Table of Contents

3.1. Syntax
3.2. Rule syntax


pam_abl.conf - Configuration file for pam_abl PAM module.


Configuration file for both the pam_abl(8) PAM module, and the pam_abl(1) command line tool.


3.1. Syntax

word        ::= /[^\s\|\/\*]+/
name        ::= word | '*'
username    ::= name
servicename ::= name
userservice ::= username
            |   username '/' servicename
namelist    ::= userservice
            |   userservice '|' namelist
userspec    ::= namelist
            |   '!' namelist
multiplier  ::= 's' | 'm' | 'h' | 'd'
number      ::= /\d+/
period      ::= number
            |   number multiplier
trigger     ::= number '/' period
triglist    ::= trigger
            |   trigger ',' triglist
userclause  ::= userspec ':' triglist
rule        ::= userclause
            |   userclause /\s+/ rule

3.2. Rule syntax

Each rule consists of a number of space separated user clauses. A user clause specifies the user (and service) names to match and a set of triggers. A simple example would be


which means block any user (*) if they are responsible for ten or more failed authentication attempts in the last hour. In place of the *, which matches any user, a list of usernames can be supplied like this


which means block the users root, dba and admin if they are responsible for ten or more failed authentication attempts in the last hour. You can also specify a service name to match against like this


which means block the users root for service sshd and dba for any service if they are responsible for three or more failed authentication attempts in the last day. Finally you can specify multiple triggers like this


which means block the user root if they are responsible for ten or more failed attempts in the last hour or twenty or more failed attempts in the last day.

Multiple rules can be provided separated by spaces like this

*:10/1h root:5/1h,10/1d

in which case all rules that match a particular user and service will be checked. The user or host will be blocked if any of the rule triggers matches. The sense of the user matching can be inverted by placing a ! in front of the rule so that


is a rule which would match for all users apart from root. It is important to treat root as a special case in the user_rule otherwise excessive attempts to authenticate as root will result in the root account being locked out even for valid holders of root credentials. The config file can contain any arguments that would be supplied via PAM config. In the config file arguments are placed on separate lines. Comments may be included after a #, and line continuation is possible by placing a back slash at the end of the line to be continued. Here is a sample /etc/security/pam_abl.conf:

# /etc/security/pam_abl.conf

All of the standard PAM arguments (debug, expose_account, no_warn, try_first_pass, use_first_pass, use_mapped_pass) are accepted; with the exception of debug and no_warn these are ignored.

The arguments that are specific to the PAM module are as follows:

Specify the directory where the Berkeley db can store its lock and log files. Make sure this directory exists and is writable.
Its value should have the following syntax "<minimum>-<maximum>". If you do not block hosts (with iptables for example) that do too many attempts, the db can easily become bloated. To prevent this we introduced this setting. As soon as there are a <maximum> number of attempts for a user/host, the number of stored attempts for this user/host is reduced to <minimum>. A <maximum> of 0 means no limits. Make sure that <minimum> is larger than any rule specified. We recommend a value of "1000-1200".
host_db, user_db
Specify the name of the databases that will be used to log failed authentication attempts. The host database is used to log the hostname responsible for a failed auth, and the user database is used to log the requested username. If host_db or user_db is omitted the corresponding auto blacklisting will be disabled.
host_purge, user_purge
Specify the length of time for which failed attempts should be kept in the databases. For rules to work correctly this must be at least as long as the longest period specified in a corresponding rule. You may wish to retain information about failed attempts for longer than this so that the pam_abl command line tool can report information over a longer period of time. The format for this item is a number with an optional multiplier suffix, s, m, h or d which correspond with seconds, minutes, hours and days. To specify seven days for example one would use 7d. Note that in normal operation pam_abl will only purge the logged data for a particular host or user if it happens to be updating it, i.e. if that host or user makes another failed attempt. To purge all old entries, the pam_abl command line tool should be used.
host_rule, user_rule
These are the rules used to determine the circumstances under which accounts are auto-blacklisted. The host_rule is used to block access to hosts that are responsible for excessive authentication failures and the user_rule is used to disable accounts for which there have been excessive authentication failures. The rule syntax is described in full below.
host_clr_cmd, host_blk_cmd, user_clr_cmd, user_blk_cmd
Deprecated for security reasons. Please use the corresponding safer option: host_clear_cmd, host_block_cmd, user_clear_cmd, user_block_cmd
host_clear_cmd, host_block_cmd, user_clear_cmd, user_block_cmd
These specify commands that will run during a check when an item switches state since its last check. host_clear_cmd and user_clear_cmd will run if the host or user is currently allowed access, but was blocked during the previous attempt. Obviously this will not work if you previously blocked them with iptables, since they will never be able to trigger this command. In this case, you can’t rely on pam_abl to unblock them. One possibility would be to use special firewall chains which can be cleared automatically with a script and a cron job. host_block_cmd and user_block_cmd are run if the current attempt causes a previously allowed host or user to be blocked. Within the commands, you can specify substitutions with %h, %u and %s, which will be replaced with the host name, user name and service currently being checked. For security reasons we do not run the command using the system call. We use the more secure fork/exec solution. This means that you can’t specify input and output redirections. Note that this also means that no escaping is done, so if you call a shell here, you might introduce a security problem. The commands should follow a special syntax (you can use the command line tool with the -d option to test the parsing of your commands) where the command and its different arguments need to be enclosed in [] and all text not enclosed in [] is simply ignored. For example: "[/usr/bin/logger] ignored [block] [user] [%u]" will run the command "/usr/bin/logger block user <current user>". If you want to specify a [, ] or \, you need to escape them with a \.
host_whitelist, user_whitelist
A semi-colon separated list of hosts or users whose attempts will not be recorded. So if an attempt is made from "" for user "root" and "root" is in the whitelist, only an attempt for his machine is recorded. If a user is whitelisted, this does not prevent his machine from being blocked. Hosts can be specified using their IP ( or using a netmask (


# /etc/security/pam_abl.conf
host_block_cmd=[/sbin/iptables] [-I] [INPUT] [-s] [%h] [-j] [DROP]
user_clear_cmd=[/usr/bin/logger] [block] [user] [%u]


pam_abl.conf(5), pam_abl(1)


Lode Mertens

Andy Armstrong

Chris Tasma