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NAME

       udp - User Datagram Protocol for IPv4

SYNOPSIS

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

       udp_socket = socket(PF_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
       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  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
       sysctl  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)
       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 ip_no_pmtu_disc sysctl, 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 possible
       to set the SO_BSDCOMPAT SOL_SOCKET option to receive remote errors only
       when  the  socket  has been connected (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.

   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.

       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.

       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)

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

COLOPHON

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       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.