How to retrieve host ip from nat virtualbox guest?

Question

I have a windows 7 host handling a VirtualBox debian guest with nat. This guest is running some web services reachable from the outside through port redirection.

I need to provide the external access to these services through urls, but I can't rely on a proper dns resolution, so I have to propagate the host ip to the guest environment. How could I do that with the minimal impact on the host?

Some clarifications: "the outside", "external access" refers to web browsers hosted at the same level as the host, ie. they can't only rely on the host address.

A drawing is worth a thousand words:

+--------------+  External                         
| Windows Host |  network   +---------------------+
| +----+       |            |      Web client     |
| |    |      NAT           |  +----------------+ |
| |    ------------------------> Page generated | |
| |    |       |            |  |by VM with URLs | |
| | VM |       |            |  |to host         | |
| |    |       |            |  +--------|-------+ |
| |    |      FWD           |           |         |
| |    <--------------------------------+         |
| |    |       |            +---------------------+
| +----+       |                                   
+--------------+

Answer 1

One way to achieve this is the trick behind sites like whatsmyip.com. all you need is a web server in your network that sends the IP-address from where it receives a request back to the client.

Remember, since you are using NAT the server sees the address of the NAT-router (in this case the VM-host) as originating address.

To accomplish this you could create a php-script (i.e.whatsmyip.php) in the web-root of your web server, like

//whatsmyip.php
<?php 
//Gets the IP address
$ip = getenv("REMOTE_ADDR") ; 
Echo "$ip; 
?> 

and access it from your guest like http://webserver/whatsmyip.php

(source: Thoughtco.com )

Now you only need to evaluate that answer in your service

Answer 2

Taking account guntbert's answer, this involves:

• Use a third party web server in the "external" network to host a remote address responder as described in the given answer.
• During the VM initialization phase (and/or periodically according to the network context), browse it to fetch the external address of the windows host
• Use the retrieved address to forge URLs while building pages.