xref: /dragonfly/share/man/man4/udp.4 (revision c37c9ab3) 
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28.\"     @(#)udp.4	8.1 (Berkeley) 6/5/93
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32.Dd June 5, 1993
33.Dt UDP 4
34.Os
35.Sh NAME
36.Nm udp
37.Nd Internet User Datagram Protocol
38.Sh SYNOPSIS
39.In sys/types.h
40.In sys/socket.h
41.In netinet/in.h
42.Ft int
43.Fn socket AF_INET SOCK_DGRAM 0
44.Sh DESCRIPTION
45.Tn UDP
46is a simple, unreliable datagram protocol which is used
47to support the
48.Dv SOCK_DGRAM
49abstraction for the Internet
50protocol family.
51.Tn UDP
52sockets are connectionless, and are
53normally used with the
54.Xr sendto 2
55and
56.Xr recvfrom 2
57calls, though the
58.Xr connect 2
59call may also be used to fix the destination for future
60packets (in which case the
61.Xr recv 2
62or
63.Xr read 2
64and
65.Xr send 2
66or
67.Xr write 2
68system calls may be used).
69.Pp
70.Tn UDP
71address formats are identical to those used by
72.Tn TCP .
73In particular
74.Tn UDP
75provides a port identifier in addition
76to the normal Internet address format.  Note 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).  In addition broadcast
89packets may be sent (assuming the underlying network supports
90this) by using a reserved
91.Dq broadcast address ;
92this address
93is network interface dependent.
94.Pp
95Options at the
96.Tn IP
97transport level may be used with
98.Tn UDP ;
99see
100.Xr ip 4 .
101.Sh ERRORS
102A socket operation may fail with one of the following errors returned:
103.Bl -tag -width Er
104.It Bq Er EISCONN
105when trying to establish a connection on a socket which
106already has one, or when trying to send a datagram with the destination
107address specified and the socket is already connected;
108.It Bq Er ENOTCONN
109when trying to send a datagram, but
110no destination address is specified, and the socket hasn't been
111connected;
112.It Bq Er ENOBUFS
113when the system runs out of memory for
114an internal data structure;
115.It Bq Er EADDRINUSE
116when an attempt
117is made to create a socket with a port which has already been
118allocated;
119.It Bq Er EADDRNOTAVAIL
120when an attempt is made to create a
121socket with a network address for which no network interface
122exists.
123.El
124.Sh MIB VARIABLES
125The
126.Nm
127protocol implements a number of variables in the
128.Li net.inet
129branch of the
130.Xr sysctl 3
131MIB.
132.Bl -tag -width UDPCTL_RECVSPACEX
133.It UDPCTL_CHECKSUM
134.Pq udp.checksum
135Enable udp checksums (enabled by default).
136.It UDPCTL_MAXDGRAM
137.Pq udp.maxdgram
138Maximum outgoing UDP datagram size
139.It UDPCTL_RECVSPACE
140.Pq udp.recvspace
141Maximum space for incoming UDP datagrams
142.It udp.log_in_vain
143For all udp datagrams, to ports on which there is no socket
144listening, log the connection attempt (disabled by default).
145.It udp.blackhole
146When a datagram is received on a port where there is no socket
147listening, do not return an ICMP port unreachable message.
148(Disabled by default.  See
149.Xr blackhole 4 . )
150.El
151.Sh SEE ALSO
152.Xr getsockopt 2 ,
153.Xr recv 2 ,
154.Xr send 2 ,
155.Xr socket 2 ,
156.Xr blackhole 4 ,
157.Xr inet 4 ,
158.Xr intro 4 ,
159.Xr ip 4
160.Sh HISTORY
161The
162.Nm
163protocol appeared in
164.Bx 4.2 .
165