pam_krb5

Section: System Administrator's Manual (8)
Updated: 2013/09/21
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NAME

pam_krb5 - Kerberos 5 authentication

SYNOPSIS

auth required //usr/$LIB/security/pam_krb5.so
session optional //usr/$LIB/security/pam_krb5.so
account sufficient //usr/$LIB/security/pam_krb5.so
password sufficient //usr/$LIB/security/pam_krb5.so

DESCRIPTION

The pam_krb5.so module is designed to allow smooth integration of Kerberos 5 password-checking for applications which use PAM. It creates session-specific credential caches. If the system is an AFS client, it will also attempt to obtain tokens for the local cell, the cell which contains the user's home directory, and any explicitly-configured cells.

When a user logs in, the module's authentication function performs a simple password check and, if possible, obtains Kerberos 5 credentials, caching them for later use. When the application requests initialization of credentials (or opens a session), the usual ticket files are created. When the application subsequently requests deletion of credentials or closing of the session, the module deletes the ticket files. When the application requests account management, if the module did not participate in authenticating the user, it will signal libpam to ignore the module. If the module did participate in authenticating the user, it will check for an expired user password and verify the user's authorization using the .k5login file of the user being authenticated, which is expected to be accessible to the module.

ARGUMENTS

debug

turns on debugging via syslog(3). Debugging messages are logged with priority LOG_DEBUG.

debug_sensitive

turns on debugging of sensitive information via syslog(3). Debug messages are logged with priority LOG_DEBUG.

afs_cells=cell.example.com[,...]

tells pam_krb5.so to obtain tokens for the named cells, in addition to the local cell, for the user. The module will guess the principal name of the AFS service for the named cells, or it can be specified by giving cell in the form cellname=principalname.

always_allow_localname

tells pam_krb5.so, when performing an authorization check using the target user's .k5login file, to always allow access when the principal name being authenticated maps to the local user's name (as configured using the auth_to_local_names and auth_to_local settings in krb5.conf(5), if your implementation provides those settings). Otherwise, if the file exists and can be read, but the principal is not explicitly listed, access is typically denied. This setting is disabled by default.

armor = true|false|service [...]

attempt to use armoring when communicating with the KDC. This option is currently mainly only useful for testing, as the keytab method should not be expected to work when the module is called from an unprivileged process, and the pkinit method requires that the KDC is properly configured to offer anonymous PKINIT, and that the client is also properly configured to trust the KDC's CA. The default is false.

armor_strategy = keytab,pkinit

controls how the module will attempt to obtain tickets for use as armor. The value should be a comma-separated list of methods. Supported methods include ketyab and pkinit. The default is keytab,pkinit.

banner=Kerberos 5

tells pam_krb5.so how to identify itself when users attempt to change their passwords. The default setting is "Kerberos 5".

ccache_dir=/tmp

tells pam_krb5.so which directory to use for storing credential caches. The default setting is /tmp.

ccname_template=DIR:/run/user/%U/krb5cc_XXXXXX

specifies the location in which to place the user's session-specific credential cache. This value is treated as a template, and these sequences are substituted:
  %u    login name

  %U    login UID

  %p    principal name

  %r    principal's realm name

  %h    home directory

  %d    the default ccache directory (as set with ccache_dir)

  %P    the current process ID

  %%    literal '%'

If the resulting template does not end with "XXXXXX", a suffix will be added to the configured value. If not set, the module attempts to read the default used by libkrb5 from krb5.conf(5), and if one is not found, the default is DIR:/run/user/%U/krb5cc_XXXXXX".

chpw_prompt

tells pam_krb5.so to allow expired passwords to be changed during authentication attempts. While this is the traditional behavior exhibited by "kinit", it is inconsistent with the behavior expected by PAM, which expects authentication to (appear to) succeed, only to have password expiration be flagged by a subsequent call to the account management function. Some applications which don't handle password expiration correctly will fail unconditionally if the user's password is expired, and this flag can be used to attempt to work around this bug in those applications. The default is false.

cred_session

specifies that pam_krb5 should create and destroy credential caches, as it does when the calling application opens and closes a PAM session, when the calling application establishes and deletes PAM credentials. This is done to compensate for applications which expect to create a credential cache but which don't use PAM session management. It is usually a harmless redundancy in applications which don't require it, so this option is enabled by default except for these services: "sshd".

external

external=sshd

tells pam_krb5.so to use Kerberos credentials provided by the calling application during session setup. This is most often useful for obtaining AFS tokens.

ignore_afs=true|false|service [...]

tells pam_krb5.so to completely ignore the presence of AFS, preventing any attempts to obtain new tokens on behalf of the calling application.

