Struggling to make a TCP server based on new socket module using use the “reuseaddr” and “reuseport” options

Questions / Help
1

I’m struggling to make the TCP server based on new socket module (from the doc) use the “reuseaddr” and “reuseport” options.

server(Addr, Port) ->  
   {ok, LSock} = socket:open(inet, stream, tcp),
   ok = socket:bind(LSock, #{family => inet,
                             reuseaddr => true,  %% this fails
                             reuseport => true,   %% this fails as well
                             port   => Port,
                             addr   => Addr}),
   ok = socket:listen(LSock),
   {ok, Sock} = socket:accept(LSock),
   {ok, Msg} = socket:recv(Sock),
   ok = socket:send(Sock, Msg),
   ok = socket:close(Sock),
   ok = socket:close(LSock).

I’d like to have a listening socket and 2 worker processes accepting connection on the same port (hence the reuseaddr).

What am i doing wrong?

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2

I’m not sure that reuseaddr and reuseport are part of sockaddr() type, but they are covered by socket_option() type. Try setting those options to true with socket:setopt/3 before binding the socket.

Also, according to Erlang -- Socket Usage those options must be set on OS level and used only with inet/inetv6 addresses.

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3

The main issue with your use of socket options is that reuseaddr and reuseport options have to be set prior to calling socket:bind/2. Try this:

{ok, LSock} = socket:open(inet, stream, tcp),
ok = socket:setopt(LSock, socket, reuseaddr, true),
ok = socket:setopt(LSock, socket, reuseport, true),
ok = socket:bind(LSock, #{family => inet, port => Port, addr => Addr}),
...

Secondly, if you need to have multiple worker processes doing the accept, you can pass the LSock to a pool of worker processes so that they are calling the socket:accept/1 on the same instance of LSock. If your idea was that the reuseaddr and reuseport will help you have multiple listening sockets open on the same address/port pair, that is not what those two options are for. These options are meant to allow your OS process to reconnect to the same address/port right after the process crashed that was previously bound to that address/port pair, so that the OS would release the socket and make it available for new binding immediately.

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