[vbox-dev] VirtualBox blocks 127/8 (loopback) subnets for intnet

Maxime Dor max at kamax.io
Tue Dec 1 13:00:00 UTC 2015


Hi George,

If you mean IPs like 127.x.x.x, they are dedicated to localhost 
communications by the TCP/IP standard and described as such in RFC 990 
<https://tools.ietf.org/html/rfc990>:

The class A network number 127 is assigned the "loopback"
          function, that is, a datagram sent by a higher level protocol
          to a network 127 address should loop back inside the host.  No
          datagram "sent" to a network 127 address should ever appear on
          any network anywhere.


You are lucky it ever worked in the first place, most likely because of 
poor software implementation.
I'm not aware of any way to make it work again and you should definitly 
NOT try to make it work again.
Use 10.0.0.0/8 if you need such a big subnet, else go with 172.16.0.0/12 
or 192.168.0.0/16

Max

On 01/12/15 13:43, george.vargh at wipro.com wrote:
>
> Hi,
>
> We are having our test topology running multiple VMs on a server.
>
> The guest VMs are linux based and the host is based on RHEL 6.4.
>
> The eth interface of VMs are interconnected via ‘intnet’ NIC settings.
>
> The VMs are using 127/8 IP addresses to communicate with each other.
>
> We are currently using VirtualBoX 3.1.8 and everything works fine.
>
> But when we upgrade to latest VirtualBoX 5.0.10, we observe that the 
> 127/8 IP addresses are being blocked and it does not reach the VM 
> interfaces.
>
> Based on our testing, it looks like this behaviour is seen from 
> VirtualBox 3.2.0 onwards.
>
> Can you please suggest if there is a way to get these IP address to be 
> allowed?
>
> Best Regards
>
> George
>
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