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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 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  un-
       specified.   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  or-
       der  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  IN-
       ADDR_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) 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
       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_ER-
       RQUEUE flag set.

   /proc interfaces
       System-wide  UDP	parameter settings can be accessed by files in the di-
       rectory /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-
			ets.

	      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  re-
	      ceiving  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	 pres-
	      sure.

   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
       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 sup-
       ported.

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 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2013-07-31				UDP(7)

NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | SEE ALSO | COLOPHON

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