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NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | SEE ALSO | COLOPHON |
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udp(7) Miscellaneous Information 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 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.
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.
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
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⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
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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-
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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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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