DOC: section for troubleshooting and offline installation in setup readme

README_SETUP.md

@@ -208,3 +208,50 @@ cd javasphinx
 git checkout 659209069603a
 pip3 install .
 ```
+
+## Installation under special conditions
+
+### Installation without or with restricted internet connection on the host system
+
+It is of course possible to install CaosDB on servers which do not have an internet connection
+or only have limited access to the internet.
+However, maven, that is used to build the CaosDB server, typically accesses the internet very often, and some reconfiguration might be necessary to prevent timeout issues and similar headache.
+
+A recommendation to proceed is:
+- Clone the repositories (caosdb-server, caosdb-mysqlbackend and subrepository caosdb-webui) on
+  a second machine that has internet connection.
+- Run `make install` and `make run` in the caosdb-server repository. This will download all required dependencies and setup all maven-related artifacts in a folder called `.m2` typically located in the home directory.
+- In addition to copying over server, mysqlbackend and webui to the host machine, also copy the complete `.m2` folder to the **home directory of the user that is running the CaosDB server**.
+- On the host machine, open the file `pom.xml` in the server repository.
+- Look for the section `<repositories>`. There should be a `<respository>` with `<id>local-maven-repo</id>`. It should be moved on top of the list of repositories, so that the xml block appears directly after `<repositories>` and before the repository maven-central.
+
+## Troubleshooting / FAQ
+
+### I set up and run the server, but I get an error that XSLT parsing failed in the WebUI
+
+You probably forgot to `make` the webui.
+
+Make sure that you:
+- Installed the WebUI according to the section [Web UI]
+- Run `make install` in the webui sub folder
+
+### How can I run the CaosDB server using ports 443 and 80?
+
+On standard linux setups, ports 443 and 80 must not be opened by non-root processes.
+
+There are multiple solutions for this problem:
+- Use iptables to redirect a low port to a high port.
+- Use `CAP_NET_BIND_SERVICE`
+- Use authbind
+
+Here are some resources to read about these options:
+- [Question on Stackoverflow](https://stackoverflow.com/questions/413807/is-there-a-way-for-non-root-processes-to-bind-to-privileged-ports-on-linux)
+- [Questoin on Superuser](https://superuser.com/questions/710253/allow-non-root-process-to-bind-to-port-80-and-443)
+- [iptables tutorial](https://www.frozentux.net/iptables-tutorial/iptables-tutorial.html#REDIRECTTARGET)
+
+Using authbind is a simple solution that is briefly described here:
+- Install the package authbind
+- Create (empty) files `/etc/authbind/byport/80` (and `.../443`), e.g. using `touch`
+- Set permissions of these files to 777
+- Run the CaosDB server using authbind with the `--deep` option:
+  `authbind --deep make run`