Update rack attack docs

doc/security/rack_attack.md

-# Rack attack
+# Rack Attack
 
-To prevent abusive clients doing damage GitLab uses rack-attack gem.
+Rack Attack, also known as Rack::Attack, is [a rubygem](https://github.com/kickstarter/rack-attack)
+that is meant to protect GitLab with the ability to customize throttling and
+blocking user IPs.
+You can prevent brute-force passwords attacks, scrapers, or any other offenders
+by throttling requests from IP addresses making large volumes of requests.
+In case you find throttling is not enough to protect you against abusive clients,
+Rack Attack offers IP whitelisting, blacklisting, Fail2ban style filtering and
+tracking.
 
-If you installed or upgraded GitLab by following the official guides this should be enabled by default.
+By default, user sign-in, user sign-up (if enabled), and user password reset is
+limited to 6 requests per minute. After trying for 6 times, the client will
+have to wait for the next minute to be able to try again.
 
-If you are missing `config/initializers/rack_attack.rb` the following steps need to be taken in order to enable protection for your GitLab instance:
+If you installed or upgraded GitLab by following the [official guides](../install/README.md)
+this should be enabled by default. If your instance is not exposed to any incoming
+connections, it is recommended to disable Rack Attack.
 
-1.  In config/application.rb find and uncomment the following line:
+For more information on how to use these options check out
+[rack-attack README](https://github.com/kickstarter/rack-attack/blob/master/README.md).
 
-        config.middleware.use Rack::Attack
+## Settings
 
-1.  Rename `config/initializers/rack_attack.rb.example` to `config/initializers/rack_attack.rb`.
+**Omnibus GitLab**
 
-1.  Review the `paths_to_be_protected` and add any other path you need protecting.
+1. Open `/etc/gitlab/gitlab.rb` with you editor
+1. Add the following:
 
-1.  Restart GitLab instance.
+    ```ruby
+    gitlab_rails['rack_attack_git_basic_auth'] = {
+      'enabled' => true,
+      'ip_whitelist' => ["127.0.0.1"],
+      'maxretry' => 10,
+      'findtime' => 60,
+      'bantime' => 3600
+    }
+    ```
 
-By default, user sign-in, user sign-up(if enabled) and user password reset is limited to 6 requests per minute. After trying for 6 times, client will have to wait for the next minute to be able to try again. These settings can be found in `config/initializers/rack_attack.rb`
+3. Reconfigure GitLab:
 
-If you want more restrictive/relaxed throttle rule change the `limit` or `period` values. For example, more relaxed throttle rule will be if you set limit: 3 and period: 1.second(this will allow 3 requests per second). You can also add other paths to the protected list by adding to `paths_to_be_protected` variable. If you change any of these settings do not forget to restart your GitLab instance.
+    ```
+    sudo gitlab-ctl reconfigure
+    ```
 
-In case you find throttling is not enough to protect you against abusive clients, rack-attack gem offers IP whitelisting, blacklisting, Fail2ban style filter and tracking.
+The following settings can be configured:
 
-For more information on how to use these options check out [rack-attack README](https://github.com/kickstarter/rack-attack/blob/master/README.md).
+- `enabled`: By default this is set to `true`. Set this to `false` to disable Rack Attack.
+- `ip_whitelist`: Whitelist any IPs from being blocked. They must be formatted as strings within a ruby array.
+   For example, `["127.0.0.1", "127.0.0.2", "127.0.0.3"]`.
+- `maxretry`: The maximum amount of times a request can be made in the
+   specified time.
+- `findtime`: The maximum amount of time failed requests can count against an IP
+   before it's blacklisted.
+- `bantime`: The total amount of time that a blacklisted IP will be blocked in
+   seconds.
+
+**Installations from source**
+
+These settings can be found in `config/initializers/rack_attack.rb`. If you are
+missing `config/initializers/rack_attack.rb`, the following steps need to be
+taken in order to enable protection for your GitLab instance:
+
+1. In `config/application.rb` find and uncomment the following line:
+
+    ```ruby
+    config.middleware.use Rack::Attack
+    ```
+
+1. Copy `config/initializers/rack_attack.rb.example` to `config/initializers/rack_attack.rb`
+1. Open `config/initializers/rack_attack.rb`, review the
+   `paths_to_be_protected`, and add any other path you need protecting
+1. Restart GitLab:
+
+    ```sh
+    sudo service gitlab restart
+    ```
+
+If you want more restrictive/relaxed throttle rules, edit
+`config/initializers/rack_attack.rb` and change the `limit` or `period` values.
+For example, more relaxed throttle rules will be if you set
+`limit: 3` and `period: 1.seconds` (this will allow 3 requests per second).
+You can also add other paths to the protected list by adding to `paths_to_be_protected`
+variable. If you change any of these settings do not forget to restart your
+GitLab instance.
+
+## Remove blocked IPs from Rack Attack via Redis
+
+In case you want to remove a blocked IP, follow these steps:
+
+1. Find the IPs that have been blocked in the production log:
+
+    ```sh
+    grep "Rack_Attack" /var/log/gitlab/gitlab-rails/production.log
+    ```
+
+2. Since the blacklist is stored in Redis, you need to open up `redis-cli`:
+
+    ```sh
+    /opt/gitlab/embedded/bin/redis-cli -s /var/opt/gitlab/redis/redis.socket
+    ```
+
+3. You can remove the block using the following syntax, replacing `<ip>` with
+   the actual IP that is blacklisted:
+
+    ```
+    del cache:gitlab:rack::attack:allow2ban:ban:<ip>
+    ```
+
+4. Confirm that the key with the IP no longer shows up:
+
+    ```
+    keys *rack::attack*
+    ```
+
+5. Optionally, add the IP to the whitelist to prevent it from being blacklisted
+   again (see [settings](#settings)).
+
+## Troubleshooting
+
+### Rack attack is blacklisting the load balancer
+
+Rack Attack may block your load balancer if all traffic appears to come from
+the load balancer. In that case, you will need to:
+
+1. [Configure `nginx[real_ip_trusted_addresses]`](https://docs.gitlab.com/omnibus/settings/nginx.html#configuring-gitlab-trusted_proxies-and-the-nginx-real_ip-module).
+   This will keep users' IPs from being listed as the load balancer IPs.
+2. Whitelist the load balancer's IP address(es) in the Rack Attack [settings](#settings).
+3. Reconfigure GitLab:
+
+    ```
+    sudo gitlab-ctl reconfigure
+    ```
+
+4. [Remove the block via Redis.](#remove-blocked-ips-from-rack-attack-via-redis)