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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>

       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 immedi-
       ately 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 /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  sock-
              ets.

              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 bites, of receive buffer 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.

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



Linux                             2008-11-21                            UDP(7)