UDP(4) BSD Kernel Interfaces Manual UDP(4)NAMEudp — Internet User Datagram Protocol
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
int
socket(AF_INET, SOCK_DGRAM, 0);
int
socket(AF_INET6, 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.
There are two UDP-level setsockopt(2)/getsockopt(2) options. UDP_OPTIONS
may be used to change the default behavior of the socket. For example:
setsockopt(s, IPPROTO_UDP, UDP_OPTIONS, NULL, 0);
The UDP_ENCAP option can be used to encapsulate ESP packets in UDP.
There are two valid enapsulation options: UDP_ENCAP_ESPINUDP_NON_IKE from
draft-ietf-ipsec-nat-t-ike-00/01 and UDP_ENCAP_ESPINUDP from draft-ietf-
ipsec-udp-encaps-06 defined in <netinet/udp.h>.
The UDP_RFC6056ALGO can be used to randomize the port selection. Valid
algorithms are described in rfc6056(7) and their respective constants are
in <netinet/rfc6056.h>. For example,
int algo = RFC6056_ALGO_RANDOM_PICK; /* see <netinet/rfc6056.h> */
setsockopt(s, IPPROTO_UDP, UDP_RFC6056ALGO, &algo, sizeof(algo));
The port selection can be also controlled at a global level for all UDP
or UDP6 sockets using the following sysctl(7) variables:
net.inet.udp.rfc6056.selected and net.inet6.udp6.rfc6056.selected respec‐
tively.
Options at the IP transport level may be used with UDP; see ip(4) or
ip6(4).
DIAGNOSTICS
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 datagram 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.
SEE ALSOgetsockopt(2), recv(2), send(2), socket(2), inet(4), inet6(4), intro(4),
ip(4), ip6(4), rfc6056(7), sysctl(7)
User Datagram Protocol, RFC, 768, August 28, 1980.
Requirements for Internet Hosts -- Communication Layers, RFC, 1122,
October 1989.
HISTORY
The udp protocol appeared in 4.2BSD.
BSD September 24, 2011 BSD
