.)
To invoke some target application by another name, the user may
symbolically link the target application with the desired name. To be
precise all the user need do is, ln -s /target/application
./preferred_name and then run ./preferred_name.
By studying the Linux-PAM configuration file(s), an attacker can choose
the preferred_name to be that of a service enjoying minimal protection;
for example a game which uses Linux-PAM to restrict access to certain
hours of the day. If the service-name were to be linked to the filename
under which the service was invoked, it is clear that the user is
effectively in the position of dictating which authentication scheme the
service uses. Needless to say, this is not a secure situation.
The conclusion is that the application developer should carefully define
the service-name of an application. The safest thing is to make it a
single hard-wired name.
4.3. The conversation function
Care should be taken to ensure that the conv() function is robust. Such a
function is provided in the library libpam_misc (see below).
4.4. The identity of the user
The Linux-PAM modules will need to determine the identity of the user who
requests a service, and the identity of the user who grants the service.
These two users will seldom be the same. Indeed there is generally a third
user identity to be considered, the new (assumed) identity of the user
once the service is granted.
The need for keeping tabs on these identities is clearly an issue of
security. One convention that is actively used by some modules is that the
identity of the user requesting a service should be the current UID (user
ID) of the running process; the identity of the privilege granting user is
the EUID (effective user ID) of the running process; the identity of the
user, under whose name the service will be executed, is given by the
contents of the PAM_USER pam_get_item(3). Note, modules can change the
values of PAM_USER and PAM_RUSER during any of the pam_*() library calls.
For this reason, the application should take care to use the
pam_get_item() every time it wishes to establish who the authenticated
user is (or will currently be).
For network-serving databases and other applications that provide their
own security model (independent of the OS kernel) the above scheme is
insufficient to identify the requesting user.
A more portable solution to storing the identity of the requesting user is
to use the PAM_RUSER pam_get_item(3). The application should supply this
value before attempting to authenticate the user with pam_authenticate().
How well this name can be trusted will ultimately be at the discretion of
the local administrator (who configures PAM for your application) and a
selected module may attempt to override the value where it can obtain more
reliable data. If an application is unable to determine the identity of
the requesting entity/user, it should not call pam_set_item(3) to set
PAM_RUSER.
In addition to the PAM_RUSER item, the application should supply the
PAM_RHOST (requesting host) item. As a general rule, the following
convention for its value can be assumed: NULL = unknown; localhost =
invoked directly from the local system; other.place.xyz = some component
of the user's connection originates from this remote/requesting host. At
present, PAM has no established convention for indicating whether the
application supports a trusted path to communication from this host.
4.5. Sufficient resources
Care should be taken to ensure that the proper execution of an application
is not compromised by a lack of system resources. If an application is
unable to open sufficient files to perform its service, it should fail
gracefully, or request additional resources. Specifically, the quantities
manipulated by the setrlimit(2) family of commands should be taken into
consideration.
This is also true of conversation prompts. The application should not
accept prompts of arbitrary length with out checking for resource
allocation failure and dealing with such extreme conditions gracefully and
in a manner that preserves the PAM API. Such tolerance may be especially
important when attempting to track a malicious adversary.
Chapter 5. A library of miscellaneous helper functions
To aid the work of the application developer a library of miscellaneous
functions is provided. It is called libpam_misc, and contains a text based
conversation function, and routines for enhancing the standard
PAM-environment variable support.
The functions, structures and macros, made available by this library can
be defined by including <security/pam_misc.h>. It should be noted that
this library is specific to Linux-PAM and is not referred to in the
defining DCE-RFC (see See also) below.
5.1. Functions supplied
5.1.1. Text based conversation function
#include <security/pam_misc.h>
int misc_conv( num_msg,
msgm,
response,
appdata_ptr);
int num_msg;
const struct pam_message **msgm;
struct pam_response **response;
void *appdata_ptr;
5.1.1.1. DESCRIPTION
The misc_conv function is part of libpam_misc and not of the standard
libpam library. This function will prompt the user with the appropriate
comments and obtain the appropriate inputs as directed by authentication
modules.
In addition to simply slotting into the appropriate pam_conv(3), this
function provides some time-out facilities. The function exports five
variables that can be used by an application programmer to limit the
amount of time this conversation function will spend waiting for the user
to type something. The five variabls are as follows:
time_t pam_misc_conv_warn_time;
This variable contains the time (as returned by time(2)) that the
user should be first warned that the clock is ticking. By default
it has the value 0, which indicates that no such warning will be
given. The application may set its value to sometime in the
future, but this should be done prior to passing control to the
Linux-PAM library.
const char *pam_misc_conv_warn_line;
Used in conjuction with pam_misc_conv_warn_time, this variable is
a pointer to the string that will be displayed when it becomes
time to warn the user that the timeout is approaching. Its default
value is a translated version of "...Time is running out...", but
this can be changed by the application prior to passing control to
Linux-PAM.
