BIO_S_DATAGRAM(3SSL) OpenSSL BIO_S_DATAGRAM(3SSL)
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_detect_peer_addr, 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);
int BIO_dgram_detect_peer_addr(BIO *bio, BIO_ADDR *peer);
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 or QUIC.
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.
For a memory-based BIO which provides datagram semantics identical to
those of BIO_s_datagram(), see BIO_s_dgram_pair(3).
This BIO supports the BIO_sendmmsg(3) and BIO_recvmmsg(3) functions.
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 either use BIO_s_dgram() only with a connected socket,
or, if using BIO_s_dgram() with an unconnected socket, to use the
BIO_sendmmsg(3) and BIO_recvmmsg(3) methods only and forego use of
BIO_read(3) and BIO_write(3). An exception is where DTLSv1_listen(3)
must be used; see DTLSv1_listen(3) for further discussion.
• Unlike BIO_read(3) and BIO_write(3), the BIO_sendmmsg(3) and
BIO_recvmmsg(3) methods are stateless and do not cause the internal
state of the BIO_s_datagram() to change.
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)).
This control does not affect the operation of BIO_sendmmsg(3) or
BIO_recvmmsg(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.
This does not affect the operation of BIO_sendmmsg(3).
BIO_recvmmsg(3) does not affect the value set by
BIO_dgram_set_peer().
BIO_dgram_detect_peer_addr (BIO_CTRL_DGRAM_DETECT_PEER_ADDR)
This is similar to BIO_dgram_get_peer() except that if the peer
address has not been set on the BIO object, an OS call such as
getpeername(2) will be attempted to try and autodetect the peer
address to which the underlying socket is connected. Other BIOs may
also implement this control if they are capable of sensing a peer
address, without necessarily also implementing BIO_dgram_set_peer()
and BIO_dgram_get_peer().
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_recvmmsg(3) is not affected by this control.
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() and
BIO_dgram_set_peer() return 1 on success and 0 on failure.
BIO_dgram_get_peer() and BIO_dgram_detect_peer_addr() return 0 on
failure and the number of bytes for the outputted address representation
(a positive value) on success.
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
BIO_sendmmsg(3), BIO_s_dgram_pair(3), 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>.
3.5.7 2026-06-09 BIO_S_DATAGRAM(3SSL)
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