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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
     unspecified.   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
     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 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_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  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
            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  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.