time_t pam_misc_conv_die_time;
This variable contains the time (as returned by time(2)) that the
will time out. By default it has the value 0, which indicates that
the conversation function will not timeout. The application may
set its value to sometime in the future, but this should be done
prior to passing control to the Linux-PAM library.
const char *pam_misc_conv_die_line;
Used in conjuction with pam_misc_conv_die_time, this variable is a
pointer to the string that will be displayed when the conversation
times out. Its default value is a translated version of "...Sorry,
your time is up!", but this can be changed by the application
prior to passing control to Linux-PAM.
int pam_misc_conv_died;
Following a return from the Linux-PAM libraray, the value of this
variable indicates whether the conversation has timed out. A value
of 1 indicates the time-out occurred.
The following two function pointers are available for supporting binary
prompts in the conversation function. They are optimized for the current
incarnation of the libpamc library and are subject to change.
int (*pam_binary_handler_fn)(void *appdata, pamc_bp_t *prompt_p);
This function pointer is initialized to NULL but can be filled
with a function that provides machine-machine (hidden) message
exchange. It is intended for use with hidden authentication
protocols such as RSA or Diffie-Hellman key exchanges. (This is
still under development.)
int (*pam_binary_handler_free)(void *appdata, pamc_bp_t *delete_me);
This function pointer is initialized to PAM_BP_RENEW(delete_me, 0,
0), but can be redefined as desired by the application.
5.1.2. Transcribing an environment to that of PAM
#include <security/pam_misc.h>
int pam_misc_paste_env( pamh,
user);
pam_handle_t *pamh;
const char * const *user;
5.1.2.1. DESCRIPTION
This function takes the supplied list of environment pointers and uploads
its contents to the PAM environment. Success is indicated by PAM_SUCCESS.
5.1.3. Liberating a locally saved environment
#include <security/pam_misc.h>
int pam_misc_drop_env( env);
char **env;
5.1.3.1. DESCRIPTION
This function is defined to complement the pam_getenvlist(3) function. It
liberates the memory associated with env, overwriting with 0 all memory
before free()ing it.
5.1.4. BSD like PAM environment variable setting
#include <security/pam_misc.h>
int pam_misc_setenv( pamh,
name,
value,
readonly);
pam_handle_t *pamh;
const char *name;
const char *value;
intreadonly;
5.1.4.1. DESCRIPTION
This function performs a task equivalent to pam_putenv(3), its syntax is,
however, more like the BSD style function; setenv(). The name and value
are concatenated with an '=' to form a name=value and passed to
pam_putenv(). If, however, the PAM variable is already set, the
replacement will only be applied if the last argument, readonly, is zero.
Chapter 6. Porting legacy applications
The point of PAM is that the application is not supposed to have any idea
how the attached authentication modules will choose to authenticate the
user. So all they can do is provide a conversation function that will talk
directly to the user(client) on the modules' behalf.
Consider the case that you plug a retinal scanner into the login program.
In this situation the user would be prompted: "please look into the
scanner". No username or password would be needed - all this information
could be deduced from the scan and a database lookup. The point is that
the retinal scanner is an ideal task for a "module".
While it is true that a pop-daemon program is designed with the POP
protocol in mind and no-one ever considered attaching a retinal scanner to
it, it is also the case that the "clean" PAM'ification of such a daemon
would allow for the possibility of a scanner module being be attached to
it. The point being that the "standard" pop-authentication protocol(s)
[which will be needed to satisfy inflexible/legacy clients] would be
supported by inserting an appropriate pam_qpopper module(s). However,
having rewritten popd once in this way any new protocols can be
implemented in-situ.
One simple test of a ported application would be to insert the pam_permit
module and see if the application demands you type a password... In such a
case, xlock would fail to lock the terminal - or would at best be a
screen-saver, ftp would give password free access to all etc.. Neither of
these is a very secure thing to do, but they do illustrate how much
flexibility PAM puts in the hands of the local admin.
The key issue, in doing things correctly, is identifying what is part of
the authentication procedure (how many passwords etc..) the exchange
protocol (prefixes to prompts etc., numbers like 331 in the case of ftpd)
and what is part of the service that the application delivers. PAM really
needs to have total control in the authentication "procedure", the
conversation function should only deal with reformatting user prompts and
extracting responses from raw input.
Chapter 7. Glossary of PAM related terms
The following are a list of terms used within this document.
Authentication token
Generally, this is a password. However, a user can authenticate
him/herself in a variety of ways. Updating the user's
authentication token thus corresponds to refreshing the object
they use to authenticate themself with the system. The word
password is avoided to keep open the possibility that the
authentication involves a retinal scan or other non-textual mode
of challenge/response.
