Duplicate Port Allocation

Published on Apr 19, 2017

Weird observation: From Clojure Repl I run a webserver application. From a browser I was able to connect to this app. An application in another JVM running some acceptance test failed to connect:

#error {
:cause "connection was closed”

This is strange since from same terminal

$ nc -z localhost 8080
Connection to localhost port 8080 [tcp/http-alt] succeeded!

Than I remembered that I configured port forwarding for Virtualbox for this port as well. Could it be that there are two bindings for the same port to different applications?

sudo lsof -i -n -P | grep TCP | grep 8080
VBoxHeadl 10270       sli   20u  IPv4 0xc0e59eafb581c8ef      0t0  TCP *:8080 (LISTEN)
java      86447       sli  303u  IPv6 0xc0e59eaf98f62d9f      0t0  TCP *:8080 (LISTEN)

There is one binding for IPv4 and one binding for IPv6. Now it was easy to find the solution to make the acceptance test pass:


The other option is to specify the loopback interface for IPv6 explicitly:


Context: On MacOS Sierra I run Docker in a VirtualBox. I use port forwarding of the VirtualBox to make docker applications accessible via localhost.