pam_abl

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^AUR package.

Configuration

Add pam_abl to the PAM auth stack

Note: This will enable pam_abl for all the services using system-remote-login (like ssh, php etc.).

Open /etc/pam.d/system-remote-login as root in your editor of choice. Add the following line above all other lines:

/etc/pam.d/system-remote-login

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

Assuming you have not made any other modifications, your /etc/pam.d/system-remote-login should now look like this:

/etc/pam.d/system-remote-login

#%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

Edit pam_abl.conf

Edit /etc/security/pam_abl.conf to customize the rules and other settings.

See man pam_abl.conf for details.

Managing the blacklist databases

Check blacklisted hosts/users

As root, simply run:

# pam_abl

Manually remove 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>

Purging old users/hosts entries

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

# pam_abl -p

Other pam_abl commands

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

$ man pam_abl

Known issues

As of current version (1.0), 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).