man7/udp.7

   1 .\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
   2 .\" Permission is granted to distribute possibly modified copies
   3 .\" of this page provided the header is included verbatim,
   4 .\" and in case of nontrivial modification author and date
   5 .\" of the modification is added to the header.
   6 .\" $Id: udp.7,v 1.7 2000/01/22 01:55:05 freitag Exp $
   7 .TH UDP  7 1998-10-02 "Linux Man Page" "Linux Programmer's Manual"
   8 .SH NAME
   9 udp \- User Datagram Protocol for IPv4
  10 .SH SYNOPSIS
  11 .B #include <sys/socket.h>
  12 .br
  13 .B #include <netinet/in.h>
  14 .br
  15 .B udp_socket = socket(PF_INET, SOCK_DGRAM, 0);
  16 .SH DESCRIPTION
  17 This is an implemention of the User Datagram Protocol described in RFC768. It
  18 implements a connectionless, unreliable datagram packet service.
  19 Packets may be reordered or duplicated before they arrive. UDP
  20 generates and checks checksums to catch transmission errors.
  21
  22 When a UDP socket is created, its local and remote addresses are unspecified.
  23 Datagrams can be sent immediately using
  24 .BR sendto (2)
  25 or
  26 .BR sendmsg (2)
  27 with a valid destination address as an argument.  When
  28 .BR connect (2)
  29 is called on the socket the default destination address is set and datagrams
  30 can now be sent using
  31 .BR send (2)
  32 or
  33 .BR write (2)
  34 without specifying an destination address.
  35 It is still possible to send to other destinations by passing an address to
  36 .BR sendto (2)
  37 or
  38 .BR sendmsg (2).
  39 In order to receive packets the socket can be bound to an local
  40 address first by using
  41 .BR bind (2).
  42 Otherwise the socket layer will automatically assign
  43 a free local port out of the range defined by
  44 .I net.ipv4.ip_local_port_range
  45 and bind the socket to
  46 .IR INADDR_ANY .
  47
  48 All receive operations return only one packet.  When the packet is smaller
  49 than the passed buffer only that much data is returned, when it is bigger
  50 the packet is truncated and the
  51 .B MSG_TRUNC
  52 flag is set.
  53 .I MSG_WAITALL
  54 is not supported.
  55
  56 IP options may be sent or received using the socket options described in
  57 .BR ip (7).
  58 They are only processed by the kernel when the appropriate sysctl
  59 is enabled (but still passed to the user even when it is turned off). See
  60 .BR ip (7).
  61
  62 When the
  63 .B MSG_DONTROUTE
  64 flag is set on sending the destination address must refer to an local
  65 interface address and the packet is only sent to that interface.
  66
  67 UDP fragments a packet when its total length exceeds the interface MTU
  68 (Maximum Transmission Unit).
  69 A more network friendly alternative is to use path MTU discovery
  70 as described in the
  71 .B IP_MTU_DISCOVER
  72 section of
  73 .BR ip (7).
  74
  75 .SH "ADDRESS FORMAT"
  76 UDP uses the IPv4
  77 .B sockaddr_in
  78 address format described in
  79 .BR ip (7).
  80
  81 .SH "ERROR HANDLING"
  82 All fatal errors will be passed to the user as an error return even
  83 when the socket is not connected. This includes asynchronous errors
  84 received from the network. You may get an error for an earlier packet
  85 that was sent on the same socket.
  86 This behaviour differs from many other BSD socket implementations
  87 which don't pass any errors unless the socket is connected.
  88 Linux's behaviour is mandated by
  89 .BR RFC1122 .
  90
  91 For compatibility with legacy code it is possible to set the
  92 .B SO_BSDCOMPAT
  93 SOL_SOCKET option to receive remote errors only when the socket has been
  94 connected (except for
  95 .B EPROTO
  96 and
  97 .BR EMSGSIZE ).
  98 It is better to fix the
  99 code to handle errors properly than to enable this option.
 100 Locally generated errors are always passed.
 101
 102 When the
 103 .B IP_RECVERR
 104 option is enabled all errors are stored in the socket error queue
 105 and can be received by
 106 .BR recvmsg (2)
 107 with the
 108 .B MSG_ERRQUEUE
 109 flag set.
 110 .SH IOCTLS
 111 These ioctls can be accessed using
 112 .BR ioctl (2).
 113 The correct syntax is:
 114 .PP
 115 .RS
 116 .nf
 117 .BI int " value";
 118 .IB error " = ioctl(" tcp_socket ", " ioctl_type ", &" value ");"
 119 .fi
 120 .RE
 121 .TP
 122 .BR FIONREAD " (" SIOCINQ )
 123 Gets a pointer to an integer as argument. Returns the size of the next
 124 pending datagram in the integer in bytes, or 0 when no datagram is pending.
 125 .TP
 126 .BR TIOCOUTQ " (" SIOCOUTQ )
 127 Returns the number of data bytes in the local send queue. Only supported
 128 with Linux 2.4 and above.
 129 .PP
 130 In addition all ioctls documented in
 131 .BR ip (7)
 132 and
 133 .BR socket (7)
 134 are supported.
 135 .SH ERRORS
 136 All errors documented for
 137 .BR socket (7)
 138 or
 139 .BR ip (7)
 140 may be returned by a send or receive on a UDP socket.
 141
 142 .B ECONNREFUSED
 143 No receiver was associated with the destination address.  This might be
 144 caused by a previous packet sent over the socket.
 145
 146 .SH VERSIONS
 147 IP_RECVERR is a new feature in Linux 2.2.
 148
 149 .SH CREDITS
 150 This man page was written by Andi Kleen.
 151
 152 .SH "SEE ALSO"
 153 .BR ip (7),
 154 .BR raw (7),
 155 .BR socket (7)
 156
 157 RFC768 for the User Datagram Protocol.
 158 .br
 159 RFC1122 for the host requirements.
 160 .br
 161 RFC1191 for a description of path MTU discovery.