Add DTLSv1_listen documentation

Adds a new man page to cover the DTLSv1_listen() function.

Reviewed-by: Andy Polyakov <appro@openssl.org>

doc/ssl/DTLSv1_listen.pod

@@ -0,0 +1,91 @@
+=pod
+
+=head1 NAME
+
+DTLSv1_listen - listen for incoming DTLS connections.
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ int DTLSv1_listen(SSL *ssl, struct sockaddr *peer);
+
+=head1 DESCRIPTION
+
+DTLSv1_listen() listens for new incoming DTLS connections. If a ClientHello is
+received that does not contain a cookie, then DTLSv1_listen() responds with a
+HelloVerifyRequest. If a ClientHello is received with a cookie that is verified
+then control is returned to user code to enable the handshake to be completed
+(for example by using SSL_accept()).
+
+=head1 NOTES
+
+Datagram based protocols can be susceptible to Denial of Service attacks. A
+DTLS attacker could, for example, submit a series of handshake initiation
+requests that cause the server to allocate state (and possibly perform
+cryptographic operations) thus consuming server resources. The attacker could
+also (with UDP) quite simply forge the source IP address in such an attack.
+
+As a counter measure to that DTLS includes a stateless cookie mechanism. The
+idea is that when a client attempts to connect to a server it sends a
+ClientHello message. The server responds with a HelloVerifyRequest which
+contains a unique cookie. The client then resends the ClientHello, but this time
+includes the cookie in the message thus proving that the client is capable of
+receiving messages sent to that address. All of this can be done by the server
+without allocating any state, and thus without consuming expensive resources.
+
+OpenSSL implements this capability via the DTLSv1_listen() function. The B<ssl>
+parameter should be a newly allocated SSL object with its read and write BIOs
+set, in the same way as might be done for a call to SSL_accept(). Typically the
+read BIO will be in an "unconnected" state and thus capable of receiving
+messages from any peer.
+
+When a ClientHello is received that contains a cookie that has been verified,
+then DTLSv1_listen() will return with the B<ssl> parameter updated into a state
+where the handshake can be continued by a call to (for example) SSL_accept().
+Additionally the B<struct sockaddr> location pointed to by B<peer> will be
+filled in with details of the peer that sent the ClientHello. Typically user
+code is expected to "connect" the underlying socket to the peer and continue the
+handshake in a connected state.
+
+Prior to calling DTLSv1_listen() user code must ensure that cookie generation
+and verification callbacks have been set up using
+SSL_CTX_set_cookie_generate_cb() and SSL_CTX_set_cookie_verify_cb()
+respectively.
+
+Since DTLSv1_listen() operates entirely statelessly whilst processing incoming
+ClientHellos it is unable to process fragmented messages (since this would
+require the allocation of state). An implication of this is that DTLSv1_listen()
+B<only> supports ClientHellos that fit inside a single datagram.
+
+=head1 RETURN VALUES
+
+From OpenSSL 1.1.0 a return value of >= 1 indicates success. In this instance
+the B<peer> value will be filled in and the B<ssl> object set up ready to
+continue the handshake.
+
+A return value of 0 indicates a non-fatal error. This could (for
+example) be because of non-blocking IO, or some invalid message having been
+received from a peer. Errors may be placed on the OpenSSL error queue with
+further information if appropriate. Typically user code is expected to retry the
+call to DTLSv1_listen() in the event of a non-fatal error. Any old errors on the
+error queue will be cleared in the subsequent call.
+
+A return value of <0 indicates a fatal error. This could (for example) be
+because of a failure to allocate sufficient memory for the operation.
+
+Prior to OpenSSL 1.1.0 fatal and non-fatal errors both produce return codes
+<= 0 (in typical implementations user code treats all errors as non-fatal),
+whilst return codes >0 indicate success.
+
+=head1 SEE ALSO
+
+L<SSL_get_error(3)|SSL_get_error(3)>, L<SSL_accept(3)|SSL_accept(3)>,
+L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>
+
+=head1 HISTORY
+
+DTLSv1_listen() was added in OpenSSL 0.9.8. Its return codes were clarified in
+OpenSSL 1.1.0.
+
+=cut

doc/ssl/ssl.pod

@@ -743,7 +743,8 @@ L<SSL_SESSION_get_time(3)>,
 L<d2i_SSL_SESSION(3)>,
 L<SSL_CTX_set_psk_client_callback(3)>,
 L<SSL_CTX_use_psk_identity_hint(3)>,
-L<SSL_get_psk_identity(3)>
+L<SSL_get_psk_identity(3)>,
+L<DTLSv1_listen(3)|DTLSv1_listen(3)>
 
 =head1 HISTORY