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). 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.
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) .
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 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.
IOCTLS
These ioctls can be accessed using
ioctl(2) . The correct syntax is:
int value;
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.
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.
