Add note to troubleshoot pgbouncer failing to connect to pg due to missing auth cidr address

doc/administration/high_availability/database.md

@@ -296,7 +296,7 @@ On each database node perform the following:
 
     # Replace XXX.XXX.XXX.XXX/YY with Network Address
     postgresql['trust_auth_cidr_addresses'] = %w(XXX.XXX.XXX.XXX/YY)
-    repmgr['trust_auth_cidr_addresses'] = %w(XXX.XXX.XXX.XXX/YY)
+    repmgr['trust_auth_cidr_addresses'] = %w(127.0.0.1/32 XXX.XXX.XXX.XXX/YY)
 
     # Replace placeholders:
     #
@@ -537,6 +537,12 @@ Ensure that all migrations ran:
 gitlab-rake gitlab:db:configure
 ```
 
+> **Note**: If you encounter a `rake aborted!` error stating that PGBouncer is failing to connect to
+PostgreSQL it may be that your PGBouncer node's IP address is missing from
+PostgreSQL's `trust_auth_cidr_addresses` in `gitlab.rb` on your database nodes. See 
+[PGBouncer error `ERROR:  pgbouncer cannot connect to server`](#pgbouncer-error-error-pgbouncer-cannot-connect-to-server) 
+in the Troubleshooting section before proceeding.
+
 #### Ensure GitLab is running
 
 At this point, your GitLab instance should be up and running. Verify you are
@@ -966,6 +972,34 @@ For PostgreSQL, it is usually safe to restart the master node by default. Automa
 
 On the consul server nodes, it is important to restart the consul service in a controlled fashion. Read our [consul documentation](consul.md#restarting-the-server-cluster) for instructions on how to restart the service.
 
+#### PGBouncer error `ERROR:  pgbouncer cannot connect to server`
+
+You may get this error when running `gitlab-rake gitlab:db:configure` or you 
+may see the error in the PGBouncer log file. 
+
+```
+PG::ConnectionBad: ERROR:  pgbouncer cannot connect to server
+```
+
+The problem may be that your PGBouncer node's IP address is not included in the
+`trust_auth_cidr_addresses` setting in `/etc/gitlab/gitlab.rb` on the database nodes. 
+
+You can confirm that this is the issue by checking the PostgreSQL log on the master
+database node. If you see the following error then `trust_auth_cidr_addresses` 
+is the problem.
+
+```
+2018-03-29_13:59:12.11776 FATAL:  no pg_hba.conf entry for host "123.123.123.123", user "pgbouncer", database "gitlabhq_production", SSL off
+```
+
+To fix the problem, add the IP address to `/etc/gitlab/gitlab.rb`. 
+
+```
+postgresql['trust_auth_cidr_addresses'] = %w(123.123.123.123/32 <other_cidrs>)
+```
+
+[Reconfigure GitLab] for the changes to take effect.
+
 #### Issues with other components
 
 If you're running into an issue with a component not outlined here, be sure to check the troubleshooting section of their specific documentation page.