Merge pull request #582 from alexfinnarn/patch-1

Update troubleshooting.md

Author: dunglas

extra/troubleshooting.md

@@ -10,7 +10,17 @@ Docker Toolbox is not supported anymore by API Platform. Please upgrade to [Dock
 
 ### Error starting userland proxy
 
-If the `app` container cannot start and display this `Error starting userland proxy: Bind for 0.0.0.0:80`, it means that port 80 is already in use.
+If the `app` container cannot start and display this `Error starting userland proxy: Bind for 0.0.0.0:80`, it means that port 80 is already in use. You can check to see which processes are currently listening on certain ports.
+
+```bash
+# Find out if any service listens on port 80.
+# You can use this command on UNIX-based OSes like MacOS and Linux.
+sudo lsof -n -i :80 | grep LISTEN
+
+# For Windows, you can use netstat. 
+# This will give you all TCP/IP network connections and not just processes listening to port 80.
+netstat -a -b
+```
 
 You can change the port to be used in the `docker-compose.yml` file (default is port 80).