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 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 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.

    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 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 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.

  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. Supported only 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)

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

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