udp(7) — Linux manual page


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

NAME         top

       udp - User Datagram Protocol for IPv4

SYNOPSIS         top

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

       udp_socket = socket(AF_INET, SOCK_DGRAM, 0);

DESCRIPTION         top

       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

       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 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
                     allocated by UDP exceeds this number, UDP starts to
                     moderate memory usage.

                     This value was introduced to follow the format of
                     tcp_mem (see tcp(7)).

              max    Number of pages allowed for queueing by all UDP

              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

       Following is a list of UDP-specific socket options.  For details of
       some other socket options that are also applicable for UDP sockets,
       see socket(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.

       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

ERRORS         top

       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.

VERSIONS         top

       IP_RECVERR is a new feature in Linux 2.2.

SEE ALSO         top

       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.

COLOPHON         top

       This page is part of release 5.08 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                            2020-04-11                           UDP(7)

Pages that refer to this page: getsockopt(2)recv(2)recvfrom(2)recvmsg(2)send(2)sendmsg(2)sendto(2)setsockopt(2)socket(2)services(5)ip(7)socket(7)udplite(7)unix(7)