This is an implemention of the User Datagram Protocol described in RFC768. 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).
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
UDP fragments a packet when its total length exceeds the interface MTU (Maximum
Transmission Unit). A more network friendly alternative is to use path MTU
discovery as described in the IP_MTU_DISCOVER section of 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 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 RFC1122.
For compatibility with legacy code it is 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). It is better to fix
the code to handle errors properly than to enable this option. Locally
generated errors are always passed.
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.