1=pod
2
3=head1 NAME
4
5BIO_s_datagram, BIO_new_dgram,
6BIO_ctrl_dgram_connect,
7BIO_ctrl_set_connected,
8BIO_dgram_recv_timedout,
9BIO_dgram_send_timedout,
10BIO_dgram_get_peer,
11BIO_dgram_set_peer,
12BIO_dgram_get_mtu_overhead - Network BIO with datagram semantics
13
14=head1 SYNOPSIS
15
16 #include <openssl/bio.h>
17
18 BIO_METHOD *BIO_s_datagram(void);
19 BIO *BIO_new_dgram(int fd, int close_flag);
20
21 int BIO_ctrl_dgram_connect(BIO *bio, const BIO_ADDR *peer);
22 int BIO_ctrl_set_connected(BIO *bio, const BIO_ADDR *peer);
23 int BIO_dgram_recv_timedout(BIO *bio);
24 int BIO_dgram_send_timedout(BIO *bio);
25 int BIO_dgram_get_peer(BIO *bio, BIO_ADDR *peer);
26 int BIO_dgram_set_peer(BIO *bio, const BIO_ADDR *peer);
27 int BIO_dgram_get_mtu_overhead(BIO *bio);
28
29=head1 DESCRIPTION
30
31BIO_s_datagram() is a BIO implementation designed for use with network sockets
32which provide datagram semantics, such as UDP sockets. It is suitable for use
33with DTLSv1.
34
35Because BIO_s_datagram() has datagram semantics, a single BIO_write() call sends
36a single datagram and a single BIO_read() call receives a single datagram. If
37the size of the buffer passed to BIO_read() is inadequate, the datagram is
38silently truncated.
39
40When using BIO_s_datagram(), it is important to note that:
41
42=over 4
43
44=item
45
46This BIO can be used with either a connected or unconnected network socket. A
47connected socket is a network socket which has had L<BIO_connect(3)> or a
48similar OS-specific function called on it. Such a socket can only receive
49datagrams from the specified peer. Any other socket is an unconnected socket and
50can receive datagrams from any host.
51
52=item
53
54Despite their naming,
55neither BIO_ctrl_dgram_connect() nor BIO_ctrl_set_connected() cause a socket
56to become connected. These controls are provided to indicate to the BIO how
57the underlying socket is configured and how it is to be used; see below.
58
59=item
60
61Use of BIO_s_datagram() with an unconnected network socket is hazardous hecause
62any successful call to BIO_read() results in the peer address used for any
63subsequent call to BIO_write() being set to the source address of the datagram
64received by that call to BIO_read(). Thus, unless the caller calls
65BIO_dgram_set_peer() immediately prior to every call to BIO_write(), or never
66calls BIO_read(), any host on the network may cause future datagrams written to
67be redirected to that host. Therefore, it is recommended that users use
68BIO_s_dgram() only with a connected socket. An exception is where
69L<DTLSv1_listen(3)> must be used; see L<DTLSv1_listen(3)> for further
70discussion.
71
72=back
73
74Various controls are available for configuring the BIO_s_datagram() using
75L<BIO_ctrl(3)>:
76
77=over 4
78
79=item BIO_ctrl_dgram_connect (BIO_CTRL_DGRAM_CONNECT)
80
81This is equivalent to calling L<BIO_dgram_set_peer(3)>.
82
83Despite its name, this function does not cause the underlying socket to become
84connected.
85
86=item BIO_ctrl_set_connected (BIO_CTRL_SET_CONNECTED)
87
88This informs the BIO_s_datagram() whether the underlying socket has been
89connected, and therefore how the BIO_s_datagram() should attempt to use the
90socket.
91
92If the I<peer> argument is non-NULL, BIO_s_datagram() assumes that the
93underlying socket has been connected and will attempt to use the socket using OS
94APIs which do not specify peer addresses (for example, send(3) and recv(3) or
95similar). The I<peer> argument should specify the peer address to which the socket
96is connected.
97
98If the I<peer> argument is NULL, BIO_s_datagram() assumes that the underlying
99socket is not connected and will attempt to use the socket using an OS APIs
100which specify peer addresses (for example, sendto(3) and recvfrom(3)).
101
102=item BIO_dgram_get_peer (BIO_CTRL_DGRAM_GET_PEER)
103
104This outputs a B<BIO_ADDR> which specifies one of the following values,
105whichever happened most recently:
106
107=over 4
108
109=item
110
111The peer address last passed to BIO_dgram_set_peer(), BIO_ctrl_dgram_connect()
112or BIO_ctrl_set_connected().
113
114=item
115
116The peer address of the datagram last received by a call to BIO_read().
117
118=back
119
120=item BIO_dgram_set_peer (BIO_CTRL_DGRAM_SET_PEER)
121
122Sets the peer address to be used for subsequent writes to this BIO.
