udp_set_membership fails for IPv6 destination addresses

[wjholden]

julia/stdlib/Sockets/src/Sockets.jl

Line 732 in 788b2c7

function udp_set_membership(sock::UDPSocket, group_addr::String,

I am not able to open join an IPv6 multicast destination. Here is an example program that first listens to on an IPv4 multicast destination, then IPv6:

using Sockets

# The the following IPv4 and IPv6 multicast destinations,
for group in [ip"239.2.0.21", @ip_str "ff05::2021"]
	# Create a socket
	s = Sockets.UDPSocket()

	# Bind this socket to all interfaces on port 2021, allowing port reuse.
	# The "ipv6only" parameter does not change the outcome.
	bind(s, ip"0.0.0.0", 2021)

	# Join the multicast destination group
	join_multicast_group(s, group)

	# Print the first ten datagrams received as strings.
	for _ in 1:10
		println(String(recv(s)))
	end

	# Leave the group
	leave_multicast_group(s, group)

	# Close the socket
	close(s)
end

Here is a simple program that sends input to the above:

using Sockets

# For the same two multicast destinations
for group in [ip"239.2.0.21", @ip_str "ff05::2021"]
	# Open a socket. There is no need to bind anything.
	s = Sockets.UDPSocket()

	# Send ten packets.
	for i in 1:10	
		send(s, group, 2021, "Test $i")
	end

	# Close the socket
	close(s)
end

The error I get is:

ERROR: IOError: uv_udp_set_membership: invalid argument (EINVAL)
Stacktrace:
 [1] uv_error at .\libuv.jl:97 [inlined]
 [2] udp_set_membership(::UDPSocket, ::String, ::Nothing, ::Int64) at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Sockets\src\Sockets.jl:740
 [3] join_multicast_group at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Sockets\src\Sockets.jl:753 [inlined]
 [4] join_multicast_group at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Sockets\src\Sockets.jl:760 [inlined]
 [5] join_multicast_group(::UDPSocket, ::IPv6) at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Sockets\src\Sockets.jl:757
 [6] top-level scope at .\REPL[3]:10

I have been poking around in Sockets.jl and https://github.com/libuv/libuv/blob/v1.x/src/win/udp.c but have not come up with anything.

I am translating a Python program into Julia. This may or may not be relevant, but in Python I had to set the address family to scoket.IPPROTO_IPV6 for IPv6 multicast (see http://svn.python.org/projects/python/trunk/Demo/sockets/mcast.py). Here is a small Python IPv6 receiver for reference:

import socket
import struct

# Open a socket initialized with an explicit IPv6 address family.
s = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)

# Allow port reuse.
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

# Bind the port on all interfaces to UDP port 2021.
s.bind(('', 2021))

# Join the multicast destination
group_bin = socket.inet_pton(socket.AF_INET6, "ff05::2021")
mreq = group_bin + struct.pack("@I", 0)
s.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_JOIN_GROUP, mreq)

for _ in range(10):
	data, _ = s.recvfrom(1500)
	print(str(data))

[wjholden]

I got it to work! The syntax for this is a little trickier than one might expect.

using Sockets

# Use the Sockets.IPv6 function to parse IPv6 addresses.
group = Sockets.IPv6("ff05::2021")
s = Sockets.UDPSocket()

# Binding fails if you try to use Sockets.IPv4("0.0.0.0"),
# with and without the ipv6only parameter.
bind(s, Sockets.IPv6("::"), 2021)

join_multicast_group(s, group)

for _ in 1:10
	println(String(recv(s)))
end

leave_multicast_group(s, group)

close(s)

To send data, I used the ncat program on my Windows computer.

"This actually works!" | ncat -v --send-only -u "ff05::2021" 2021

I found that IPv6 multicast group joins fail if you try to bind to all interfaces with Sockets.IPv4("0.0.0.0"). The bind function only worked for me with Sockets.IPv6("::").

[StefanKarpinski]

That code looks pretty reasonable to me. Any suggestions for what to improve here?

[wjholden]

I think a small section in the documentation on UDP multicast could be useful to others. I have added a short section and submitted a pull request. (I am completely new to contributing to someone else's open-source project, so if there is something I can do better about this please let me know).

[quildtide]

I have to thank this post for helping me figure out what I was doing wrong. I was mistakenly trying to bind to the multicast address itself instead of 0.0.0.0 (for IPv4), and that returns false unless one calls join_multicast_group first, which doesn't work. (My networking knowledge is weak enough that I don't quite understand if that's not intended behavior)

This issue and the referenced documentation pull request are the only examples of working Julia multicasting on the indexed internet that I know of.

Thanks for the documentation addition; I think it'll help a lot of people once 1.7 hits the live documentation.

[wjholden]

The documentation is now merged at https://docs.julialang.org/en/v1/manual/networking-and-streams/#Multicast. I will close this issue.