Add documentation for UNIX socket connections

Author: nathanjrobertson

docs/sql.md

@@ -180,6 +180,37 @@ Example query - SHA512 of salt + password, stored as salt (32 bytes) + sha256(sa
     )
 ```
 
+Connecting with UNIX Domain Sockets (Local Sockets)
+---------------------------------------------------
+
+When on a UNIX-like platform (Linux, *BSD, etc), and when your SQL database server is running on the same host as the web server
+hosting SimpleSAMLphp, it is possible to use UNIX domain sockets instead of TCP sockets for the database connection. This
+configuration should result in marginally better performance and security (when configured correctly).
+
+Here is an example using PostgreSQL:
+
+```php
+    'example-unix-socket-sql' => [
+        'sqlauth:SQL',
+        'dsn' => 'pgsql:host=/var/run/postgresql;dbname=simplesaml',
+        'username' => 'www-data',
+        'password' => 'this-is-ignored',
+        'query' => 'SELECT uid, givenName, email, eduPersonPrincipalName FROM users WHERE uid = :username ' .
+            'AND password = SHA2(CONCAT((SELECT salt FROM users WHERE uid = :username), :password), 256);',
+    ],
+```
+
+Configuration is largely the same as TCP sockets (documented above), with the differences being:
+
+`dsn`
+:   The key difference is that the `host` parameter. This needs to be the **directory** that contains the socket file used to connect to the PostgreSQL server. For example, actual socket file might be `/var/run/postgresql/.s.PGSQL.5432`, so `host=/var/run/postgresql` is the parameter that you need. If you're struggling to find where the socket is, the `unix_socket_directories` parameter in the server `postgresql.conf` is where that location is configured.
+
+`username`
+:   The UNIX username of the user running SimpleSAMLphp (ie. the web server user or the php-fpm user, depending on your setup).
+
+`password`
+:   Required, but the value you specify is ignored (so you can put any placeholder string value in there). All authentication for UNIX domain sockets are done by the operating system kernel.
+
 Security considerations
 -----------------------