ignore_k5login

specifies that pam_krb5 should skip checking the user's .k5login file to verify that the principal name of the client being authenticated is authorized to access the user account. (Actually, the check is performed by a function offered by the Kerberos library, which controls which files it will consult.) The default is to perform the check.

ignore_unknown_principals

ignore_unknown_spn

ignore_unknown_upn

specifies that not pam_krb5 should return a PAM_IGNORE code to libpam instead of PAM_USER_UNKNOWN for users for whom the determined principal name is expired or does not exist.

keytab=FILE:/etc/krb5.keytab

tells pam_krb5.so the location of a keytab to use when validating credentials obtained from KDCs.

minimum_uid=0

tells pam_krb5.so to ignore authentication attempts by users with UIDs below the specified number.

multiple_ccaches

specifies that pam_krb5 should maintain multiple credential caches for this service, because it both sets credentials and opens a PAM session, but it sets the KRB5CCNAME variable after doing only one of the two. This option is usually not necessary for most services.

no_initial_prompt

tells pam_krb5.so to not ask for a password before attempting authentication, and to instead allow the Kerberos library to trigger a request for a password only in cases where one is needed.

no_subsequent_prompt

tells pam_krb5.so to only provide the previously-entered password in response to any request for a password which the Kerberos library might make. If the calling application does not properly support PAM conversations (possibly due to limitations of a network protocol which it is serving), this may be need to be used to prevent the application from supplying the user's current password in a password-changing situations when a new password is called for.

no_user_check

tells pam_krb5.so to not check if a user exists on the local system, to skip authorization checks using the user's .k5login file, and to create ccaches owned by the current process's UID. This is useful for situations where a non-privileged server process needs to use Kerberized services on behalf of remote users who may not have local access. Note that such a server should have an encrypted connection with its client in order to avoid allowing the user's password to be eavesdropped.

no_validate

no_validate=vlock

tells pam_krb5.so to not attempt to use the local keytab to verify that the TGT obtained from the realm's servers has not been spoofed. The libdefaults verify_ap_req_nofail setting can affect whether or not errors reading the keytab which are encountered during validation will be suppressed.

null_afs

tells pam_krb5.so, when it attempts to set tokens, to try to get credentials for services with names which resemble afs@REALM before attempting to get credentials for services with names resembling afs/cell@REALM. The default is to assume that the cell's name is the instance in the AFS service's Kerberos principal name.

preauth_options=[]

controls the preauthentication options which pam_krb5 passes to libkrb5, if the system-defaults need to be overridden. The list is treated as a template, and these sequences are substituted:

  %u    login name

  %U    login UID

  %p    principal name

  %r    principal's realm name

  %h    home directory

  %d    the default ccache directory

  %P    the current process ID

  %%    literal '%'

A list of recognized values should be listed in the kinit(1) manual page as parameters for its -X option.

pwhelp=filename

specifies the name of a text file whose contents will be displayed to clients who attempt to change their passwords. There is no default.

realm=realm

overrides the default realm set in /etc/krb5.conf, which pam_krb5.so will attempt to authenticate users to.

tokens

tokens=imap

signals that pam_krb5.so should create a new AFS PAG and obtain AFS tokens during authentication in addition to session setup. This is primarily useful in server applications which need to access a user's files but which do not open PAM sessions before doing so. A properly-written server will not need this flag set in order to function correctly.

trace

turns on libkrb5's library tracing. Trace messages are logged to syslog(3) with priority LOG_DEBUG.

try_first_pass

tells pam_krb5.so to check the previously-entered password as with use_first_pass, but to prompt the user for another one if the previously-entered one fails. This is the default mode of operation.

use_first_pass

tells pam_krb5.so to get the user's entered password as it was stored by a module listed earlier in the stack, usually pam_unix or pam_pwdb, instead of prompting the user for it.

use_authtok

tells pam_krb5.so to never prompt for new passwords when changing passwords. This is useful if you are using pam_cracklib or pam_passwdqc to try to enforce use of less-easy-to-guess passwords.

use_shmem

use_shmem=sshd

tells pam_krb5.so to pass credentials from the authentication service function to the session management service function using shared memory, or to do so for specific services.

validate_user_user

validate_user_user=gnome-screensaver

specifies that, when attempting validation of the TGT, the module should attempt user-to-user authentication using a previously-obtainted TGT in the default ccache if validation can't be performed using a keytab.

FILES

/etc/krb5.conf

BUGS

Probably, but let's hope not. If you find any, please file them in the bug database at http://bugzilla.redhat.com/ against the "pam_krb5" component.

AUTHOR

Nalin Dahyabhai <nalin@redhat.com>