Clarify request of client certificates. This is a FAQ.

FAQ

@@ -47,6 +47,7 @@ OpenSSL  -  Frequently Asked Questions
 * Why do I get errors about unknown algorithms?
 * Why can't the OpenSSH configure script detect OpenSSL?
 * Can I use OpenSSL's SSL library with non-blocking I/O?
+* Why doesn't my server application receive a client certificate?
 
 ===============================================================================
 
@@ -519,5 +520,12 @@ requiring a bi-directional message exchange; both SSL_read() and
 SSL_write() will try to continue any pending handshake.
 
 
+* Why doesn't my server application receive a client certificate?
+
+Due to the TLS protocol definition, a client will only send a certificate,
+if explicitely asked by the server. Use the SSL_VERIFY_PEER flag of the
+SSL_CTX_set_verify() function to enable the use of client certificates.
+
+
 ===============================================================================
 

doc/ssl/SSL_get_peer_certificate.pod

@@ -17,6 +17,12 @@ peer presented. If the peer did not present a certificate, NULL is returned.
 
 =head1 NOTES
 
+Due to the protocol definition, a TLS/SSL server will always send a
+certificate, if present. A client will only send a certificate when
+explicitely requested to do so by the server (see
+L<SSL_CTX_set_verify(3)|SSL_CTX_set_verify(3)>). If an anonymous cipher
+is used, no certificates are sent.
+
 That a certificate is returned does not indicate information about the
 verification state, use L<SSL_get_verify_result(3)|SSL_get_verify_result(3)>
 to check the verification state.
@@ -43,6 +49,7 @@ The return value points to the certificate presented by the peer.
 
 =head1 SEE ALSO
 
-L<ssl(3)|ssl(3)>, L<SSL_get_verify_result(3)|SSL_get_verify_result(3)>
+L<ssl(3)|ssl(3)>, L<SSL_get_verify_result(3)|SSL_get_verify_result(3)>,
+L<SSL_CTX_set_verify(3)|SSL_CTX_set_verify(3)>
 
 =cut