UDP(7)                      Linux Programmer's Manual                     UDP(7)

       udp - User Datagram Protocol for IPv4

       #include <sys/socket.h>
       #include <netinet/in.h>
       #include <netinet/udp.h>

       udp_socket = socket(AF_INET, SOCK_DGRAM, 0);

       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 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)
       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

       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

                     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.  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, see

       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.

       These ioctls can be accessed using ioctl(2).  The correct syntax is:

              int value;
              error = ioctl(udp_socket, ioctl_type, &value);

              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.

              Returns the number of data bytes in the local send queue.
              Supported only with Linux 2.4 and above.

       In addition, all ioctls documented in ip(7) and socket(7) are supported.

       All errors documented for socket(7) or ip(7) may be returned by a send or
       receive on a UDP socket.

              No receiver was associated with the destination address.  This
              might be caused by a previous packet sent over the socket.

       IP_RECVERR is a new feature in Linux 2.2.

       ip(7), raw(7), socket(7), udplite(7)

       The kernel source file Documentation/networking/ip-sysctl.txt.

       RFC 768 for the User Datagram Protocol.
       RFC 1122 for the host requirements.
       RFC 1191 for a description of path MTU discovery.

       This page is part of release 5.13 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

Linux                              2021-03-22                             UDP(7)