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 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) 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
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.
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 behaviour differs from many other BSD socket implementations
which don’t pass any errors unless the socket is connected.
Linux’s behaviour 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.
