First attempt on DNSBL documentation

[ci skip]

doc/README.md

@@ -67,6 +67,7 @@
 - [Reply by email](incoming_email/README.md) Allow users to comment on issues and merge requests by replying to notification emails.
 - [Migrate GitLab CI to CE/EE](migrate_ci_to_ce/README.md) Follow this guide to migrate your existing GitLab CI data to GitLab CE/EE.
 - [Git LFS configuration](workflow/lfs/lfs_administration.md)
+- [DNS-based Blackhole List](administration/dnsbl.md) Check against DNSBLs at sign-up.
 ## Contributor documentation

doc/administration/dnsbl.md

+# DNS-based Blackhole List
+
+_**Note:** This feature was [introduced][ce-2455] in GitLab 8.4._
+
+---
+
+GitLab supports a DNSBL mechanism which checks for blacklisted IPs during
+sign-up. In conjunction with [reCAPTCHA](../integration/recaptcha.md), it
+serves as a mean to fight against spam on GitLab instances that have public
+sign-up enabled.
+
+Excerpt from [Wikipedia][wiki-dnsbl]:
+
+> A DNS-based Blackhole List (DNSBL) or Real-time Blackhole List (RBL) is an
+> effort to stop email spamming. It is a "blacklist" of locations on the
+> Internet reputed to send email spam. The locations consist of IP addresses
+> which are most often used to publish the addresses of computers or networks
+> linked to spamming; most mail server software can be configured to reject or
+> flag messages which have been sent from a site listed on one or more such
+> lists. The term "Blackhole List" is sometimes interchanged with the term
+> "blacklist" and "blocklist".
+
+## How DNSBL works
+
+| Attribute | Description |
+| --------- | ----------- |
+| threshold | |
+| domain    | |
+| weight    | |
+
+## Enable DNSBL
+
+The configuration is done via `gitlab.yml` and access to the server that hosts
+GitLab is required.
+
+### Enable DNSBL on source installations
+
+Make sure your `/home/git/gitlab/config/gitlab.yml` is updated and then edit it
+to match your preferences (see [How DNSBL works](#how-dnsbl-works)).
+
+There are some defaults in place which you can use as-is. The minimal change
+you need to do, is set `enabled` to `true`:
+
+```yaml
+dnsbl_check:
+  enabled: true
+  treshold: 0.3
+  lists:
+    - domain: list.blogspambl.com
+      weight: 6
+    - domain: all.s5h.net
+      weight: 4
+```
+
+### Enable DNSBL on Omnibus installations
+
+
+[ce-2455]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2455
+[wiki-dnsbl]: https://en.wikipedia.org/wiki/DNSBL "Wikipedia on DNS-based Blackhole Lists"