Pam abl

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Summary help replacing me
Using pam_abl to increase sshd security
Related
SSH
Using SSH Keys
A Cure for the Common SSH Login Attack

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.

Installation

Install the pam_abl PKGBUILD from the AUR using Makepkg.

Configuration

Add pam_abl to the PAM auth stack

Open /etc/pam.d/sshd 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 /etc/pam.d/sshd should now look like this:

#%PAM-1.0
auth            required        pam_abl.so config=/etc/security/pam_abl.conf
auth            include         system-login
account         include         system-login
password        include         system-login
session         include         system-login

Create pam_abl.conf

Create /etc/security/pam_abl.conf as root using your editor of choice.

Note: As of pam_abl 0.4.3, an extra line is required in pam_abl.conf to specify db_home. Auxiliary database files are now created in that directory.

A sample /etc/security/pam_abl.conf is as follows:

# /etc/security/pam_abl.conf
db_home=/var/lib/abl/
host_db=/var/lib/abl/hosts.db
host_purge=7d
host_rule=*:10/1h
user_db=/var/lib/abl/users.db
user_purge=7d
user_rule=!root:10/1h

The paths given in host_db and user_db specify where the blacklists should be stored. Typical paths are /var/lib/abl/hosts.db and /var/lib/abl/users.db, respectively.

The values given in host_purge and user_purge specify the time period before hosts/users are removed from the blacklist. Values are specifed as <number><suffix> where suffix can be any of s, m, h, or d for units of seconds, minutes, hours or days, respectively.

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

For example, the rule *:10/1h specifies that for any user, ten failed login attempts within an hour will get the host blacklisted. The rule !root:10/1h 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:

user_rule=!root:10/1h,20/1d

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 /etc/security/pam_abl.conf.

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>

or

# 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>

or

# 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

Known Issues

The current version (0.5.0) of pam_abl has a problem that can affect its ability to blacklist under specific conditions.

Due to the way sshd operates and the way pam modules are passed information, failure of a given attempt is not logged until either a second attempt is made or the connection is closed. This means that long as the attacker only makes one attempt per connection, and never closes any connections, no failures are ever logged.

In practice, the sshd_config settings "MaxStartups" (default 10) and to a lesser degree "LoginGraceTime" (default 120s) limit the viability of this approach, but it still could be used to squeeze out more attempts then you specify.

In the meantime, the workaround is to set "MaxAuthTries" to 1 (or expect that an additional "MaxStartups" number of attempts could be made above and beyond what you specify in your pam_abl config).