Update linux.md

Adding default ports in linux buidl environment information

Author: michal-at-travisci

user/reference/linux.md

@@ -24,6 +24,30 @@ To use our Ubuntu Linux build infrastructure, you can choose between the distrib
 
 We use Ubuntu Xenial 16.04 as default. You find more about packages, tools and settings in [Ubuntu Xenial 16.04](/user/reference/xenial/).
 
+### Ports
+
+Travis CI Linux build images do utilize default open ports as follows:
+
+53 for dns, 
+22 for ssh, 
+323 for ntp  
+68 for dhcp
+
+On top of that, some of the services pre-installed in the build image or installed during execution of the build job execution as dependencies (e.g. database engine or docker) may occupy additrrional ports. 
+Travis CI build image configuratione doesn't override such defaults.
+
+If in any case during a build job you run into a situation a port you want to use is occupied/not available, please re-run the build using 
+following `nestat` call in your `.travis.yml` at the beginning of the script phase or directly before the step, which you suspect clashes over occupied ports.
+
+```yaml
+script:
+# select proper step in your build execution and add this line before it
+ - sudo netstat -tulpn
+```
+{: data-file=".travis.yml"}
+
+to show all open ports occupied during the build job runtime.
+
 ### Using Xenial
 
 To use Ubuntu Xenial, add the following to your `.travis.yml`.