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 Clien-
tHello 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(3)).
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 initia-
tion requests that cause the server to allocate state (and possibly per-
form 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 con-
suming 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 veri-
fied, then DTLSv1_listen() will return with the ssl parameter updated
into a state where the handshake can be continued by a call to (for exam-
ple) 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 "con-
nect" the underlying socket to the peer and continue the handshake in a
Prior to calling DTLSv1_listen() user code must ensure that cookie gener-
ation 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 exam-
ple) 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.
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 5.5 March 27, 2018 DragonFly 5.5