udp(7)



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.   Sup-
              ported only 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 4.16 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                             2017-09-15                            UDP(7)

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