NAME

       BIO_s_datagram, BIO_new_dgram, BIO_ctrl_dgram_connect, BIO_ctrl_set_connected,
       BIO_dgram_recv_timedout, BIO_dgram_send_timedout, BIO_dgram_get_peer, BIO_dgram_set_peer,
       BIO_dgram_get_mtu_overhead - Network BIO with datagram semantics

SYNOPSIS

        #include <openssl/bio.h>

        BIO_METHOD *BIO_s_datagram(void);
        BIO *BIO_new_dgram(int fd, int close_flag);

        int BIO_ctrl_dgram_connect(BIO *bio, const BIO_ADDR *peer);
        int BIO_ctrl_set_connected(BIO *bio, const BIO_ADDR *peer);
        int BIO_dgram_recv_timedout(BIO *bio);
        int BIO_dgram_send_timedout(BIO *bio);
        int BIO_dgram_get_peer(BIO *bio, BIO_ADDR *peer);
        int BIO_dgram_set_peer(BIO *bio, const BIO_ADDR *peer);
        int BIO_dgram_get_mtu_overhead(BIO *bio);

DESCRIPTION

       BIO_s_datagram() is a BIO implementation designed for use with network sockets which
       provide datagram semantics, such as UDP sockets. It is suitable for use with DTLSv1.

       Because BIO_s_datagram() has datagram semantics, a single BIO_write() call sends a single
       datagram and a single BIO_read() call receives a single datagram. If the size of the
       buffer passed to BIO_read() is inadequate, the datagram is silently truncated.

       When using BIO_s_datagram(), it is important to note that:

       •   This BIO can be used with either a connected or unconnected network socket. A
           connected socket is a network socket which has had BIO_connect(3) or a similar OS-
           specific function called on it. Such a socket can only receive datagrams from the
           specified peer. Any other socket is an unconnected socket and can receive datagrams
           from any host.

       •   Despite their naming, neither BIO_ctrl_dgram_connect() nor BIO_ctrl_set_connected()
           cause a socket to become connected. These controls are provided to indicate to the BIO
           how the underlying socket is configured and how it is to be used; see below.

       •   Use of BIO_s_datagram() with an unconnected network socket is hazardous hecause any
           successful call to BIO_read() results in the peer address used for any subsequent call
           to BIO_write() being set to the source address of the datagram received by that call
           to BIO_read(). Thus, unless the caller calls BIO_dgram_set_peer() immediately prior to
           every call to BIO_write(), or never calls BIO_read(), any host on the network may
           cause future datagrams written to be redirected to that host. Therefore, it is
           recommended that users use BIO_s_dgram() only with a connected socket. An exception is
           where DTLSv1_listen(3) must be used; see DTLSv1_listen(3) for further discussion.

       Various controls are available for configuring the BIO_s_datagram() using BIO_ctrl(3):

       BIO_ctrl_dgram_connect (BIO_CTRL_DGRAM_CONNECT)
           This is equivalent to calling BIO_dgram_set_peer(3).

           Despite its name, this function does not cause the underlying socket to become
           connected.

       BIO_ctrl_set_connected (BIO_CTRL_SET_CONNECTED)
           This informs the BIO_s_datagram() whether the underlying socket has been connected,
           and therefore how the BIO_s_datagram() should attempt to use the socket.

           If the peer argument is non-NULL, BIO_s_datagram() assumes that the underlying socket
           has been connected and will attempt to use the socket using OS APIs which do not
           specify peer addresses (for example, send(3) and recv(3) or similar). The peer
           argument should specify the peer address to which the socket is connected.

           If the peer argument is NULL, BIO_s_datagram() assumes that the underlying socket is
           not connected and will attempt to use the socket using an OS APIs which specify peer
           addresses (for example, sendto(3) and recvfrom(3)).

       BIO_dgram_get_peer (BIO_CTRL_DGRAM_GET_PEER)
           This outputs a BIO_ADDR which specifies one of the following values, whichever
           happened most recently:

           •   The peer address last passed to BIO_dgram_set_peer(), BIO_ctrl_dgram_connect() or
               BIO_ctrl_set_connected().

           •   The peer address of the datagram last received by a call to BIO_read().

       BIO_dgram_set_peer (BIO_CTRL_DGRAM_SET_PEER)
           Sets the peer address to be used for subsequent writes to this BIO.

