pam - Pluggable Authentication Modules Library
PAM is a system of libraries that handle the authentication tasks of
applications (services) on the system. The library provides a stable
general interface (Application Programming Interface - API) that
privilege granting programs (such as login(1) and su(1)) defer to to
perform standard authentication tasks.
Initialization and Cleanup
The pam_start(3) 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 pam_end(3) 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handle pamh is no longer valid and all memory associated with it will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. It can be called at any time to terminate a PAM
The pam_authenticate(3) 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
The pam_setcred(3) function manages the userscredentials.
The pam_acct_mgmt(3) 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
The pam_chauthtok(3) function is used to change the authentication
token for a given user on request or because the token has expired.
The pam_open_session(3) 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).
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. See pam_conv(3) for details.
The pam_set_item(3) and pam_get_item(3) functions allows applications
and PAM service modules to set and retrieve PAM informations.
The pam_get_user(3) function is the preferred method to obtain the
The pam_set_data(3) and pam_get_data(3) functions allows PAM service
modules to set and retrieve free-form data from one invocation to
Environment and Error Management
The pam_putenv(3), pam_getenv(3) and pam_getenvlist(3) functions are
for maintaining a set of private environment variables.
The pam_strerror(3) function returns a pointer to a string describing
the given PAM error code.
The following return codes are known by PAM:
Critical error, immediate abort.
User account has expired.
Authentication service cannot retrieve authentication info.
Authentication token aging disabled.
Authentication token manipulation error.
Authentication token expired.
Authentication token lock busy.
Authentication information cannot be recovered.
Memory buffer error.
Failure setting user credentials.
User credentials expired.
Insufficient credentials to access authentication data.
Authentication service cannot retrieve user credentials.
The return value should be ignored by PAM dispatch.
Have exhausted maximum number of retries for service.
Module is unknown.
Authentication token is no longer valid; new one required.
No module specific data is present.
Failed to load module.
Error in service module.
Cannot make/remove an entry for the specified session.
Symbol not found.
Failed preliminary check by password service.
User not known to the underlying authentication module.
pam_acct_mgmt(3), pam_authenticate(3), pam_chauthtok(3),
pam_close_session(3), pam_conv(3), pam_end(3), pam_get_data(3),
pam_getenv(3), pam_getenvlist(3), pam_get_item(3), pam_get_user(3),
pam_open_session(3), pam_putenv(3), pam_set_data(3), pam_set_item(3),
pam_setcred(3), pam_start(3), pam_strerror(3)
The libpam interfaces are only thread-safe if each thread within the
multithreaded application uses its own PAM handle.