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.\"     @(#)udp.4	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.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 .