Here is my setup to connecting VirtualBox guest OS to GNS3 network simulator. I use Ubuntu as guest OS and Cisco router C2600 series.
By default, Host-only adapter will be set to IP address 192.168.56.2 255.255.255.0 (DHCP),so I set my router to IP address 192.168.56.3 255.255.255.0.
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| Configure guest virtual OS to use Virtualbox host-only adapter |
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| Add VirtualBox Host interface to GNS3 Host |
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| Connect host to router, make sure the interface name is correct |
Now run virtualbox guest OS and router on GNS3, than try ping test connection.
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| Ping test from guest virtualbox |
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| Ping test from GNS3 router |
Have fun :)