udp(7) — Linux manual page


udp(7)              Miscellaneous Information 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
              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
              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 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.

       UDP_SEGMENT (since Linux 4.18)
              Enables UDP segmentation offload.  Segmentation offload
              reduces send(2) cost by transferring multiple datagrams
              worth of data as a single large packet through the kernel
              transmit path, even when that exceeds MTU.  As late as
              possible, the large packet is split by segment size into a
              series of datagrams.  This segmentation offload step is
              deferred to hardware if supported, else performed in
              software.  This option takes a value in the range
              [0, USHRT_MAX] that sets the segment size: the size of
              datagram payload, excluding the UDP header.  The segment
              size must be chosen such that at most 64 datagrams are
              sent in a single call and that the datagrams after
              segmentation meet the same MTU rules that apply to
              datagrams sent without this option.  Segmentation offload
              depends on checksum offload, as datagram checksums are
              computed after segmentation.  The option may also be set
              for individual sendmsg(2) calls by passing it as a
              cmsg(3).  A value of zero disables the feature.  This
              option should not be used in code intended to be portable.

       UDP_GRO (since Linux 5.0)
              Enables UDP receive offload.  If enabled, the socket may
              receive multiple datagrams worth of data as a single large
              buffer, together with a cmsg(3) that holds the segment
              size.  This option is the inverse of segmentation offload.
              It reduces receive cost by handling multiple datagrams
              worth of data as a single large packet in the kernel
              receive path, even when that exceeds MTU.  This option
              should not be used in code intended to be portable.

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

              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

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 the man-pages (Linux kernel and C library
       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report
       for this manual page, see
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
       fetched from
       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
       2024-06-26.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to

Linux man-pages 6.9.1          2024-05-02                         udp(7)

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