[dhcpv6] Handle SOLICIT with rapid commit in NewReplyFromMessage

NewReplyFromMessage creates a reply from a message. With this patch it
can handle Solicit messages with rapid-commit.

RFC3315 states that:
```
   If the client has included a Rapid Commit option in its Solicit
   message, the client terminates the waiting process as soon as a Reply
   message with a Rapid Commit option is received.
```

Signed-off-by: Andrea Barberio <insomniac@slackware.it>

2 files changed, 16 insertions, 4 deletions

diff --git a/dhcpv6/dhcpv6_test.go b/dhcpv6/dhcpv6_test.go
index 10a7296..5de2294 100644
--- a/dhcpv6/dhcpv6_test.go
+++ b/dhcpv6/dhcpv6_test.go
@@ -195,6 +195,12 @@ func TestNewReplyFromMessage(t *testing.T) {
 	msg.MessageType = MessageTypeSolicit
 	rep, err = NewReplyFromMessage(&msg)
 	require.Error(t, err)
+
+	msg.MessageType = MessageTypeSolicit
+	msg.AddOption(&OptionGeneric{OptionCode: OptionRapidCommit})
+	rep, err = NewReplyFromMessage(&msg)
+	require.NoError(t, err)
+	msg.Options.Del(OptionRapidCommit)
 }
 
 func TestNewMessageTypeSolicit(t *testing.T) {
diff --git a/dhcpv6/dhcpv6message.go b/dhcpv6/dhcpv6message.go
index 707a65d..f4f5eb1 100644
--- a/dhcpv6/dhcpv6message.go
+++ b/dhcpv6/dhcpv6message.go
@@ -161,17 +161,23 @@ func NewRequestFromAdvertise(adv *Message, modifiers ...Modifier) (*Message, err
 }
 
 // NewReplyFromMessage creates a new REPLY packet based on a
-// Message. The function is to be used when generating a reply to
-// REQUEST, CONFIRM, RENEW, REBIND, RELEASE and INFORMATION-REQUEST packets.
+// Message. The function is to be used when generating a reply to a SOLICIT with
+// rapid-commit, REQUEST, CONFIRM, RENEW, REBIND, RELEASE and INFORMATION-REQUEST
+// packets.
 func NewReplyFromMessage(msg *Message, modifiers ...Modifier) (*Message, error) {
 	if msg == nil {
-		return nil, errors.New("Message cannot be nil")
+		return nil, errors.New("message cannot be nil")
 	}
 	switch msg.Type() {
+	case MessageTypeSolicit:
+		if msg.GetOneOption(OptionRapidCommit) == nil {
+			return nil, errors.New("cannot create REPLY from a SOLICIT without rapid-commit option")
+		}
+		modifiers = append([]Modifier{WithRapidCommit}, modifiers...)
 	case MessageTypeRequest, MessageTypeConfirm, MessageTypeRenew,
 		MessageTypeRebind, MessageTypeRelease, MessageTypeInformationRequest:
 	default:
-		return nil, errors.New("Cannot create REPLY from the passed message type set")
+		return nil, errors.New("cannot create REPLY from the passed message type set")
 	}
 
 	// build REPLY from MESSAGE