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UDP(7)				    Linux Programmer's Manual				   UDP(7)

NAME
       udp - User Datagram Protocol for IPv4

SYNOPSIS
       #include <sys/socket.h>
       #include <netinet/in.h>
       #include <netinet/udp.h>

       udp_socket = socket(AF_INET, SOCK_DGRAM, 0);

DESCRIPTION
       This  is  an implementation of the User Datagram Protocol described in RFC 768.	It imple-
       ments a connectionless, unreliable datagram packet service.  Packets may be  reordered  or
       duplicated  before  they arrive.  UDP generates and checks checksums to catch transmission
       errors.

       When a UDP socket is created, its local and remote addresses are  unspecified.	Datagrams
       can  be sent immediately using sendto(2) or sendmsg(2) with a valid destination address as
       an argument.  When connect(2) is called on the socket, the default destination address  is
       set  and datagrams can now be sent using send(2) or write(2) without specifying a destina-
       tion address.  It is still possible to send to other destinations by passing an address to
       sendto(2)  or sendmsg(2).  In order to receive packets, the socket can be bound to a local
       address first by using bind(2).	Otherwise the socket layer will  automatically	assign	a
       free  local  port  out  of the range defined by /proc/sys/net/ipv4/ip_local_port_range and
       bind the socket to INADDR_ANY.

       All receive operations return only one packet.  When the packet is smaller than the passed
       buffer,	only  that  much data is returned; when it is bigger, the packet is truncated and
       the MSG_TRUNC flag is set.  MSG_WAITALL is not supported.

       IP options may be sent or received using the socket options described in ip(7).	They  are
       processed  by  the  kernel only when the appropriate /proc parameter is enabled (but still
       passed to the user even when it is turned off).	See ip(7).

       When the MSG_DONTROUTE flag is set on sending, the destination address  must  refer  to	a
       local interface address and the packet is sent only to that interface.

       By default, Linux UDP does path MTU (Maximum Transmission Unit) discovery.  This means the
       kernel will keep track of the MTU to a specific target IP address and return EMSGSIZE when
       a  UDP  packet  write  exceeds it.  When this happens, the application should decrease the
       packet size.  Path MTU discovery can be also turned off using the  IP_MTU_DISCOVER  socket
       option or the /proc/sys/net/ipv4/ip_no_pmtu_disc file; see ip(7) for details.  When turned
       off, UDP will fragment outgoing UDP packets that exceed the interface MTU.  However,  dis-
       abling it is not recommended for performance and reliability reasons.

   Address format
       UDP uses the IPv4 sockaddr_in address format described in ip(7).

   Error handling
       All fatal errors will be passed to the user as an error return even when the socket is not
       connected.  This includes asynchronous errors received from the network.  You may  get  an
       error  for an earlier packet that was sent on the same socket.  This behavior differs from
       many other BSD socket implementations which don't pass any errors  unless  the  socket  is
       connected.  Linux's behavior is mandated by RFC 1122.

       For  compatibility  with  legacy  code,	in  Linux  2.0 and 2.2 it was possible to set the
       SO_BSDCOMPAT SOL_SOCKET option to receive remote errors only when the socket has been con-
       nected  (except	for  EPROTO  and  EMSGSIZE).  Locally generated errors are always passed.
       Support for this socket option was removed in later kernels;  see  socket(7)  for  further
       information.

       When  the  IP_RECVERR  option is enabled, all errors are stored in the socket error queue,
       and can be received by recvmsg(2) with the MSG_ERRQUEUE flag set.

   /proc interfaces
       System-wide  UDP  parameter  settings  can  be  accessed  by  files   in   the	directory
       /proc/sys/net/ipv4/.

       udp_mem (since Linux 2.6.25)
	      This is a vector of three integers governing the number of pages allowed for queue-
	      ing by all UDP sockets.

	      min	Below this number  of  pages,  UDP  is	not  bothered  about  its  memory
			appetite.   When  the amount of memory allocated by UDP exceeds this num-
			ber, UDP starts to moderate memory usage.

	      pressure	This value was introduced to follow the format of tcp_mem (see tcp(7)).

	      max	Number of pages allowed for queueing by all UDP sockets.

	      Defaults values for these three items are calculated at boot time from  the  amount
	      of available memory.

       udp_rmem_min (integer; default value: PAGE_SIZE; since Linux 2.6.25)
	      Minimal size, in bytes, of receive buffers used by UDP sockets in moderation.  Each
	      UDP socket is able to use the size for receiving data, even if total pages  of  UDP
	      sockets exceed udp_mem pressure.

       udp_wmem_min (integer; default value: PAGE_SIZE; since Linux 2.6.25)
	      Minimal size, in bytes, of send buffer used by UDP sockets in moderation.  Each UDP
	      socket is able to use the size for sending data, even if total pages of UDP sockets
	      exceed udp_mem pressure.

   Socket options
       To  set	or  get a UDP socket option, call getsockopt(2) to read or setsockopt(2) to write
       the option with the option level argument set to  IPPROTO_UDP.	Unless	otherwise  noted,
       optval is a pointer to an int.

       UDP_CORK (since Linux 2.5.44)
	      If  this option is enabled, then all data output on this socket is accumulated into
	      a single datagram that is transmitted when the option  is  disabled.   This  option
	      should not be used in code intended to be portable.

   Ioctls
       These ioctls can be accessed using ioctl(2).  The correct syntax is:

	      int value;
	      error = ioctl(udp_socket, ioctl_type, &value);

       FIONREAD (SIOCINQ)
	      Gets  a  pointer	to  an integer as argument.  Returns the size of the next pending
	      datagram in the integer in bytes, or 0 when no datagram is pending.  Warning: Using
	      FIONREAD,  it  is  impossible  to distinguish the case where no datagram is pending
	      from the case where the next pending datagram contains zero bytes of data.   It  is
	      safer to use select(2), poll(2), or epoll(7) to distinguish these cases.

       TIOCOUTQ (SIOCOUTQ)
	      Returns  the  number  of	data  bytes in the local send queue.  Only supported with
	      Linux 2.4 and above.

       In addition all ioctls documented in ip(7) and socket(7) are supported.

ERRORS
       All errors documented for socket(7) or ip(7) may be returned by a send or receive on a UDP
       socket.

       ECONNREFUSED
	      No receiver was associated with the destination address.	This might be caused by a
	      previous packet sent over the socket.

VERSIONS
       IP_RECVERR is a new feature in Linux 2.2.

SEE ALSO
       ip(7), raw(7), socket(7), udplite(7)

       RFC 768 for the User Datagram Protocol.
       RFC 1122 for the host requirements.
       RFC 1191 for a description of path MTU discovery.

COLOPHON
       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux					    2013-07-31					   UDP(7)
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