ssh: add support for banners

According to RFC 4252 section 5.4, the banner is sent between the
ssh-connection request and responding to user authentication.

Original support for server sending banner by joshua stein <jcs@jcs.org>

Fixes golang/go#19567

Change-Id: I68944a7f4711c0623759f6a59023e8e45a8781aa
Reviewed-on: https://go-review.googlesource.com/65271
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>

ssh/server.go

@@ -95,6 +95,10 @@ type ServerConfig struct {
 	// Note that RFC 4253 section 4.2 requires that this string start with
 	// "SSH-2.0-".
 	ServerVersion string
+
+	// BannerCallback, if present, is called and the return string is sent to
+	// the client after key exchange completed but before authentication.
+	BannerCallback func(conn ConnMetadata) string
 }
 
 // AddHostKey adds a private key as a host key. If an existing host
@@ -343,6 +347,19 @@ userAuthLoop:
 		}
 
 		s.user = userAuthReq.User
+
+		if authFailures == 0 && config.BannerCallback != nil {
+			msg := config.BannerCallback(s)
+			if msg != "" {
+				bannerMsg := &userAuthBannerMsg{
+					Message: msg,
+				}
+				if err := s.transport.writePacket(Marshal(bannerMsg)); err != nil {
+					return nil, err
+				}
+			}
+		}
+
 		perms = nil
 		authErr := errors.New("no auth passed yet")