dns: document usage profiles & Android automatic mode (#1242)

* dns: document usage profiles & Android automatic mode

Resolves: #1239

* dns: fix typo, sslstrip

* dns: add space between SSL and strip

_includes/sections/dns.html

@@ -408,7 +408,11 @@ github="https://github.com/jedisct1/dnscrypt-proxy"
 <h4>Terms</h4>
 
 <ul>
-  <li>DNS-over-TLS (DoT) - A security protocol for encrypted DNS on a dedicated port 853. Some providers support port 443 which generally works everywhere while port 853 is often blocked by restrictive firewalls.</li>
+  <li>DNS-over-TLS (DoT) - A security protocol for encrypted DNS on a dedicated port 853. Some providers support port 443 which generally works everywhere while port 853 is often blocked by restrictive firewalls. DoT has two modes:</li>
+    <ul>
+        <li>Oppurtunistic mode: the client attempts to form a DNS-over-TLS connection to the server on port 853 without performing certificate validation. If it fails, it will use unencrypted DNS. <span class="badge badge-warning" data-toggle="tooltip" data-original-title="In other words automatic mode leaves your DNS traffic vulnerable to SSL strip and MITM attacks"><i class="fas fa-exclamation-triangle"></i></span></li>
+        <li>Strict mode: the client connects to a specific hostname and performs certificate validation for it. If it fails, no DNS queries are made until it succeeds.</li>
+    </ul>
   <li>DNS-over-HTTPS (DoH) - Similar to DoT, but uses HTTPS instead, being indistinguishable from "normal" HTTPS traffic on port 443. <span class="badge badge-warning" data-toggle="tooltip" data-original-title="DoH contains metadata such as user-agent (which may include system information) that is sent to the DNS server."><a href="https://tools.ietf.org/html/rfc8484#section-8.2"><i class="fas fa-exclamation-triangle"></i></a></span></li>
   <li>DNSCrypt - An older yet robust method of encrypting DNS.</li>
 </ul>
@@ -444,6 +448,9 @@ github="https://github.com/jedisct1/dnscrypt-proxy"
   <li><strong>Encrypted DNS clients for mobile:</strong>
     <ul>
       <li><em>Android 9</em> comes with a DoT client by <a href="https://support.google.com/android/answer/9089903">default</a>. <span class="badge badge-warning" data-toggle="tooltip" data-original-title="...but with some caveats"><a href="https://www.quad9.net/private-dns-quad9-android9/"><i class="fas fa-exclamation-triangle"></i></a></span></li>
+        <ul>
+          <li>We recommend selecting <em>Private DNS provider hostname</em> and entering the DoT address from documentation of your DoT provider to enable strict mode (see Terms above). <span class="badge badge-warning" data-toggle="tooltip" data-original-title="If you are on a network blocking access to port 853, Android will error about the network not having internet connectivity."><i class="fas fa-exclamation-triangle"></i></span></li>
+        </ul>
       <li><em><a href="https://apps.apple.com/app/id1452162351">DNSCloak</a></em> - An <a href="https://github.com/s-s/dnscloak">open-source</a> DNSCrypt and DoH client for iOS by <td><a data-toggle="tooltip" data-placement="bottom" data-original-title='"A charitable non-profit host organization for international Free Software projects."' href="https://techcultivation.org/">the Center for the Cultivation of Technology gemeinnuetzige GmbH</a>.</li>
       <li><em><a href="https://git.frostnerd.com/PublicAndroidApps/smokescreen/blob/master/README.md">Nebulo</a></em> - An open-source application for Android supporting DoH and DoT. It also supports caching DNS responses and locally logging DNS queries.</li>
     </ul>