pam_krb5

Section: System Administrator's Manual (8)
Updated: 2009/12/11
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NAME

pam_krb5 - Kerberos 5 authentication

SYNOPSIS

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

addressless

tells pam_krb5.so to obtain credentials without address lists. This may be necessary if your network uses NAT, and should otherwise not be used. This option is deprecated in favor of the noaddresses flag in the libdefaults section of krb5.conf(5).

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.

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=FILE:%d/krb5cc_%U_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    realm name

  %h    home directory

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

  %P    the current process ID

  %%    literal '%'

The default setting is "FILE:%d/krb5cc_%U_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 services in this list: "sshd".

existing_ticket

tells pam_krb5.so to accept the presence of pre-existing Kerberos credentials provided by the calling application in the default credential cache as sufficient to authenticate the user, and to skip any account management checks.

DANGER! Unless validation is also in use, it is relatively easy to produce a credential cache which looks "good enough" to fool pam_krb5.so.

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.

forwardable

tells pam_krb5.so that credentials it obtains should be forwardable. This option is deprecated in favor of the forwardable option in the libdefaults section of krb5.conf(5).

hosts=host[,...]

tells pam_krb5.so to obtain credentials using the addresses of the given hosts in addition to the addresses of interfaces on the local workstation. For example, if your workstation is behind a masquerading firewall, specifying the firewall's outward-facing address here should allow Kerberos authentication to succeed. This option is deprecated in favor of the extra_addresses flag in the libdefaults section of krb5.conf(5).

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 ccache files 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    realm name

  %h    home directory

  %d    the default ccache directory

  %P    the current process ID

  %%    literal '%'

proxiable

tells pam_krb5.so that credentials it obtains should be proxiable. This option is deprecated in favor of the proxiable option in the libdefaults section of krb5.conf(5).

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.

renew_lifetime=36000

sets the default renewable lifetime for credentials. This option is deprecated in favor of the renew_lifetime option in the libdefaults section of krb5.conf(5).

ticket_lifetime=36000

sets the default lifetime for credentials.

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.

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.

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>

This document was created by man2html, using the manual pages.
Time: 05:34:28 GMT, December 24, 2015