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

       udp - User Datagram Protocol for IPv4

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

       udp_socket = socket(AF_INET, SOCK_DGRAM, 0);

       This  is	 an  implementation of the User Datagram Protocol described in
       RFC 768.	 It implements 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  con‐
       nect(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	speci‐
       fying  a	 destination  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
       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 only processed by the kernel 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 only sent to
       that interface.

       By default, Linux UDP does path MTU (Maximum Transmission Unit) discov‐
       ery.   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_DIS‐
       COVER 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, disabling 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 possi‐
       ble to set the SO_BSDCOMPAT SOL_SOCKET option to receive remote	errors
       only  when  the	socket has been connected (except for EPROTO and EMSG‐
       SIZE).  Locally generated errors are always passed.  Support  for  this
       socket  option  was removed in later kernels; see socket(7) for further

       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 queueing by all UDP sockets.

	      min	Below this number of pages, UDP is not bothered	 about
			its  memory appetite.  When the amount of memory allo‐
			cated by UDP exceeds this number, UDP starts to moder‐
			ate 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	 sock‐

	      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 bites, of receive buffer used by UDP sockets in
	      moderation.  Each UDP socket is able to use the size for receiv‐
	      ing data, even if total pages  of	 UDP  sockets  exceed  udp_mem

       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 pres‐

   Socket Options
       To set or get a UDP socket option, call getsockopt(2) to read  or  set‐
       sockopt(2)  to  write  the option with the option level argument set to

       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.

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

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

	      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.

	      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.

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

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

       IP_RECVERR is a new feature in Linux 2.2.

       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.

       This page is part of release 3.22 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				  2008-11-21				UDP(7)

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