           Warning: When used with an unconnected network socket, the value set may be modified
           by future calls to BIO_read(3), making use of BIO_s_datagram() hazardous when used
           with unconnected network sockets; see above.

       BIO_dgram_recv_timeout (BIO_CTRL_DGRAM_GET_RECV_TIMER_EXP)
           Returns 1 if the last I/O operation performed on the BIO (for example, via a call to
           BIO_read(3)) may have been caused by a receive timeout.

       BIO_dgram_send_timedout (BIO_CTRL_DGRAM_GET_SEND_TIMER_EXP)
           Returns 1 if the last I/O operation performed on the BIO (for example, via a call to
           BIO_write(3)) may have been caused by a send timeout.

       BIO_dgram_get_mtu_overhead (BIO_CTRL_DGRAM_GET_MTU_OVERHEAD)
           Returns a quantity in bytes which is a rough estimate of the number of bytes of
           overhead which should typically be added to a datagram payload size in order to
           estimate the final size of the Layer 3 (e.g. IP) packet which will contain the
           datagram. In most cases, the maximum datagram payload size which can be transmitted
           can be determined by determining the link MTU in bytes and subtracting the value
           returned by this call.

           The value returned by this call depends on the network layer protocol being used.

           The value returned is not fully reliable because datagram overheads can be higher in
           atypical network configurations, for example where IPv6 extension headers or IPv4
           options are used.

       BIO_CTRL_DGRAM_SET_DONT_FRAG
           If num is nonzero, configures the underlying network socket to enable Don't Fragment
           mode, in which datagrams will be set with the IP Don't Fragment (DF) bit set. If num
           is zero, Don't Fragment mode is disabled.

       BIO_CTRL_DGRAM_QUERY_MTU
           Queries the OS for its assessment of the Path MTU for the destination to which the
           underlying network socket, and returns that Path MTU in bytes. This control can only
           be used with a connected socket.

           This is not supported on all platforms and depends on OS support being available.
           Returns 0 on failure.

       BIO_CTRL_DGRAM_MTU_DISCOVER
           This control requests that Path MTU discovery be enabled on the underlying network
           socket.

       BIO_CTRL_DGRAM_GET_FALLBACK_MTU
           Returns the estimated minimum size of datagram payload which should always be
           supported on the BIO. This size is determined by the minimum MTU required to be
           supported by the applicable underlying network layer. Use of datagrams of this size
           may lead to suboptimal performance, but should be routable in all circumstances. The
           value returned is the datagram payload size in bytes and does not include the size of
           layer 3 or layer 4 protocol headers.

       BIO_CTRL_DGRAM_MTU_EXCEEDED
           Returns 1 if the last attempted write to the BIO failed due to the size of the
           attempted write exceeding the applicable MTU.

       BIO_CTRL_DGRAM_SET_NEXT_TIMEOUT
           Accepts a pointer to a struct timeval. If the time specified is zero, disables receive
           timeouts. Otherwise, configures the specified time interval as the receive timeout for
           the socket for the purposes of future BIO_read(3) calls.

       BIO_CTRL_DGRAM_SET_PEEK_MODE
           If num is nonzero, enables peek mode; otherwise, disables peek mode. Where peek mode
           is enabled, calls to BIO_read(3) read datagrams from the underlying network socket in
           peek mode, meaning that a future call to BIO_read(3) will yield the same datagram
           until peek mode is disabled.

       BIO_new_dgram() is a helper function which instantiates a BIO_s_datagram() and sets the
       BIO to use the socket given in fd by calling BIO_set_fd().

RETURN VALUES

       BIO_s_datagram() returns a BIO method.

       BIO_new_dgram() returns a BIO on success and NULL on failure.

       BIO_ctrl_dgram_connect(), BIO_ctrl_set_connected(), BIO_dgram_get_peer(),
       BIO_dgram_set_peer() return 1 on success and 0 on failure.

       BIO_dgram_recv_timedout() and BIO_dgram_send_timedout() return 0 or 1 depending on the
       circumstance; see discussion above.

       BIO_dgram_get_mtu_overhead() returns a value in bytes.

SEE ALSO

       DTLSv1_listen(3), bio(7)

COPYRIGHT

       Copyright 2022-2023 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (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 <https://www.openssl.org/source/license.html>.