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

Tue Dec 1 14:54:54 GMT 2015

Hi Maxime,

Thanks a lot for the response.

This is quite an old software which has been using this series of IP address (like 127.2.x.x and 127.3.x.x) for VM to VM communication.
Unfortunately the software has a lot of dependency on this and same software code is shared between many products.
So it looks tough to redesign this part.

We were running this successfully on VirtualBox 3.0.14 and VirtualBox 3.1.8 without any issues.
Recently the linux kernel in guest VM was upgraded from 3.0.14 to 3.14.75.

After this we started seeing strange disk corruption issues on VirtualBox 3.1.8
i.e we do parallel copy of large files (around 2GB) to 5 VMs in parallel and in one of the VMs (randomly) we see that the md5sum is different for the transferred file.
The file size is shown correctly after the copy.
The strange part is that on each subsequent 'md5sum' of the same file (without again copying) yields different results.
The disk for each VM is stored as separate vdi files on the host disk.
We use SATA controller configuration on the VM for disk access.

This issue is not seen on VirtualBox 5.0.10.
But then we have the issue that VM to VM communication is failing since 127.x.x.x is getting blocked on VirtualBox 5.0.10.

Best Regards
George

From: Maxime Dor [mailto:max at kamax.io]
Sent: 01 December 2015 18:30
To: vbox-dev at virtualbox.org
Subject: Re: [vbox-dev] VirtualBox blocks 127/8 (loopback) subnets for intnet

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<mailto: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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