DragonFly On-Line Manual Pages
DTLSV1_LISTEN(3) DragonFly Library Functions Manual DTLSV1_LISTEN(3)
DTLSv1_listen - listen for incoming DTLS connections
DTLSv1_listen(SSL *ssl, struct sockaddr *peer);
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
DTLSv1_listen() is currently implemented as a macro.
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
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(3). 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 ssl parameter updated
into a state where the handshake can be continued by a call to (for
example) SSL_accept(3). Additionally the struct sockaddr pointed to by
peer will be filled in with details of the peer that sent the
ClientHello. It is the calling code's responsibility to ensure that the
peer location is sufficiently large to accommodate the addressing scheme
in use. For example this might be done by allocating space for a struct
sockaddr_storage and casting the pointer to it to a struct sockaddr * for
the call to DTLSv1_listen(). 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()
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
From OpenSSL 1.1.0 a return value of >= 1 indicates success. In this
instance the peer value will be filled in and the 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
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.
BIO_new(3), ssl(3), SSL_accept(3), SSL_get_error(3)
DTLSv1_listen() first appeared in OpenSSL 0.9.8m and has been available
since OpenBSD 4.9.
DragonFly 6.1-DEVELOPMENT March 27, 2018 DragonFly 6.1-DEVELOPMENT