UDPSocket represents a UDP/IP socket.

Class Methods

Creates a new UDPSocket object.

address_family should be an integer, a string or a symbol: Socket::AF_INET, “AF_INET”, :INET, etc.

require 'socket'                   #=> #<UDPSocket:fd 3> #=> #<UDPSocket:fd 4>
Instance Methods

Binds udpsocket to host:port.

u1 =
u1.bind("", 4913)
u1.send "message-to-self", 0, "", 4913
p u1.recvfrom(10) #=> ["message-to", ["AF_INET", 4913, "localhost", ""]]

Connects udpsocket to host:port.

This makes possible to send without destination address.

u1 =
u1.bind("", 4913)
u2 =
u2.connect("", 4913)
u2.send "uuuu", 0
p u1.recvfrom(10) #=> ["uuuu", ["AF_INET", 33230, "localhost", ""]]

Receives up to maxlen bytes from udpsocket using recvfrom(2) after O_NONBLOCK is set for the underlying file descriptor. flags is zero or more of the MSG_ options. The first element of the results, mesg, is the data received. The second element, sender_inet_addr, is an array to represent the sender address.

When recvfrom(2) returns 0, Socket#recv_nonblock returns nil. In most cases it means the connection was closed, but it may also mean an empty packet was received, as the underlying API makes it impossible to distinguish these two cases.


  • maxlen - the number of bytes to receive from the socket

  • flags - zero or more of the MSG_ options

  • outbuf - destination String buffer

  • options - keyword hash, supporting ‘exception: false`


require 'socket'
s1 =
s1.bind("", 0)
s2 =
s2.bind("", 0)
s1.send "aaa", 0
begin # emulate blocking recvfrom
  p s2.recvfrom_nonblock(10)  #=> ["aaa", ["AF_INET", 33302, "localhost.localdomain", ""]]
rescue IO::WaitReadable[s2])

Refer to Socket#recvfrom for the exceptions that may be thrown if the call to recvfrom_nonblock fails.

UDPSocket#recvfrom_nonblock may raise any error corresponding to recvfrom(2) failure, including Errno::EWOULDBLOCK.

If the exception is Errno::EWOULDBLOCK or Errno::EAGAIN, it is extended by IO::WaitReadable. So IO::WaitReadable can be used to rescue the exceptions for retrying recvfrom_nonblock.

By specifying a keyword argument exception to false, you can indicate that recvfrom_nonblock should not raise an IO::WaitReadable exception, but return the symbol :wait_readable instead.


Sends mesg via udpsocket.

flags should be a bitwise OR of Socket::MSG_* constants.

u1 =
u1.bind("", 4913)

u2 =
u2.send "hi", 0, "", 4913

mesg, addr = u1.recvfrom(10)
u1.send mesg, 0, addr[3], addr[1]

p u2.recv(100) #=> "hi"