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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)udp.4 8.1 (Berkeley) 6/5/93 29.\" 30.Dd August 1, 2022 31.Dt UDP 4 32.Os 33.Sh NAME 34.Nm udp 35.Nd Internet User Datagram Protocol 36.Sh SYNOPSIS 37.In sys/types.h 38.In sys/socket.h 39.In netinet/in.h 40.Ft int 41.Fn socket AF_INET SOCK_DGRAM 0 42.Sh DESCRIPTION 43.Tn UDP 44is a simple, unreliable datagram protocol which is used 45to support the 46.Dv SOCK_DGRAM 47abstraction for the Internet 48protocol family. 49.Tn UDP 50sockets are connectionless, and are 51normally used with the 52.Xr sendto 2 53and 54.Xr recvfrom 2 55calls, though the 56.Xr connect 2 57call may also be used to fix the destination for future 58packets (in which case the 59.Xr recv 2 60or 61.Xr read 2 62and 63.Xr send 2 64or 65.Xr write 2 66system calls may be used). 67.Pp 68.Tn UDP 69address formats are identical to those used by 70.Tn TCP . 71In particular 72.Tn UDP 73provides a port identifier in addition 74to the normal Internet address format. 75Note that the 76.Tn UDP 77port 78space is separate from the 79.Tn TCP 80port space (i.e., a 81.Tn UDP 82port 83may not be 84.Dq connected 85to a 86.Tn TCP 87port). 88In addition broadcast 89packets may be sent (assuming the underlying network supports 90this) by using a reserved 91.Dq broadcast address ; 92this address 93is network interface dependent. 94.Pp 95Options at the 96.Tn IP 97transport level may be used with 98.Tn UDP ; 99see 100.Xr ip 4 . 101.Tn UDP_ENCAP 102socket option may be used at the 103.Tn IPPROTO_UDP 104level to encapsulate 105.Tn ESP 106packets in 107.Tn UDP . 108Only one value is supported for this option: 109.Tn UDP_ENCAP_ESPINUDP 110from RFC 3948, defined in 111.In netinet/udp.h . 112.Sh MIB (sysctl) Variables 113The 114.Nm 115protocol implements a number of variables in the 116.Va net.inet.udp 117branch of the 118.Xr sysctl 3 119MIB, which can be also read or modified with 120.Xr sysctl 8 : 121.Bl -tag -width ".Va log_in_vain" 122.It Va blackhole 123When a datagram is received on a port where there is no socket 124listening, do not return an ICMP port unreachable message. 125(Disabled by default. 126See 127.Xr blackhole 4 . ) 128.It Va checksum 129Enable UDP checksums (enabled by default). 130.It Va log_in_vain 131For all UDP datagrams, to ports on which there is no socket 132listening, log the connection attempt (disabled by default). 133.It Va maxdgram 134Maximum outgoing UDP datagram size 135.It Va recvspace 136Maximum space for incoming UDP datagrams 137.El 138.Sh ERRORS 139A socket operation may fail with one of the following errors returned: 140.Bl -tag -width Er 141.It Bq Er EISCONN 142when trying to establish a connection on a socket which 143already has one, or when trying to send a datagram with the destination 144address specified and the socket is already connected; 145.It Bq Er ENOTCONN 146when trying to send a datagram, but 147no destination address is specified, and the socket has not been 148connected; 149.It Bq Er ENOBUFS 150when the system runs out of memory for 151an internal data structure; 152.It Bq Er EADDRINUSE 153when an attempt 154is made to create a socket with a port which has already been 155allocated; 156.It Bq Er EADDRNOTAVAIL 157when an attempt is made to create a 158socket with a network address for which no network interface 159exists. 160.El 161.Sh SEE ALSO 162.Xr getsockopt 2 , 163.Xr recv 2 , 164.Xr send 2 , 165.Xr socket 2 , 166.Xr blackhole 4 , 167.Xr inet 4 , 168.Xr intro 4 , 169.Xr ip 4 , 170.Xr udplite 4 171.Sh HISTORY 172The 173.Nm 174protocol appeared in 175.Bx 4.2 . 176