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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)udp.4 8.1 (Berkeley) 6/5/93 29.\" $FreeBSD$ 30.\" 31.Dd August 1, 2022 32.Dt UDP 4 33.Os 34.Sh NAME 35.Nm udp 36.Nd Internet User Datagram Protocol 37.Sh SYNOPSIS 38.In sys/types.h 39.In sys/socket.h 40.In netinet/in.h 41.Ft int 42.Fn socket AF_INET SOCK_DGRAM 0 43.Sh DESCRIPTION 44.Tn UDP 45is a simple, unreliable datagram protocol which is used 46to support the 47.Dv SOCK_DGRAM 48abstraction for the Internet 49protocol family. 50.Tn UDP 51sockets are connectionless, and are 52normally used with the 53.Xr sendto 2 54and 55.Xr recvfrom 2 56calls, though the 57.Xr connect 2 58call may also be used to fix the destination for future 59packets (in which case the 60.Xr recv 2 61or 62.Xr read 2 63and 64.Xr send 2 65or 66.Xr write 2 67system calls may be used). 68.Pp 69.Tn UDP 70address formats are identical to those used by 71.Tn TCP . 72In particular 73.Tn UDP 74provides a port identifier in addition 75to the normal Internet address format. 76Note that the 77.Tn UDP 78port 79space is separate from the 80.Tn TCP 81port space (i.e., a 82.Tn UDP 83port 84may not be 85.Dq connected 86to a 87.Tn TCP 88port). 89In addition broadcast 90packets may be sent (assuming the underlying network supports 91this) by using a reserved 92.Dq broadcast address ; 93this address 94is network interface dependent. 95.Pp 96Options at the 97.Tn IP 98transport level may be used with 99.Tn UDP ; 100see 101.Xr ip 4 . 102.Tn UDP_ENCAP 103socket option may be used at the 104.Tn IPPROTO_UDP 105level to encapsulate 106.Tn ESP 107packets in 108.Tn UDP . 109Only one value is supported for this option: 110.Tn UDP_ENCAP_ESPINUDP 111from RFC 3948, defined in 112.In netinet/udp.h . 113.Sh MIB (sysctl) Variables 114The 115.Nm 116protocol implements a number of variables in the 117.Va net.inet.udp 118branch of the 119.Xr sysctl 3 120MIB, which can be also read or modified with 121.Xr sysctl 8 : 122.Bl -tag -width ".Va log_in_vain" 123.It Va blackhole 124When a datagram is received on a port where there is no socket 125listening, do not return an ICMP port unreachable message. 126(Disabled by default. 127See 128.Xr blackhole 4 . ) 129.It Va checksum 130Enable UDP checksums (enabled by default). 131.It Va log_in_vain 132For all UDP datagrams, to ports on which there is no socket 133listening, log the connection attempt (disabled by default). 134.It Va maxdgram 135Maximum outgoing UDP datagram size 136.It Va recvspace 137Maximum space for incoming UDP datagrams 138.El 139.Sh ERRORS 140A socket operation may fail with one of the following errors returned: 141.Bl -tag -width Er 142.It Bq Er EISCONN 143when trying to establish a connection on a socket which 144already has one, or when trying to send a datagram with the destination 145address specified and the socket is already connected; 146.It Bq Er ENOTCONN 147when trying to send a datagram, but 148no destination address is specified, and the socket has not been 149connected; 150.It Bq Er ENOBUFS 151when the system runs out of memory for 152an internal data structure; 153.It Bq Er EADDRINUSE 154when an attempt 155is made to create a socket with a port which has already been 156allocated; 157.It Bq Er EADDRNOTAVAIL 158when an attempt is made to create a 159socket with a network address for which no network interface 160exists. 161.El 162.Sh SEE ALSO 163.Xr getsockopt 2 , 164.Xr recv 2 , 165.Xr send 2 , 166.Xr socket 2 , 167.Xr blackhole 4 , 168.Xr inet 4 , 169.Xr intro 4 , 170.Xr ip 4 , 171.Xr udplite 4 172.Sh HISTORY 173The 174.Nm 175protocol appeared in 176.Bx 4.2 . 177