.\" Copyright (c) 1983, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)udp.4	8.1 (Berkeley) 6/5/93
.\"
.Dd February 6, 2017
.Dt UDP 4
.Os
.Sh NAME
.Nm udp
.Nd Internet User Datagram Protocol
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In netinet/in.h
.Ft int
.Fn socket AF_INET SOCK_DGRAM 0
.Sh DESCRIPTION
.Tn UDP
is a simple, unreliable datagram protocol which is used
to support the
.Dv SOCK_DGRAM
abstraction for the Internet
protocol family.
.Tn UDP
sockets are connectionless, and are
normally used with the
.Xr sendto 2
and
.Xr recvfrom 2
calls, though the
.Xr connect 2
call may also be used to fix the destination for future
packets (in which case the
.Xr recv 2
or
.Xr read 2
and
.Xr send 2
or
.Xr write 2
system calls may be used).
.Pp
.Tn UDP
address formats are identical to those used by
.Tn TCP .
In particular
.Tn UDP
provides a port identifier in addition
to the normal Internet address format.
Note that the
.Tn UDP
port
space is separate from the
.Tn TCP
port space (i.e., a
.Tn UDP
port
may not be
.Dq connected
to a
.Tn TCP
port).
In addition broadcast
packets may be sent (assuming the underlying network supports
this) by using a reserved
.Dq broadcast address ;
this address
is network interface dependent.
.Pp
Options at the
.Tn IP
transport level may be used with
.Tn UDP ;
see
.Xr ip 4 .
.Tn UDP_ENCAP
socket option may be used at the
.Tn IPPROTO_UDP
level to encapsulate
.Tn ESP
packets in
.Tn UDP .
Only one value is supported for this option:
.Tn UDP_ENCAP_ESPINUDP
from RFC 3948, defined in
.In netinet/udp.h .
.Sh MIB VARIABLES
The
.Nm
protocol implements a number of variables in the
.Li net.inet
branch of the
.Xr sysctl 3
MIB.
.Bl -tag -width UDPCTL_RECVSPACEX
.It UDPCTL_CHECKSUM
.Pq udp.checksum
Enable udp checksums (enabled by default).
.It UDPCTL_MAXDGRAM
.Pq udp.maxdgram
Maximum outgoing UDP datagram size
.It UDPCTL_RECVSPACE
.Pq udp.recvspace
Maximum space for incoming UDP datagrams
.It udp.log_in_vain
For all udp datagrams, to ports on which there is no socket
listening, log the connection attempt (disabled by default).
.It udp.blackhole
When a datagram is received on a port where there is no socket
listening, do not return an ICMP port unreachable message.
(Disabled by default.
See
.Xr blackhole 4 . )
.El
.Sh ERRORS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width Er
.It Bq Er EISCONN
when trying to establish a connection on a socket which
already has one, or when trying to send a datagram with the destination
address specified and the socket is already connected;
.It Bq Er ENOTCONN
when trying to send a datagram, but
no destination address is specified, and the socket has not been
connected;
.It Bq Er ENOBUFS
when the system runs out of memory for
an internal data structure;
.It Bq Er EADDRINUSE
when an attempt
is made to create a socket with a port which has already been
allocated;
.It Bq Er EADDRNOTAVAIL
when an attempt is made to create a
socket with a network address for which no network interface
exists.
.El
.Sh SEE ALSO
.Xr getsockopt 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr socket 2 ,
.Xr blackhole 4 ,
.Xr inet 4 ,
.Xr intro 4 ,
.Xr ip 4 ,
.Xr udplite 4
.Sh HISTORY
The
.Nm
protocol appeared in
.Bx 4.2 .