PAM_ABL(8) Linux-PAM Manual PAM_ABL(8)
NAME
pam_abl - PAM Auto Blacklist Module
SYNOPSIS
Provides auto blacklisting of hosts and users responsible for repeated failed authentication attempts. Generally configured so that
blacklisted users still see normal login prompts but are guaranteed to fail to authenticate.
This functionality is only available to services which call PAM as root. If pam_abl is called for uid != 0 it will silently succeed.
DESCRIPTION
Brute force password discovery attacks involve repeated attempts to authenticate against a service using a dictionary of common passwords.
While it is desirable to enforce strong passwords for users this is not always possible and in cases where a weak password has been used
brute force attacks can be effective.
The pam_abl module monitors failed authentication attempts and automatically blacklists those hosts (and accounts) that are responsible for
large numbers of failed attempts. Once a host is blacklisted it is guaranteed to fail authentication even if the correct credentials are
provided.
Blacklisting is triggered when the number of failed authentication attempts in a particular period of time exceeds a predefined limit.
Hosts which stop attempting to authenticate will, after a period of time, be un-blacklisted.
Commands can be specified which will be run when a host or user switches state from being blocked to clear or clear to blocked. See below
or the pam_abl.conf(5) manpage for the details.
If pam_abl is called for uid != 0 it will silently succeed. If this was not the case it would be possible for a malicious local user to
poison the pam_abl data by, for example, discovering the names of the hosts from which root typically logs in and then constructing PAM
authentication code to lock out root login attempts from those hosts.
OPTIONS
Name Arguments Description
debug None Enable debug output to syslog.
expose_account None Ignored
no_warn None Disable warnings which are otherwise
output to syslog. try_first_pass
None Ignored
use_first_pass None Ignored
use_mapped_pass None Ignored
config Path to the configuration file. The configuration file contains
additional arguments. In order for
the pam_abl command line tool to
work correctly most of the
configuration should be placed in
the config file rather than being
provided by arguments. The format of
the config file is described below.
host_db Path to host database file. Path to the Berkeley DB which is
used to log the host responsible for
failed authentication attempts.
host_purge Purge time for the host database. Defines how long failed hosts are
retained in the host database.
Defaults to 1 day.
host_rule Rule for host blacklisting. The rule (see below for format)
which defines the conditions under
which a failed hosts will be
blackisted.
host_blk_cmd Host block command Command that should be run when a
host is checked, and is currently
blocked. Within the command, the
strings %u, %h and %s are
substituted with username, host and
service. Not all need to be used. If
substitution is called for, but
there is no iformation for that
field, the command will not run.
host_clr_cmd Host block command Command that should be run when a
host is checked, and is currently
clear. Within the command, the
strings %u, %h and %s are
substituted with username, host and
service. Not all need to be used. If
substitution is called for, but
there is no iformation for that
field, the command will not run.
user_db Path to user database file. Path to the Berkeley DB which is
used to log the user responsible for
failed authentication attempts.
user_purge Purge time for the user database. Defines how long failed users are
retained in the user database.
Defaults to 1 day.
user_rule Rule for user blacklisting. The rule (see below for format)
which defines the conditions under
which a failed users will be
blackisted.
user_blk_cmd Host block command Command that should be run when a
user is checked, and is currently
blocked. Within the command, the
strings %u, %h and %s are
substituted with username, host and
service. Not all need to be used. If
substitution is called for, but
there is no iformation for that
field, the command will not run.
user_clr_cmd Host block command Command that should be run when a
user is checked, and is currently
clear. Within the command, the
strings %u, %h and %s are
substituted with username, host and
service. Not all need to be used. If
substitution is called for, but
there is no iformation for that
field, the command will not run.
USAGE
Typically pam_abl.so is added to the auth stack as a required module just before whatever modules actually peform authentication. Here's a
fragment of the PAM config for a production server that is running pam_abl:
auth required /lib/security/pam_env.so auth required /lib/security/pam_abl.so config=/etc/security/pam_abl.conf auth sufficient
/lib/security/pam_unix.so likeauth nullok auth required /lib/security/pam_deny.so
Although all of accepted arguments can be supplied here they will usually be placed in a separate config file and linked to using the
config argument as in the above example. The pam_abl command line tool reads the external config file (/etc/security/pam_abl.conf in this
case) to find the databases so in order for it work correctly an external config should be used.
EXAMPLES
auth required /lib/security/pam_env.so
auth required /lib/security/pam_abl.so config=/etc/security/pam_abl.conf
auth sufficient /lib/security/pam_unix.so likeauth nullok
auth required /lib/security/pam_deny.so
SEE ALSO
pam_abl.conf(5), pam_abl(1)
AUTHORS
Andy Armstrong <andy@hexten.net>
Chris Tasma <pam-abl@deksai.com>
GNU
01/13/2010 PAM_ABL(8)
