Highlights:
- windows 10 host machine
- ubuntu vagrant box (virtualbox) as guest vm
- using vagrant port forwarding as like this:
config.vm.network "forwarded_port", guest: 1234, host: 12340 - IDE: IntelliJ IDEA with Ruby plugin
The Issue:
I've tried to set up remote ruby debug following this guide and getting an error in IDE: "failed to find free socket port for process dispatcher". It looks this issue is not IntelliJ-specific, I was able to reproduce it with latest RubyMine as well.
From IDEA's log
2017-07-07 21:53:03,515 [8879188] INFO - tion.impl.ExecutionManagerImpl - Failed to find free socket port for process dispatcher
com.intellij.execution.ExecutionException: Failed to find free socket port for process dispatcher
at org.jetbrains.plugins.ruby.ruby.debugger.RubyProcessDispatcher.<init>(RubyProcessDispatcher.java:46)
at org.jetbrains.plugins.ruby.ruby.debugger.RubyRemoteDebugRunner.doExecute(RubyRemoteDebugRunner.java:62)
...
Caused by: java.net.BindException: Address already in use: JVM_Bind
at java.net.TwoStacksPlainSocketImpl.socketBind(Native Method)
at java.net.TwoStacksPlainSocketImpl.socketBind(TwoStacksPlainSocketImpl.java:137)
...
I can understand it says Address already in use: JVM_Bind, but how remote debug supposed to work at all then? (I mean Is there any way to access guest vm port not forwarding it before? Clearly no) Any help to solve this issue is much appreciated.
For me the issue was due to another debug session that was open in the background. To prevent that from happening again (and also close all other currently open sessions, once you run the configuration again) select "Single instance only" in the Debug Configuration: 
来源：https://stackoverflow.com/questions/45084093/failed-to-find-free-socket-port-for-process-dispatcher-when-trying-remote-debug