VPN: IPsec - add DPD so we have aplace holder to mention the "enable" flag

source/manual/vpnet.rst

@@ -188,6 +188,21 @@ The default behavior of our firewall is to block inbound traffic, which also mea
 be allowed explicitly, the :menuselection:`Firewall --> Rules --> IPsec` menu items offer access to the IPsec traffic policies.
 
 
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+Dead Peer Detection (DPD)
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+
+Dead Peer Detection (DPD) is a method of detecting a dead IKE peer by sending periodic R-U-THERE messages to the remote expecting R-U-THERE-ACK
+messages in return as specified by `RFC 3706 <https://www.ietf.org/rfc/rfc3706.txt>`__.
+
+When a peer is assumed dead, an action may be specified, such as closing the CHILD_SA or re-negotiate the CHILD_SA under a fresh IKE_SA.
+
+.. Note::
+
+  DPD is disabled by default, when using connections, make sure to specify a :code:`DPD delay (s)` > 0 to enable the feature.
+  Actions may be specified on its children.
+
+
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 Implementation schemes
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@@ -240,6 +255,7 @@ The advantage of this type of setup is one can use standard or advanced routing
     matches both source and destination `[#] <https://github.com/freebsd/freebsd-src/blob/c8ee75f2315e8267ad814dc5b4645ef205f0e0e1/sys/net/if_ipsec.c#L479>`__
     before accepting the traffic and has no knowledge about any external changes.
 
+
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 Road Warriors / Mobile users
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