UDP(4)                 FreeBSD Kernel Interfaces Manual                 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).
     UDP_ENCAP socket option may be used at the IPPROTO_UDP level to
     encapsulate ESP packets in UDP.  Only one value is supported for this
     option: UDP_ENCAP_ESPINUDP from RFC 3948, defined in <netinet/udp.h>.

FIB support

     UDP sockets are FIB-aware.  They inherit the FIB of the process which
     created the socket.  By default, a UDP socket bound to an address can
     receive datagrams originating from any FIB.  If the
     net.inet.udp.bind_all_fibs tunable is set to 0, all UDP sockets will
     receive only datagrams originating from the same FIB as the socket.  In
     this mode, multiple sockets can be bound to the same address, so long as
     each socket belongs to a different FIB, similar to the behavior of the
     SO_REUSEPORT option.

MIB (sysctl) Variables

     The udp protocol implements a number of variables in the net.inet.udp
     branch of the sysctl(3) MIB, which can be also read or modified with
     sysctl(8):

     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).)

     checksum     Enable UDP checksums (enabled by default).

     log_in_vain  For all UDP datagrams, to ports on which there is no socket
                  listening, log the connection attempt (disabled by default).

     maxdgram     Maximum outgoing UDP datagram size

     recvspace    Maximum space for incoming UDP datagrams

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
                        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 has not been
                        connected;

     [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
                        network address for which no network interface exists.

SEE ALSO

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

HISTORY

     The udp protocol appeared in 4.2BSD.

FreeBSD 15.1-STABLE-HBSD          May 7, 2026                           UDP(4)