MANUAL.rst: Improve with Perf tips and SSL notes

Write up some tips in the manual on how to improve performance and some
notes on how we currently use SSL, to be clear and transparent on what
level of security users get by the various means of connecting via
SSL/TLS.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

docs/MANUAL.rst

@@ -300,5 +300,92 @@ KNOWN BUGS
   * IDLE may only work "once" per refresh.  If you encounter this bug,
     please send a report to the list!
 
-SEE ALSO
+
-========
+Synchronization Performance
+===========================
+
+By default, we use fairly conservative settings that are good for
+syncing but that might not be the best performing one. Once you got
+everything set up and running, you might want to look into speeding up
+your synchronization. Here are a couple of hints and tips on how to
+achieve this.
+
+ 1) Use maxconnections > 1. By default we only use one connection to an
+    IMAP server. Using 2 or even 3 speeds things up considerably in most
+    cases. This setting goes into the [Repository XXX] section.
+
+ 2) Use folderfilters. The quickest sync is a sync that can ignore some
+    folders. I sort my inbox into monthly folders, and ignore every
+    folder that is more than 2-3 months old, this lets me only inspect a
+    fraction of my Mails on every sync. If you haven't done this yet, do
+    it :). See the folderfilter section the example offlineimap.conf.
+
+ 3) The default status cache is a plain text file that will write out
+    the complete file for each single new message (or even changed flag)
+    to a temporary file. If you have plenty of files in a folder, this
+    is a few hundred kilo to megabytes for each mail and is bound to
+    make things slower. I recommend to use the sqlite backend for
+    that. See the status_backend = sqlite setting in the example
+    offlineimap.conf. You will need to have python-sqlite installed in
+    order to use this. This will save you plenty of disk activity. Do
+    note that the sqlite backend is still considered experimental as it
+    has only been included recently (although a loss of your status
+    cache should not be a tragedy as that file can be rebuild
+    automatically)
+
+ 4) Use quick sync. A regular sync will request all flags and all UIDs
+    of all mails in each folder which takes quite some time. A 'quick'
+    sync only compares the number of messages in a folder on the IMAP
+    side (it will detect flag changes on the Maildir side of things
+    though). A quick sync on my smallish account will take 7 seconds
+    rather than 40 seconds. Eg, I run a cron script that does a regular
+    sync once a day, and does quick syncs inbetween.
+
+ 5) Turn off fsync. In the [general] section you can set fsync to True
+    or False. If you want to play 110% safe and wait for all operations
+    to hit the disk before continueing, you can set this to True. If you
+    set it to False, you lose some of that safety trading it for speed.
+
+Security and SSL
+================
+
+Some words on OfflineImap and its use of SSL/TLS. By default, we will
+connect using any method that openssl supports, that is SSLv2, SSLv3, or
+TLSv1. Do note that SSLv2 is notoriously insecure and deprecated.
+Unfortunately, python2 does not offer easy ways to disable SSLv2. It is
+recommended you test your setup and make sure that the mail server does
+not use an SSLv2 connection. Use e.g. "openssl s_client -host
+mail.server -port 443" to find out the connection that is used by
+default.
+
+Certificate checking
+^^^^^^^^^^^^^^^^^^^^
+
+Unfortunately, by default we will not verify the certificate of an IMAP
+TLS/SSL server we connect to, so connecting by SSL is no guarantee
+against man-in-the-middle attacks. While verifying a server certificate
+fingerprint is being planned, it is not implemented yet. There is
+currently only one safe way to ensure that you connect to the correct
+server in an encrypted manner: You can specify a 'sslcacertfile' setting
+in your repository section of offlineimap.conf pointing to a file that
+contains (among others) a CA Certificate in PEM format which validating
+your server certificate. In this case, we will check that: 1) The server
+SSL certificate is validated by the CA Certificate 2) The server host
+name matches the SSL certificate 3) The server certificate is not past
+its expiration date. The FAQ contains an entry on how to create your own
+certificate and CA certificate.
+
+StartTLS
+^^^^^^^^
+
+If you have not configured your account to connect via SSL anyway,
+OfflineImap will still attempt to set up an SSL connection via the
+STARTTLS function, in case the imap server supports it. Do note, that
+there is no certificate or fingerprint checking involved at all, when
+using STARTTLS (the underlying imaplib library does not support this
+yet). This means that you will be protected against passively listening
+eavesdroppers and they will not be able to see your password or email
+contents. However, this will not protect you from active attacks, such
+as Man-In-The-Middle attacks which cause you to connect to the wrong
+server and pretend to be your mail server. DO NOT RELY ON STARTTLS AS A
+SAFE CONNECTION GUARANTEEING THE AUTHENTICITY OF YOUR IMAP SERVER!