Difference between revisions of "Pam abl"

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{{Article summary start|Summary}}
{{Article summary start|Summary}}

Revision as of 18:06, 10 June 2011

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Pam_abl provides another layer of security against brute-force SSH password guessing. It allows you to set a maximum number of unsuccessful login attempts within a given time period, after which a host and/or user is blacklisted. Once a host/user is blacklisted, all authentication attempts will fail even if the correct password is given. Hosts/users which stop attempting to login for a specified period of time will be removed from the blacklist.


Install the pam_abl PKGBUILD from the AUR using Makepkg.


Add pam_abl to the PAM auth stack

Open Template:Codeline as root in your editor of choice. Add the following line above all other lines:

auth            required        pam_abl.so config=/etc/security/pam_abl.conf

Assuming you haven't made any other modifications, your Template:Codeline should now look like this:

#auth           required        pam_securetty.so        #Disable remote root
auth            required        pam_abl.so config=/etc/security/pam_abl.conf
auth            required        pam_unix.so
auth            required        pam_env.so
account         required        pam_nologin.so
account         required        pam_unix.so
account         required        pam_time.so
password        required        pam_unix.so
session         required        pam_unix_session.so
session         required        pam_limits.so

Create pam_abl.conf

Create Template:Codeline as root using your editor of choice.

A sample Template:Codeline is as follows:

# /etc/security/pam_abl.conf

The paths given in Template:Codeline and Template:Codeline specify where the blacklists should be stored. Typical paths are Template:Codeline and Template:Codeline, respectively.

The values given in Template:Codeline and Template:Codeline specify the time period before hosts/users are removed from the blacklist. Values are specifed as Template:Codeline where suffix can be any of Template:Codeline, Template:Codeline, Template:Codeline, or Template:Codeline for units of seconds, minutes, hours or days, respectively.

The rules specified in Template:Codeline and Template:Codeline are specified as Template:Codeline. Template:Codeline is a list of user names separated by |s. The special user name Template:Codeline matches all users, and prefixing a user by a Template:Codeline matches all users except the one named. Template:Codeline is the number of attempts allowed before a user/host is blacklisted, and Template:Codeline specifies the period in which the attempts must occur. The same time suffixes as described above also apply to Template:Codeline.

For example, the rule Template:Codeline specifies that for any user, ten failed login attempts within an hour will get the host blacklisted. The rule Template:Codeline specifies that for any user except root, ten failed login attempts within an hour will get the user blacklisted, regardless of the host the attempts are coming from.

Warning: Whether or not to include root in the user_rule must be carefully considered. Not including root has obvious security implications. On the other hand, including root gives hackers the ability to block anyone from logging in as root by making repeated failed attempts.

Multiple conditions can be given to the same set of users using comma separation:


Multiple rules can be specified using space separation:

user_rule=!root:10/1h root:25/1h

If you only want pam_abl to blacklist one of users or hosts, simply omit the appropriate lines from Template:Codeline.

Create the blacklist databases

As root, create the directory for the database (assuming you specified the recommended path above):

# mkdir /var/lib/abl

As root, run the pam_abl utility to initialize the databases:

# pam_abl

That's it! Pam_abl should now be working. Since PAM is not a daemon, nothing needs to be restarted for these changes to take effect. It's strongly recommended to verify that pam_abl is working by purposely getting a remote host blacklisted. Don't worry though! For directions on how to manually remove a host or user from the blacklist, see below.

Managing the blacklist databases

Check blacklisted hosts/users

As root, simply run:

# pam_abl
Note: As pam_abl does not run as a daemon, it performs "lazy purging" of the blacklist. In other words, it does not remove users/hosts from the blacklist until an authentication attempt occurs. This does not affect functionality, although it will frequently cause extra failures to show up when running the above command. To force a purge, run:
# pam_abl -p

Manually removed a host or user from the blacklist

As root, simply run:

# pam_abl -w -U <user>


# pam_abl -w -H <host>

Using * as a wildcard to match multiple hosts/users is allowed in both of the above commands.

Manually add a host or user to the blacklist

As root, simply run:

# pam_abl -f -U <user>


# pam_abl -f -H <host>

Other pam_abl commands

Like virtually all linux utilities, a manpage is available to see all options:

$ man pam_abl