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DTLSv1_listen(3) OpenSSL DTLSv1_listen(3)
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SSL_stateless, DTLSv1_listen - Statelessly listen for incoming
int SSL_stateless(SSL *s);
int DTLSv1_listen(SSL *ssl, BIO_ADDR *peer);
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()).
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 countermeasure 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 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 these functions will return with the ssl parameter
updated into a state where the handshake can be continued by a call to
(for example) SSL_accept(). Additionally, for DTLSv1_listen(), the
BIO_ADDR pointed to by peer will be filled in with details of the peer
that sent the ClientHello. If the underlying BIO is unable to obtain
the BIO_ADDR of the peer (for example because the BIO does not support
this), then *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 and verification callbacks have been set up using
SSL_CTX_set_cookie_generate_cb(3) and SSL_CTX_set_cookie_verify_cb(3)
respectively. For SSL_stateless(),
SSL_CTX_set_stateless_cookie_verify_cb(3) must be used instead.
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() 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.
For SSL_stateless() a return value of 1 indicates success and the ssl
object will be set up ready to continue the handshake. A return value
of 0 or -1 indicates failure. If the value is 0 then a
HelloRetryRequest was sent. A value of -1 indicates any other error.
User code may retry the SSL_stateless() call.
For DTLSv1_listen() a return value of >= 1 indicates success. The ssl
object will be set up ready to continue the handshake. the peer value
will also be filled in.
A return value of 0 indicates a non-fatal error. This could (for
example) be because of nonblocking 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-
A return value of <0 indicates a fatal error. This could (for example)
be because of a failure to allocate sufficient memory for the
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
SSL_accept(3), ssl(7), bio(7)
The SSL_stateless() function was added in OpenSSL 1.1.1.
The DTLSv1_listen() return codes were clarified in OpenSSL 1.1.0. The
type of "peer" also changed in OpenSSL 1.1.0.
Copyright 2015-2020 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
in the file LICENSE in the source distribution or at
1.1.1i 2020-12-10 DTLSv1_listen(3)