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udp(4)

NAME

     udp -- Internet User Datagram Protocol

SYNOPSIS

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>

     int
     socket(AF_INET, SOCK_DGRAM, 0);

DESCRIPTION

     UDP is a simple, unreliable datagram protocol which is used to support
     the SOCK_DGRAM abstraction for the Internet protocol family.  UDP sockets
     are connectionless, and are normally used with the sendto(2) and
     recvfrom(2) calls, though the connect(2) call may also be used to fix the
     destination for future packets (in which case the recv(2) or read(2) and
     send(2) or write(2) system calls may be used).

     UDP address formats are identical to those used by TCP.  In particular
     UDP provides a port identifier in addition to the normal Internet address
     format.  Note that the UDP port space is separate from the TCP port space
     (i.e., a UDP port may not be ``connected'' to a TCP port).  In addition
     broadcast packets may be sent (assuming the underlying network supports
     this) by using a reserved ``broadcast address''; this address is network
     interface dependent.

     Options at the IP transport level may be used with UDP; see ip(4).

ERRORS

     A socket operation may fail with one of the following errors returned:

     [EISCONN]		when trying to establish a connection on a socket
			which already has one, or when trying to send a data-
			gram with the destination address specified and the
			socket is already connected;

     [ENOTCONN] 	when trying to send a datagram, but no destination
			address is specified, and the socket hasn't been con-
			nected;

     [ENOBUFS]		when the system runs out of memory for an internal
			data structure;

     [EADDRINUSE]	when an attempt is made to create a socket with a port
			which has already been allocated;

     [EADDRNOTAVAIL]	when an attempt is made to create a socket with a net-
			work address for which no network interface exists.

MIB VARIABLES

     The udp protocol implements a number of variables in the net.inet branch
     of the sysctl(3) MIB.

     UDPCTL_CHECKSUM	(udp.checksum) Enable udp checksums (enabled by
			default).


     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
			blackhole(4).)

SEE ALSO

     getsockopt(2), recv(2), send(2), socket(2), blackhole(4), inet(4),
     intro(4), ip(4)

HISTORY

     The udp protocol appeared in 4.2BSD.

FreeBSD 5.4			 June 5, 1993			   FreeBSD 5.4

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