UDP(7) Linux Programmer's Manual UDP(7)
udp - User Datagram Protocol for IPv4
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 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 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
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 processed by the kernel only 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 sent only to
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_DIS-
COVER 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 possi-
ble to set the SO_BSDCOMPAT SOL_SOCKET option to receive remote errors
only when the socket has been connected (except for EPROTO and EMSG-
SIZE). Locally generated errors are always passed. Support for this
socket option was removed in later kernels; see socket(7) for further
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
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 allo-
cated by UDP exceeds this number, UDP starts to moder-
ate 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 sock-
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_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 pres-
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. Unless otherwise noted, optval is a pointer to an int.
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:
error = ioctl(udp_socket, ioctl_type, &value);
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. Warning: Using FIONREAD, it is impossible
to distinguish the case where no datagram is pending from the
case where the next pending datagram contains zero bytes of
data. It is safer to use select(2), poll(2), or epoll(7) to
distinguish these cases.
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 sup-
All errors documented for socket(7) or ip(7) may be returned by a send
or receive on a UDP socket.
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.
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