Connecting to wildcard ip 0.0.0.0 works in a container, but not on the host

Question

I am having a strange behaviour on my akka Project. To summarize: I have two Actorsystems running on my hostsystem and want to connect from one to another to send messages. Now the Problem is, when I use the following akka configuration on my client:

akka {
  actor {
    provider = remote
  }
  remote {
    enabled-transports = ["akka.remote.netty.tcp"]
    netty.tcp {
      hostname = "127.0.0.1"
      port = 0
    }
 }
}

and the following on my server actorsystem:

akka {
  loglevel = "INFO"
  actor {
    provider = "akka.remote.RemoteActorRefProvider"
  }
  remote {
    enabled-transports = ["akka.remote.netty.tcp"]
    netty.tcp {
      hostname = "127.0.0.1"
      port = 5150
    }
    log-sent-messages = on
    log-received-messages = on
  }
}

I cannot connect to my server actorsystem using an actorselection as follows:

val selection = context.actorSelection("akka.tcp://[email protected]:5150/user/receiver")

I would expect it to work with a wildcard IP address, because it means that it tried to connect to each IPv4 the hostsystem has. I am getting following error:

[WARN] [05/06/2019 08:57:03.738] [New I/O boss #3] [NettyTransport(akka://ClientSystem)] Remote connection to [null] failed with java.net.ConnectException: Connection refused: no further information: /0.0.0.0:5150
[WARN] [05/06/2019 08:57:03.738] [ClientSystem-akka.remote.default-remote-dispatcher-13] [akka.tcp://[email protected]:8346/system/endpointManager/reliableEndpointWriter-akka.tcp%3A%2F%2FServerSystem%400.0.0.0%3A5150-0] Association with remote system [akka.tcp://[email protected]:5150] has failed, address is now gated for [5000] ms. Reason: [Association failed with [akka.tcp://[email protected]:5150]] Caused by: [java.net.ConnectException: Connection refused: no further information: /0.0.0.0:5150]

On the other hand it is working just fine if I change 127.0.0.1 on the server config to 0.0.0.0 when I run it inside a docker container and connect to it using the 0.0.0.0 address in the actor selection on my client, which is running on the host. So here it seems to work binding and connecting to the wildcard IP.

Does anyone have an explanation to this behaviour?

Answer 1

I think this is explained by this superuser.com answer.

127.0.0.1 is a private loopback network and can only be used to connect to other processes on the same computer. So if you bind the receiving socket to that address, you will not be able to receive connections from other computers.

0.0.0.0 is a pseudo-address that has various meanings depending on context. For a server it means that the socket is available from any incoming IP address on any network. For a client it probably means the default IP address of the current machine, though this depends on how the library chooses to treat it.