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authorBernhard Schmidt <berni@debian.org>2020-08-15 21:29:50 +0200
committerBernhard Schmidt <berni@debian.org>2020-08-15 21:29:50 +0200
commit1079962e4c06f88a54e50d997c1b7e84303d30b4 (patch)
tree4d019426928435425214ccedd6f89b70dbdf035d /doc/man-sections/renegotiation.rst
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+Data Channel Renegotiation
+--------------------------
+
+When running OpenVPN in client/server mode, the data channel will use a
+separate ephemeral encryption key which is rotated at regular intervals.
+
+--reneg-bytes n
+ Renegotiate data channel key after ``n`` bytes sent or received
+ (disabled by default with an exception, see below). OpenVPN allows the
+ lifetime of a key to be expressed as a number of bytes
+ encrypted/decrypted, a number of packets, or a number of seconds. A key
+ renegotiation will be forced if any of these three criteria are met by
+ either peer.
+
+ If using ciphers with cipher block sizes less than 128-bits,
+ ``--reneg-bytes`` is set to 64MB by default, unless it is explicitly
+ disabled by setting the value to :code:`0`, but this is
+ **HIGHLY DISCOURAGED** as this is designed to add some protection against
+ the SWEET32 attack vector. For more information see the ``--cipher``
+ option.
+
+--reneg-pkts n
+ Renegotiate data channel key after **n** packets sent and received
+ (disabled by default).
+
+--reneg-sec args
+ Renegotiate data channel key after at most ``max`` seconds
+ (default :code:`3600`) and at least ``min`` seconds (default is 90% of
+ ``max`` for servers, and equal to ``max`` for clients).
+ ::
+
+ reneg-sec max [min]
+
+ The effective ``--reneg-sec`` value used is per session
+ pseudo-uniform-randomized between ``min`` and ``max``.
+
+ With the default value of :code:`3600` this results in an effective per
+ session value in the range of :code:`3240`..:code:`3600` seconds for
+ servers, or just 3600 for clients.
+
+ When using dual-factor authentication, note that this default value may
+ cause the end user to be challenged to reauthorize once per hour.
+
+ Also, keep in mind that this option can be used on both the client and
+ server, and whichever uses the lower value will be the one to trigger
+ the renegotiation. A common mistake is to set ``--reneg-sec`` to a
+ higher value on either the client or server, while the other side of the
+ connection is still using the default value of :code:`3600` seconds,
+ meaning that the renegotiation will still occur once per :code:`3600`
+ seconds. The solution is to increase --reneg-sec on both the client and
+ server, or set it to :code:`0` on one side of the connection (to
+ disable), and to your chosen value on the other side.