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NAME

       udp - User Datagram Protocol for IPv4

SYNOPSIS

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

   /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
                        allocated  by  UDP  exceeds this number, 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.

       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 3.27 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/.