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Simple login lock down is a way to protect your WordPress blog from brute force login attacks.
How it works: 
1. An attacker attempts to login and fails 
2. Simple Login Lockdown record that failed login 
3. After a certain number of failed attemps (defaults to five), further attemps to access the wp-login.php page are blocked for a time (defaults to one hour).
If you happen to forget your password and make a failed login attemp yourself, the plugin will clear out the lockdown count data on successful login.
Note: This uses $_SERVER[‘REMOTE_ADDR’] directly. If you’re behind a proxy (load balancer, etc), it’s not going to work as expected. Eg. Several folks could be attempting logins at once, and all fail. As such, the plugin would pick up on all those requests coming from the same IP — the load balancer — and lock the login down. No good. If you’re using a load balancer or in some other situation where you’re behind a proxy, use this as an example and write your own. Or filter the IP as your desire using <code>cd_sll_pre_ip</code>.
<h3>Hooks</h3>
<pre><code>simple_login_lockdown_ip -- Alter the requesting IP address. Might be useful if you site is behind a proxy or load balancer.

simple_login_lockdown_allow_ip -- Allows you to "whitelist" an IP address. It first when a log attempt fails before the attempt count is incremented. Return true and no count will be taken for the IP.

simple_login_lockdown_should_die -- A filter that allows you to prevent the login page from `die`ing if a the requesting IP is temporarily blacklisted or the login limit has been reached.

simple_login_lockdown_count_reached -- Fires when the requesting IP has reached its count and will be added to the blacklist for your time limit.

simple_login_lockdown_attempt -- Fires when a login attempt is made but the requestin IP is blocked to to excessive requests.

simple_login_lockdown_response -- Change the HTTP response code of that gets sent when a blacklisted IP attempts to login.

simple_login_lockdown_time_values -- Allows you to alter values in the login lockdown time dropdown in the admin area.
</code></pre>

Simple Login Lockdown

Simple Login Lockdown prevents brute force login attacks/attempts on your WordPress installation.

Simple Login Lockdown vs Really Simple Security – Simple and Performant Security (formerly Really Simple SSL)