xref: /freebsd/share/man/man4/udp.4 (revision 0d66206fff44f864ea8a4b220c3a53b4caa959a0)
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28.\"     @(#)udp.4	8.1 (Berkeley) 6/5/93
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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