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UDP(7)                     Linux Programmer's ManualUDP(7)NAME       udp - User Datagram Protocol for IPv4SYNOPSIS       #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  usingsendto(2) orsendmsg(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 usingsend(2) orwrite(2) without  speci-       fying  a  destination  address.   It is still possible to send to other       destinations by passing an address tosendto(2) orsendmsg(2).  In  or-       der  to  receive  packets,  the  socket can be bound to a local address       first by usingbind(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       inip(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).  Seeip(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; seeip(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 inip(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; seesocket(7)  for  further       information.       When  the  IP_RECVERR  option  is enabled, all errors are stored in the       socket error queue, and can be received byrecvmsg(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 (seetcp(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, callgetsockopt(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.       Following  is  a  list  of UDP-specific socket options.  For details of       some other socket options that are also applicable for UDP sockets, seesocket(7).       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 usingioctl(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 useselect(2),poll(2), orepoll(7) to              distinguish these cases.       TIOCOUTQ (SIOCOUTQ)              Returns the number of data bytes in the local send queue.   Sup-              ported only with Linux 2.4 and above.       In  addition,  all  ioctls  documented  inip(7) andsocket(7) are sup-       ported.ERRORS       All errors documented forsocket(7) orip(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 ALSOip(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 5.05 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       https://www.kernel.org/doc/man-pages/.Linux                             2019-03-06UDP(7)