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Linux PAM.D man page

PAM.CONF(5) Linux-PAM Manual PAM.CONF(5)

NAME
pam.conf, pam.d - PAM configuration files

DESCRIPTION
When a PAM aware privilege granting application is started, it
activates its attachment to the PAM-API. This activation performs a
number of tasks, the most important being the reading of the
configuration file(s): /etc/pam.conf. Alternatively, this may be the
contents of the /etc/pam.d/ directory. The presence of this directory
will cause Linux-PAM to ignore /etc/pam.conf.

These files list the PAMs that will do the authentication tasks
required by this service, and the appropriate behavior of the PAM-API
in the event that individual PAMs fail.

The syntax of the /etc/pam.conf configuration file is as follows. The
file is made up of a list of rules, each rule is typically placed on a
single line, but may be extended with an escaped end of line: ‘\<LF>’.
Comments are preceded with ‘#’ marks and extend to the next end of
line.

The format of each rule is a space separated collection of tokens, the
first three being case-insensitive:

service type control module-path module-arguments

The syntax of files contained in the /etc/pam.d/ directory, are
identical except for the absence of any service field. In this case,
the service is the name of the file in the /etc/pam.d/ directory. This
filename must be in lower case.

An important feature of PAM, is that a number of rules may be stacked
to combine the services of a number of PAMs for a given authentication
task.

The service is typically the familiar name of the corresponding
application: login and su are good examples. The service-name, other,
is reserved for giving default rules. Only lines that mention the
current service (or in the absence of such, the other entries) will be
associated with the given service-application.

The type is the management group that the rule corresponds to. It is
used to specify which of the management groups the subsequent module is
to be associated with. Valid entries are:

account
this module type performs non-authentication based account
management. It is typically used to restrict/permit access to a
service based on the time of day, currently available system
resources (maximum number of users) or perhaps the location of the
applicant user -- ’root’ login only on the console.

auth
this module type provides two aspects of authenticating the user.
Firstly, it establishes that the user is who they claim to be, by
instructing the application to prompt the user for a password or
other means of identification. Secondly, the module can grant group
membership or other privileges through its credential granting
properties.

password
this module type is required for updating the authentication token
associated with the user. Typically, there is one module for each
’challenge/response’ based authentication (auth) type.

session
this module type is associated with doing things that need to be
done for the user before/after they can be given service. Such
things include the logging of information concerning the
opening/closing of some data exchange with a user, mounting
directories, etc.

The third field, control, indicates the behavior of the PAM-API should
the module fail to succeed in its authentication task. There are two
types of syntax for this control field: the simple one has a single
simple keyword; the more complicated one involves a square-bracketed
selection of value=action pairs.

For the simple (historical) syntax valid control values are:

required
failure of such a PAM will ultimately lead to the PAM-API returning
failure but only after the remaining stacked modules (for this
service and type) have been invoked.

requisite
like required, however, in the case that such a module returns a
failure, control is directly returned to the application. The return
value is that associated with the first required or requisite module
to fail. Note, this flag can be used to protect against the
possibility of a user getting the opportunity to enter a password
over an unsafe medium. It is conceivable that such behavior might
inform an attacker of valid accounts on a system. This possibility
should be weighed against the not insignificant concerns of exposing
a sensitive password in a hostile environment.

sufficient
success of such a module is enough to satisfy the authentication
requirements of the stack of modules (if a prior required module has
failed the success of this one is ignored). A failure of this module
is not deemed as fatal to satisfying the application that this type
has succeeded.

optional
the success or failure of this module is only important if it is the
only module in the stack associated with this service+type.

include
include all lines of given type from the configuration file
specified as an argument to this control.

For the more complicated syntax valid control values have the following
form:

[value1=action1 value2=action2 ...]

Where valueN corresponds to the return code from the function invoked
in the module for which the line is defined. It is selected from one of
these: success, open_err, symbol_err, service_err, system_err, buf_err,
perm_denied, auth_err, cred_insufficient, authinfo_unavail,
user_unknown, maxtries, new_authtok_reqd, acct_expired, session_err,
cred_unavail, cred_expired, cred_err, no_module_data, conv_err,
authtok_err, authtok_recover_err, authtok_lock_busy,
authtok_disable_aging, try_again, ignore, abort, authtok_expired,
module_unknown, bad_item and default.

The last of these, default, implies ’all valueN’s not mentioned
explicitly. Note, the full list of PAM errors is available in
/usr/include/security/_pam_types.h. The actionN can be: an unsigned
integer, n, signifying an action of ’jump over the next n modules in
the stack’, or take one of the following forms:

ignore
when used with a stack of modules, the module’s return status will
not contribute to the return code the application obtains.

bad
this action indicates that the return code should be thought of as
indicative of the module failing. If this module is the first in the
stack to fail, its status value will be used for that of the whole
stack.

die
equivalent to bad with the side effect of terminating the module
stack and PAM immediately returning to the application.

ok this tells PAM that the administrator thinks this return code should
contribute directly to the return code of the full stack of modules.
In other words, if the former state of the stack would lead to a
return of PAM_SUCCESS, the module’s return code will override this
value. Note, if the former state of the stack holds some value that
is indicative of a modules failure, this ’ok’ value will not be used
to override that value.

done
equivalent to ok with the side effect of terminating the module
stack and PAM immediately returning to the application.

reset
clear all memory of the state of the module stack and start again
with the next stacked module.

Each of the four keywords: required; requisite; sufficient; and
optional, have an equivalent expression in terms of the [...] syntax.
They are as follows:

required
[success=ok new_authtok_reqd=ok ignore=ignore default=bad]

requisite
[success=ok new_authtok_reqd=ok ignore=ignore default=die]

sufficient
[success=done new_authtok_reqd=done default=ignore]

optional
[success=ok new_authtok_reqd=ok default=ignore]

module-path is either the full filename of the PAM to be used by the
application (it begins with a ’/’), or a relative pathname from the
default module location: /lib/security/ or /lib64/security/, depending
on the architecture.

module-arguments are a space separated list of tokens that can be used
to modify the specific behavior of the given PAM. Such arguments will
be documented for each individual module. Note, if you wish to include
spaces in an argument, you should surround that argument with square
brackets.

squid auth required pam_mysql.so user=passwd_query passwd=mada \
db=eminence [query=select user_name from internet_service \
where user_name=’%u’ and password=PASSWORD(’%p’) and \
service=’web_proxy’]

When using this convention, you can include ‘[’ characters inside the
string, and if you wish to include a ‘]’ character inside the string
that will survive the argument parsing, you should use ‘\[’. In other
words:

[..[..\]..] --> ..[..]..

Any line in (one of) the configuration file(s), that is not formatted
correctly, will generally tend (erring on the side of caution) to make
the authentication process fail. A corresponding error is written to
the system log files with a call to syslog(3).

More flexible than the single configuration file is it to configure
libpam via the contents of the /etc/pam.d/ directory. In this case the
directory is filled with files each of which has a filename equal to a
service-name (in lower-case): it is the personal configuration file for
the named service.

The syntax of each file in /etc/pam.d/ is similar to that of the
/etc/pam.conf file and is made up of lines of the following form:

type control module-path module-arguments

The only difference being that the service-name is not present. The
service-name is of course the name of the given configuration file. For
example, /etc/pam.d/login contains the configuration for the login
service.

SEE ALSO
pam(3), PAM(8), pam_start(3)

Linux-PAM Manual 06/27/2006 PAM.CONF(5)


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