Updated: 2021/Apr/14

UDP(4)                       Device Drivers Manual                      UDP(4)

     udp - Internet User Datagram Protocol

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

     socket(AF_INET, SOCK_DGRAM, 0);

     socket(AF_INET6, SOCK_DGRAM, 0);

     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 is one valid encapsulation option: UDP_ENCAP_ESPINUDP from RFC3948
     defined in <netinet/udp.h>.

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

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

     [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
                      network address for which no network interface exists.

     [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;

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

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

     getsockopt(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.

     The udp protocol appeared in 4.2BSD.

NetBSD 9.99                      May 31, 2018                      NetBSD 9.99