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
with a valid destination address as an argument. When connect
called on the socket, the default destination address is set and datagrams can
now be sent using send
(2) or write
(2) without specifying 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
(2). Otherwise, the socket layer will automatically assign a free
local port out of the range defined by
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
(7). They are processed by the kernel only when the appropriate
parameter is enabled (but still passed to the user even when it
is turned off). See ip
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 that
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
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
UDP uses the IPv4 sockaddr_in
address format described in ip
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
). 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
System-wide UDP parameter settings can be accessed by files in the directory
- 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.
- Below this number of pages, UDP is not bothered about its memory appetite.
When the amount of memory allocated by UDP exceeds this number, UDP starts
to moderate memory usage.
- This value was introduced to follow the format of tcp_mem (see
- Number of pages allowed for queueing by all UDP sockets.
- 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
- 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_mem pressure.
- udp_wmem_min (integer; default value: PAGE_SIZE; since Linux
- 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 pressure.
To set or get a UDP socket option, call getsockopt
(2) to read or
(2) to write the option with the option level argument set to
. Unless otherwise noted, optval
is a pointer to an
Following is a list of UDP-specific socket options. For details of some other
socket options that are also applicable for UDP sockets, see socket
- 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
These ioctls can be accessed using ioctl
(2). The correct syntax is:
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. 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
- TIOCOUTQ (SIOCOUTQ)
- Returns the number of data bytes in the local send queue. Supported only
with Linux 2.4 and above.
In addition, all ioctls documented in ip
(7) and socket
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.
is a new feature in Linux 2.2.
RFC 768 for the User Datagram Protocol.
RFC 1122 for the host requirements.
RFC 1191 for a description of path MTU discovery.