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.
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 login session.
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 or to the superior PAM stack. 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
if such a module succeeds and no prior required module has
failed the PAM framework returns success to the application
or to the superior PAM stack immediately without calling any
further modules in the stack. A failure of a sufficient
module is ignored and processing of the PAM module stack
continues unaffected.
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.
substack
include all lines of given type from the configuration file
specified as an argument to this control. This differs from
include in that evaluation of the done and die actions in a
substack does not cause skipping the rest of the complete
module stack, but only of the substack. Jumps in a substack
also can not make evaluation jump out of it, and the whole
substack is counted as one module when the jump is done in a
parent stack. The reset action will reset the state of a
module stack to the state it was in as of beginning of the
substack evaluation.
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, 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 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.
N (an unsigned integer)
equivalent to ok with the side effect of jumping over the
next N modules in the stack. Note that N equal to 0 is not
allowed (and it would be identical to ok in such case).
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.
