Updated: 2022/Sep/29

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BIO_s_datagram(3)                   OpenSSL                  BIO_s_datagram(3)



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

LIBRARY
       libcrypto, -lcrypto

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>.



3.0.12                            2023-05-07                 BIO_s_datagram(3)