udp(7) - Linux manual page (original) (raw)
udp(7) Miscellaneous Information Manual udp(7)
NAME top
udp - User Datagram Protocol for IPv4SYNOPSIS top
**#include <sys/socket.h>**
**#include <netinet/in.h>**
**#include <netinet/udp.h>**
_udpsocket_ **= 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)](../man2/sendto.2.html) or
[sendmsg(2)](../man2/sendmsg.2.html) with a valid destination address as an argument. When
[connect(2)](../man2/connect.2.html) is called on the socket, the default destination
address is set and datagrams can now be sent using [send(2)](../man2/send.2.html) or
[write(2)](../man2/write.2.html) without specifying a destination address. It is still
possible to send to other destinations by passing an address to
[sendto(2)](../man2/sendto.2.html) or [sendmsg(2)](../man2/sendmsg.2.html). In order to receive packets, the socket
can be bound to a local address first by using [bind(2)](../man2/bind.2.html).
Otherwise, the socket layer will automatically assign a free local
port out of the range defined by
_/proc/sys/net/ipv4/iplocalportrange_ 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)](../man7/ip.7.html). 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)](../man7/ip.7.html).
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/ipnopmtudisc_ file; see [ip(7)](../man7/ip.7.html) 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 sockaddrin 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)](../man7/socket.7.html) 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)](../man2/recvmsg.2.html) 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/.
_udpmem_ (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
_tcpmem_ (see [tcp(7)](../man7/tcp.7.html)).
_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.
_udprmemmin_ (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 _udpmem_ pressure.
_udpwmemmin_ (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
_udpmem_ 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)](../man7/socket.7.html).
**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)](../man2/send.2.html) 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)](../man2/sendmsg.2.html) calls by passing it as a [cmsg(3)](../man3/cmsg.3.html). 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)](../man3/cmsg.3.html) 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(**_udpsocket_**,** _ioctltype_**, &**_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)](../man2/select.2.html),
[poll(2)](../man2/poll.2.html), or [epoll(7)](../man7/epoll.7.html) 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)](../man7/ip.7.html) and [socket(7)](../man7/socket.7.html) are
supported.ERRORS top
All errors documented for [socket(7)](../man7/socket.7.html) or [ip(7)](../man7/ip.7.html) 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 top
**IP_RECVERR** is a new feature in Linux 2.2.SEE ALSO top
[ip(7)](../man7/ip.7.html), [raw(7)](../man7/raw.7.html), [socket(7)](../man7/socket.7.html), [udplite(7)](../man7/udplite.7.html)
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
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man-pages@man7.orgLinux man-pages 6.10 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)