OPNproxy - extend docs a bit

source/vendor/deciso/opnproxy.rst

@@ -23,6 +23,11 @@ Before installing and using this plugin, make sure your web proxy is configured
     or make sure the same username exists locally to map groups too.
 
 
+.. Warning::
+    Do not install other plugins or configuration files hooking into the proxy as these might interfere with the working
+    of the system.
+
+
 Installation
 ---------------------------
 
@@ -67,6 +72,26 @@ to import/add the users in OPNsense in order to user their authorisation setting
     (it's not possible to block no bump sites in full inspection mode)
 
 
+Transparant proxies
+---------------------------
+
+It is possible to use the proxy in transparant mode, but there are some constraints and ceveats to take into account when doing so.
+This paragraph tries to explain them one by one.
+
+* Using "Log SNI information only" is not supported in a useful way. As the browser is not aware of the proxy, it will request
+  access to an ip address in stead of a hostname. With full intercept mode, this is not really an issue as the next request will
+  be the actual question and does contain the hostname, but without interception, you can only filter on ip address which is often not very useful.
+* The client has to trust the CA which the proxy uses to automatically create certificates, which means all TLS requests will be signed by the firewall instead of the
+  actual trustee.
+* User based authentication is not possible, as the client doesn't know it's being intercepted, it's also not possible to
+  request a username and password. OPNproxy only supports basic authentication.
+
+
+.. Note::
+  When changing the "Log SNI information only"  option, you have to restart the proxy as well. As the apply button will not
+  reload the proxy in full.
+
+
 Authentication options
 ---------------------------