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Automatic SSH Blocking


4.0 - Updated on 06/19/2021 by Denman, Jon

3.0 - Authored on 09/11/2019 by Frederick, Jon

Global network access

Because the Khoury Community is global, the College has chosen to make many of our systems available from off-campus without restriction. An unfortunate side effect of this is that many of these systems are the focus of constant automated intrusion attempts by those who would like to make illegitimate use of the College's resources to further their own ends.

In order to balance the remote access needs of the College community with the security necessary to provide useful (eg: safe, secure) services, the Systems group has implemented various security measures designed to detect and block these remote attackers. One such measure is a system which tracks failed ssh logins, and temporarily bans IP addresses from further connection attempts after they have exceeded a certain threshold for failed logins.


Your IP address will be banned for 10 hours after 5 unsuccessful attempts.

Accidental Lockouts

If you believe you may have accidentally locked yourself out, please read the following notes carefully:

  1. Blocks are implemented on per-host basis. ie: If you lock yourself out of login, that doesn't affect your ability to access lab machines.
  2. Blocks are implemented for ssh only. ie: Getting blocked from ssh on login will not affect your ability to browse, or your ability to access any other Khoury resources.
  3. Blocks are by ip address. They are not by account. This often manifests as an ability to ssh from some network locations, but not others.

The implications of the above points are as follows:

  1. If you think you've been blocked on a specific host, the fastest and easiest way to get back up and running is to try another host.* More information on publicly accessible hosts is available on our Linux at Khoury page.
  2. Changing your password will NOT undo a block on your IP address.

*: Note that anything that you can do on one Khoury Linux host you can do on another Khoury Linux host. And you can always ssh from one Khoury Linux host to another (eg: if you need to reconnect to a long running process).

If necessary, we can check to see if your IP address is blocked on a given host (and potentially unblock it*), but we'll need your public IP address. ie: The IP address that we (and the rest of the world) see on inbound requests from your computer. Due to Network Address Translation (common on many home and business networks) and other similar technologies, this may or may not be the IP address your computer sees itself as having.

In order to check your public IP address, use a site such as,, or to determine your public IP, then please mail your IP to, with a request that we check to see if it has been blocked from a given ssh server (eg:

*: Note, because you can easily switch from using one ssh server to another, and because blocks will automatically be expired within 24 hours, we generally only unblock IP addresses for special and extreme cases. As noted above: If you think you've locked yourself, please just use another machine!