Proxy: add documentation for proxy custom error pages, closes https://github.com/opnsense/core/issues/4174

source/manual/proxy.rst

@@ -33,10 +33,13 @@ Currently these include:
 * Local user manager
 * No authentication
 
+These options can be found in the :menuselection:`Web Proxy -> Administration -> Forward Proxy -> Authentication Settings` section.
+
 --------------
 Access Control
 --------------
-OPNsense supports fine grained access control, base upon:
+OPNsense supports fine grained access control, which can be configured in :menuselection:`Web Proxy -> Administration -> Forward Proxy -> Access Control List`
+containing the following (Advanced) options:
 
 * Subnets
 * Ports
@@ -46,6 +49,7 @@ OPNsense supports fine grained access control, base upon:
 * Blacklists
 * Browser/User Agents
 
+
 ------------------
 Traffic Management
 ------------------
@@ -57,6 +61,8 @@ shaping features. Additionally it includes its own options:
 * Overall bandwidth throttling
 * Per host bandwidth throttling
 
+These options can be found in the :menuselection:`Web Proxy -> Administration -> General Proxy Settings -> Traffic Management Settings` section.
+
 
 -------------------------
 Category Based Web Filter
@@ -96,6 +102,44 @@ configuration via WPAD / PAC.
     should proxy the connection or avoid configuring the applicance from an
     untrusted network.
 
+-----------------------
+Custom error pages
+-----------------------
+
+Error pages can be customized for your own needs.
+In order to do so, go to :menuselection:`Web Proxy -> Administration -> General Proxy Settings` and select "Custom" in "User error pages",
+after which an additional tab will be visible named "Error Pages".
+
+.. raw:: html
+
+    Go to this tab and use the top download <i class="fa fa-download fa-fw"></i> icon to receive a zip file containing
+    all available error pages and associated cascading style sheets.
+    <br/><br/>
+    After altering the files, zip them again and upload using the file selector option  <i class="fa  fa-folder-o fa-fw"></i> on the same tab
+    followed by the upload button  <i class="fa fa-upload fa-fw"></i>.
+    <br/><br/>
+    The reset <i class="fa fa-fw fa-remove"></i> button can be used to remove your custom template from the configuration, after
+    which the download option would return the standard OPNsense template.
+
+
+There are some rules to take into account when creating custom themed error pages:
+
+* extend the :code:`errorpage.css` file to theme your pages
+* make sure error pages don't require anything else than css, images should be converted to base64 and provided inline.
+  Not only is this faster to handle than separate image files it also prevents rendering issues in case images can't be accessed.
+* only existing error pages will be processed, if filenames won't match, the files won't be written to disk. you can use the download button
+  to inspect what's being deployed (it will return a combined set of custom and standard files)
+* it's best not to include files that are not altered, this saves room in the configurartion and prevents defauls from being overwritten.
+
+.. Tip::
+
+    If you only want to change the background image, it's usually enough to upload the css file in a directory and leave out all the html files.
+
+.. Tip::
+
+    To convert images to inline base64 tags, there are quite some online tools available, which can easily be found using google.
+    As an example, our logo can be found in the the css file looking like :code:`background: url('data:image/svg+xml;base64,PD9...) no-repeat left;`
+
 -----------------------
 Configuration / How-tos
 -----------------------