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NAME
udp - User Datagram Protocol for IPv4
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
udp_socket = socket(PF_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 con
nect(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 speci‐
fying an 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 an local address
first by using bind(2). Otherwise the socket layer will automatically
assign a free local port out of the range defined by
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
sysctl 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 an local interface address and the packet is only sent to
that interface.
By default Linux UDP does path MTU (Maximum Transmission Unit) discov‐
ery. 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 ip_no_pmtu_disc sysctl, 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 informa‐
tion.
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.
Socket Options
To set or get a UDP socket option, call getsockopt(2) to read or set
sockopt(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.
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.
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.
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.
ip(7), raw(7), socket(7)
RFC 768 for the User Datagram Protocol.
RFC 1122 for the host requirements.
RFC 1191 for a description of path MTU discovery.