Merge pull request #11 from syncthing/Zillode-cookies

Explain CSRF tokens & cookies

users/faq.rst

@@ -217,6 +217,33 @@ The CSRF tokens are stored using cookies. Therefore, if you get the message
 ``Syncthing seems to be experiencing a problem processing your request``, you
 should verify the cookie settings of your browser.
 
+Why can I not open two Syncthing sessions in the same browser?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The Syncthing GUI uses cookies to maintain a session. You can't have more than
+one session open against the same website in the same browser, much like you
+can't log in as two different users on this forum at the same time from the
+same browser, so that may cause this error message. This means you should not
+browse a local Syncthing instance (typically localhost:8384) and a remote
+Syncthing instance (e.g. localhost:9090 as suggested previously) at the same
+time. Often, this results in partial functionality (e.g. ignore patterns cannot
+be editted) or in an error message such as ``Syncthing seems to be experiencing
+a problem processing your request`` when updating a particular setting.
+
+In 99% of the cases of this issue are:
+
+1. Using some sort of proxy/SSH tunneling which causes the cookies to be issued
+   under an incorrect domain causing the browser to refuse them. Close your
+   browser and only keep a single Syncthing Web GUI open at each time.
+2. Having some privacy add-on enabled in the browser causing cookies to be
+   refused.
+3. Browser failing to provide the initially provided Basic Auth header to
+   re-authenticate given syncthing was restarted and the session cookie is no
+   longer valid. Refreshing the page usually fixes this.
+4. Having multiple sessions for the same site.
+
+To resolve this, please use a seperate browser. For example use Firefox for one
+session and Konqueror/Chromium/.. for another.
+
 Why do I see Syncthing twice in task manager?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~