NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | SEE ALSO | COLOPHON
UDP(7) Linux Programmer's Manual UDP(7)
udp - User Datagram Protocol for IPv4
#include <sys/socket.h>
#include <netinet/in.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 only processed by the kernel 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 only sent 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.
UDP uses the IPv4 sockaddr_in address format described in ip(7).
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.
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.
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.
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);
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.
TIOCOUTQ (SIOCOUTQ)
Returns the number of data bytes in the local send queue. Only
supported 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)
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 release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2009-09-30 UDP(7)