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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
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
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:
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.
this module type provides two aspects of authenticating the user.
things include the logging of information concerning the
opening/closing of some data exchange with a user, mounting
If the type value from the list above is prepended with a - character
the PAM library will not log to the system log if it is not possible to
load the module because it is missing in the system. This can be useful
especially for modules which are not always installed on the system and
are not required for correct authentication and authorization of the
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:
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.
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.
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. If the module succeeds the PAM framework
returns success to the application immediately without trying any
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 all lines of given type from the configuration file
specified as an argument to this control.
[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, conv_again, incomplete, 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:
when used with a stack of modules, the module?s return status will
not contribute to the return code the application obtains.
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
equivalent to bad with the side effect of terminating the module
stack and PAM immediately returning to the application.
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.
equivalent to ok with the side effect of terminating the module
stack and PAM immediately returning to the application.
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.
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
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 \
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
[..[..\]..] --> ..[..]..
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
pam(3), PAM(8), pam_start(3)