Document -verify_return_error option.

(cherry picked from commit 4e6c12f308)

doc/apps/s_client.pod

@@ -10,6 +10,7 @@ s_client - SSL/TLS client program
 B<openssl> B<s_client>
 [B<-connect host:port>]
 [B<-verify depth>]
+[B<-verify_return_error>]
 [B<-cert filename>]
 [B<-certform DER|PEM>]
 [B<-key filename>]
@@ -93,6 +94,11 @@ Currently the verify operation continues after errors so all the problems
 with a certificate chain can be seen. As a side effect the connection
 will never fail due to a server certificate verify failure.
 
+=item B<-verify_return_error>
+
+Return verification errors instead of continuing. This will typically
+abort the handshake with a fatal error.
+
 =item B<-CApath directory>
 
 The directory to use for server certificate verification. This directory
@@ -305,6 +311,13 @@ Since the SSLv23 client hello cannot include compression methods or extensions
 these will only be supported if its use is disabled, for example by using the
 B<-no_sslv2> option.
 
+The B<s_client> utility is a test tool and is designed to continue the
+handshake after any certificate verification errors. As a result it will
+accept any certificate chain (trusted or not) sent by the peer. None test
+applications should B<not> do this as it makes them vulnerable to a MITM
+attack. This behaviour can be changed by with the B<-verify_return_error>
+option: any verify errors are then returned aborting the handshake.
+
 =head1 BUGS
 
 Because this program has a lot of options and also because some of
@@ -312,9 +325,6 @@ the techniques used are rather old, the C source of s_client is rather
 hard to read and not a model of how things should be done. A typical
 SSL client program would be much simpler.
 
-The B<-verify> option should really exit if the server verification
-fails.
-
 The B<-prexit> option is a bit of a hack. We should really report
 information whenever a session is renegotiated.