=head1 NAME
-DTLSv1_listen - listen for incoming DTLS connections
+SSL_stateless,
+DTLSv1_listen
+- Statelessly listen for incoming connections
=head1 SYNOPSIS
#include <openssl/ssl.h>
+ int SSL_stateless(SSL *s);
int DTLSv1_listen(SSL *ssl, BIO_ADDR *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()).
+SSL_stateless() statelessly listens for new incoming TLSv1.3 connections.
+DTLSv1_listen() statelessly listens for new incoming DTLS connections. If a
+ClientHello is received that does not contain a cookie, then they respond with a
+request for a new ClientHello that does contain a cookie. If a ClientHello is
+received with a cookie that is verified then the function returns in order 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.
+Some transport protocols (such as UDP) can be susceptible to amplification
+attacks. Unlike TCP there is no initial connection setup in UDP that
+validates that the client can actually receive messages on its advertised source
+address. An attacker could forge its source IP address and then send handshake
+initiation messages to the server. The server would then send its response to
+the forged source IP. If the response messages are larger than the original
+message then the amplification attack has succeeded.
+
+If DTLS is used over UDP (or any datagram based protocol that does not validate
+the source IP) then it is susceptible to this type of attack. TLSv1.3 is
+designed to operate over a stream-based transport protocol (such as TCP).
+If TCP is being used then there is no need to use SSL_stateless(). However some
+stream-based transport protocols (e.g. QUIC) may not validate the source
+address. In this case a TLSv1.3 application would be susceptible to this attack.
+
+As a counter measure to this issue TLSv1.3 and DTLS include 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 HelloRetryRequest (in
+TLSv1.3) or a HelloVerifyRequest (in DTLS) 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 SSL_stateless() and DTLSv1_listen()
+functions. 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, for DTLS, 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
+then these functions 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<BIO_ADDR> pointed to by B<peer> will be filled in with
-details of the peer that sent the ClientHello. If the underlying BIO is unable
-to obtain the B<BIO_ADDR> of the peer (for example because the BIO does not
-support this), then B<*peer> will be cleared and the family set to AF_UNSPEC.
-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
+Additionally, for DTLSv1_listen(), the B<BIO_ADDR> pointed to by B<peer> will be
+filled in with details of the peer that sent the ClientHello. If the underlying
+BIO is unable to obtain the B<BIO_ADDR> of the peer (for example because the BIO
+does not support this), then B<*peer> will be cleared and the family set to
+AF_UNSPEC. Typically user code is expected to "connect" the underlying socket to
+the peer and continue the handshake in a connected state.
+
+Prior to calling these functions 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.
require the allocation of state). An implication of this is that DTLSv1_listen()
B<only> supports ClientHellos that fit inside a single datagram.
+For SSL_stateless() if an entire ClientHello message cannot be read without the
+"read" BIO becoming empty then the SSL_stateless() call will fail. It is the
+application's responsibility to ensure that data read from the "read" BIO during
+a single SSL_stateless() call is all from the same peer.
+
+SSL_stateless() will fail (with a 0 return value) if some TLS version less than
+TLSv1.3 is used.
+
+Both SSL_stateless() and DTLSv1_listen() will clear the error queue when they
+start.
+
=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.
+For SSL_stateless() a return value of 1 indicates success and the B<ssl> object
+will be set up ready to continue the handshake. A return value of 0 indicates
+failure. User code may retry the SSL_stateless() call.
+
+For DTLSv1_listen() a return value of >= 1 indicates success. The B<ssl> object
+will be set up ready to continue the handshake. the B<peer> value will also be
+filled in.
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.
+call to DTLSv1_listen() in the event of a non-fatal error.
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.
+For DTLSv1_listen(), 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
=head1 HISTORY
+SSL_stateless() was first added in OpenSSL 1.1.1.
+
DTLSv1_listen() return codes were clarified in OpenSSL 1.1.0. The type of "peer"
also changed in OpenSSL 1.1.0.
=head1 COPYRIGHT
-Copyright 2015-2016 The OpenSSL Project Authors. All Rights Reserved.
+Copyright 2015-2017 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
projects / oweals / openssl.git / commitdiff