man: Update description of the esp keyword

Clarifies how DH groups are applied, updates the proposal selection
description and ESN can now also be configured for IKEv1.

References #1039.

diff --git a/man/ipsec.conf.5.in b/man/ipsec.conf.5.in
index 54440c0..6d99e13 100644 (file)
--- a/man/ipsec.conf.5.in
+++ b/man/ipsec.conf.5.in
@@ -247,7 +247,9 @@ can be added at the end.
 If
 .B dh-group
 is specified, CHILD_SA/Quick Mode setup and rekeying include a separate
-Diffie-Hellman exchange.
+Diffie-Hellman exchange (refer to the
+.B esp
+keyword for details).
 .TP
 .BR also " = <name>"
 includes conn section
@@ -410,18 +412,27 @@ exclamation mark
 can be added at the end.
 
 .BR Note :
-As a responder the daemon accepts the first supported proposal received from
-the peer. In order to restrict a responder to only accept specific cipher
-suites, the strict flag
+As a responder, the daemon defaults to selecting the first configured proposal
+that's also supported by the peer. This may be changed via
+.BR strongswan.conf (5)
+to selecting the first acceptable proposal sent by the peer instead. In order to
+restrict a responder to only accept specific cipher suites, the strict flag
 .RB ( ! ,
 exclamation mark) can be used, e.g: aes256-sha512-modp4096!
-.br
+
 If
 .B dh-group
-is specified, CHILD_SA/Quick Mode setup and rekeying include a separate
-Diffie-Hellman exchange.  Valid values for
+is specified, CHILD_SA/Quick Mode rekeying and initial negotiation use a
+separate Diffie-Hellman exchange using the specified group. However, for IKEv2,
+the keys of the CHILD_SA created implicitly with the IKE_SA will always be
+derived from the IKE_SA's key material. So any DH group specified here will only
+apply when the CHILD_SA is later rekeyed or is created with a separate
+CREATE_CHILD_SA exchange.  Therefore, a proposal mismatch might not immediately
+be noticed when the SA is established, but may later cause rekeying to fail.
+
+Valid values for
 .B esnmode
-(IKEv2 only) are
+are
 .B esn
 and
 .BR noesn .