Credentials
Having successfully authenticated the user, PAM is able to
establish certain characteristics/attributes of the user. These
are termed credentials. Examples of which are group memberships to
perform privileged tasks with, and tickets in the form of
environment variables etc. . Some user-credentials, such as the
user's UID and GID (plus default group memberships) are not deemed
to be PAM-credentials. It is the responsibility of the application
to grant these directly.
Chapter 8. An example application
To get a flavor of the way a Linux-PAM application is written we include
the following example. It prompts the user for their password and
indicates whether their account is valid on the standard output, its
return code also indicates the success (0 for success; 1 for failure).
/*
This program was contributed by Shane Watts
[modifications by AGM and kukuk]
You need to add the following (or equivalent) to the
/etc/pam.d/check_user file:
# check authorization
auth required pam_unix.so
account required pam_unix.so
*/
#include <security/pam_appl.h>
#include <security/pam_misc.h>
#include <stdio.h>
static struct pam_conv conv = {
misc_conv,
NULL
};
int main(int argc, char *argv[])
{
pam_handle_t *pamh=NULL;
int retval;
const char *user="nobody";
if(argc == 2) {
user = argv[1];
}
if(argc > 2) {
fprintf(stderr, "Usage: check_user [username]\n");
exit(1);
}
retval = pam_start("check_user", user, &conv, &pamh);
if (retval == PAM_SUCCESS)
retval = pam_authenticate(pamh, 0); /* is user really user? */
if (retval == PAM_SUCCESS)
retval = pam_acct_mgmt(pamh, 0); /* permitted access? */
/* This is where we have been authorized or not. */
if (retval == PAM_SUCCESS) {
fprintf(stdout, "Authenticated\n");
} else {
fprintf(stdout, "Not Authenticated\n");
}
if (pam_end(pamh,retval) != PAM_SUCCESS) { /* close Linux-PAM */
pamh = NULL;
fprintf(stderr, "check_user: failed to release authenticator\n");
exit(1);
}
return ( retval == PAM_SUCCESS ? 0:1 ); /* indicate success */
}
Chapter 9. Files
/usr/include/security/pam_appl.h
Header file with interfaces for Linux-PAM applications.
/usr/include/security/pam_misc.h
Header file for useful library functions for making applications
easier to write.
Chapter 10. See also
* The Linux-PAM System Administrators' Guide.
* The Linux-PAM Module Writers' Guide.
* The V. Samar and R. Schemers (SunSoft), ``UNIFIED LOGIN WITH PLUGGABLE
AUTHENTICATION MODULES'', Open Software Foundation Request For
Comments 86.0, October 1995.
Chapter 11. Author/acknowledgments
This document was written by Andrew G. Morgan (morgan@kernel.org) with
many contributions from Chris Adams, Peter Allgeyer, Tim Baverstock, Tim
Berger, Craig S. Bell, Derrick J. Brashear, Ben Buxton, Seth Chaiklin,
Oliver Crow, Chris Dent, Marc Ewing, Cristian Gafton, Emmanuel Galanos,
Brad M. Garcia, Eric Hester, Roger Hu, Eric Jacksch, Michael K. Johnson,
David Kinchlea, Olaf Kirch, Marcin Korzonek, Thorsten Kukuk, Stephen
Langasek, Nicolai Langfeldt, Elliot Lee, Luke Kenneth Casson Leighton, Al
Longyear, Ingo Luetkebohle, Marek Michalkiewicz, Robert Milkowski, Aleph
One, Martin Pool, Sean Reifschneider, Jan Rekorajski, Erik Troan, Theodore
Ts'o, Jeff Uphoff, Myles Uyema, Savochkin Andrey Vladimirovich, Ronald
Wahl, David Wood, John Wilmes, Joseph S. D. Yao and Alex O. Yuriev.
Thanks are also due to Sun Microsystems, especially to Vipin Samar and
Charlie Lai for their advice. At an early stage in the development of
Linux-PAM, Sun graciously made the documentation for their implementation
of PAM available. This act greatly accelerated the development of
Linux-PAM.
Chapter 12. Copyright information for this document
Copyright (c) 2006 Thorsten Kukuk <kukuk@thkukuk.de>
Copyright (c) 1996-2002 Andrew G. Morgan <morgan@kernel.org>
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:
1. Redistributions of source code must retain the above copyright
notice, and the entire permission notice in its entirety,
including the disclaimer of warranties.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
3. The name of the author may not be used to endorse or promote
products derived from this software without specific prior
written permission.
Alternatively, this product may be distributed under the terms of the GNU
General Public License (GPL), in which case the provisions of the GNU GPL
are required instead of the above restrictions. (This clause is necessary
due to a potential bad interaction between the GNU GPL and the
restrictions contained in a BSD-style copyright.)
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IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
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ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR
TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
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