123
124Warning: When used with an unconnected network socket, the value set may be
125modified by future calls to L<BIO_read(3)>, making use of BIO_s_datagram()
126hazardous when used with unconnected network sockets; see above.
127
128=item BIO_dgram_recv_timeout (BIO_CTRL_DGRAM_GET_RECV_TIMER_EXP)
129
130Returns 1 if the last I/O operation performed on the BIO (for example, via a
131call to L<BIO_read(3)>) may have been caused by a receive timeout.
132
133=item BIO_dgram_send_timedout (BIO_CTRL_DGRAM_GET_SEND_TIMER_EXP)
134
135Returns 1 if the last I/O operation performed on the BIO (for example, via a
136call to L<BIO_write(3)>) may have been caused by a send timeout.
137
138=item BIO_dgram_get_mtu_overhead (BIO_CTRL_DGRAM_GET_MTU_OVERHEAD)
139
140Returns a quantity in bytes which is a rough estimate of the number of bytes of
141overhead which should typically be added to a datagram payload size in order to
142estimate the final size of the Layer 3 (e.g. IP) packet which will contain the
143datagram. In most cases, the maximum datagram payload size which can be
144transmitted can be determined by determining the link MTU in bytes and
145subtracting the value returned by this call.
146
147The value returned by this call depends on the network layer protocol being
148used.
149
150The value returned is not fully reliable because datagram overheads can be
151higher in atypical network configurations, for example where IPv6 extension
152headers or IPv4 options are used.
153
154=item BIO_CTRL_DGRAM_SET_DONT_FRAG
155
156If I<num> is nonzero, configures the underlying network socket to enable Don't
157Fragment mode, in which datagrams will be set with the IP Don't Fragment (DF)
158bit set. If I<num> is zero, Don't Fragment mode is disabled.
159
160=item BIO_CTRL_DGRAM_QUERY_MTU
161
162Queries the OS for its assessment of the Path MTU for the destination to which
163the underlying network socket, and returns that Path MTU in bytes. This control
164can only be used with a connected socket.
165
166This is not supported on all platforms and depends on OS support being
167available. Returns 0 on failure.
168
169=item BIO_CTRL_DGRAM_MTU_DISCOVER
170
171This control requests that Path MTU discovery be enabled on the underlying
172network socket.
173
174=item BIO_CTRL_DGRAM_GET_FALLBACK_MTU
175
176Returns the estimated minimum size of datagram payload which should always be
177supported on the BIO. This size is determined by the minimum MTU required to be
178supported by the applicable underlying network layer. Use of datagrams of this
179size may lead to suboptimal performance, but should be routable in all
180circumstances. The value returned is the datagram payload size in bytes and does
181not include the size of layer 3 or layer 4 protocol headers.
182
183=item BIO_CTRL_DGRAM_MTU_EXCEEDED
184
185Returns 1 if the last attempted write to the BIO failed due to the size of the
186attempted write exceeding the applicable MTU.
187
188=item BIO_CTRL_DGRAM_SET_NEXT_TIMEOUT
189
190Accepts a pointer to a B<struct timeval>. If the time specified is zero,
191disables receive timeouts. Otherwise, configures the specified time interval as
192the receive timeout for the socket for the purposes of future L<BIO_read(3)>
193calls.
194
195=item BIO_CTRL_DGRAM_SET_PEEK_MODE
196
197If B<num> is nonzero, enables peek mode; otherwise, disables peek mode. Where
198peek mode is enabled, calls to L<BIO_read(3)> read datagrams from the underlying
199network socket in peek mode, meaning that a future call to L<BIO_read(3)> will
200yield the same datagram until peek mode is disabled.
201
202=back
203
204BIO_new_dgram() is a helper function which instantiates a BIO_s_datagram() and
205sets the BIO to use the socket given in I<fd> by calling BIO_set_fd().
206
207=head1 RETURN VALUES
208
209BIO_s_datagram() returns a BIO method.
210
211BIO_new_dgram() returns a BIO on success and NULL on failure.
212
213BIO_ctrl_dgram_connect(), BIO_ctrl_set_connected(),
214BIO_dgram_get_peer(), BIO_dgram_set_peer() return 1 on success and 0 on failure.
215
216BIO_dgram_recv_timedout() and BIO_dgram_send_timedout() return 0 or 1 depending
217on the circumstance; see discussion above.
218
219BIO_dgram_get_mtu_overhead() returns a value in bytes.
220
221=head1 SEE ALSO
222
223L<DTLSv1_listen(3)>, L<bio(7)>
224
225=head1 COPYRIGHT
226
227Copyright 2022-2023 The OpenSSL Project Authors. All Rights Reserved.
228
229Licensed under the Apache License 2.0 (the "License").  You may not use
230this file except in compliance with the License.  You can obtain a copy
231in the file LICENSE in the source distribution or at
232L<https://www.openssl.org/source/license.html>.
233
234=cut
235