Re: [Xen-users] Virtual Machine - Use the IP assigned through vif configuration

[Felix Kuperjans <felix@xxxxxxxxxxxxxxxxxx>]

Hi,

it is correct, that the vif configuration's ip=... option is mainly used
internally by the vif-scripts, as it also resides in the network backend
xenstore part.

It seems to be possible to read it from the VM though, once the xenstore
utilities are installed. I attached a small script for this.
There are some pitfalls though:
* It can only detect the IP, not netmasks or gateway addresses (because
the ip=... option only stores the IP and there are no netmask=... etc.
options)
* xenstore utilities are usually not installed on a VM
* I actually don't know if a VM can access its domain ID in a better way
than this script does (iterating + test) - I'd appreciate some hints on
how this can be optimized.

What I actually did with some of my VMs and which works better is using
the kernel ip=... command line parameter.
It is documented at:
https://www.mjmwired.net/kernel/Documentation/filesystems/nfs/nfsroot.txt
Modern kernels however often don't follow this directive directly, if
either the network card driver is a module or the corresponding kernel
feature is disabled, which is done in some distribution kernels.

The trick is, to use the parameter and to read it during bootup from
/proc/cmdline. As it's even standardized by the kernel it is a good way
to configure at least the primary interface. In Xen, it can be provided
using either:
extra = "ip=..."
cmdline = "root=/dev/xvda1 ip=..." (newer Xen versions)

A small example on reading it:
sed -ne
"s/.*ip=\([.0-9]\+\):\([.0-9]\+\):\([.0-9]\+\):\([.0-9]\+\):.*/\1
netmask \4 gateway \3/p" </proc/cmdline

This information can also easily be processed in network scripts and
does not require additional software in the domU.

Cheers
Felix

> Hi,
>
>> I have a Xen environment with bridged networking and statically-assigned
>> IPs.
>>
>> I am trying to set the virtual machines IPs through the XL Configuration
>> File, using the vif = ['... ip=192.168.0.X ...'] statement.
>> The problem is that I don't know what to put in the
>> '/etc/network/interfaces' file to make the VM automatically get the
>> Xen-assigned IP address.
> As far as I know, the "ip"-option is only routed into the vif-script in
> dom0 (or the network backend domU). Thus it is more or less unusable in
> the domU. If I am wrong, please correct me, I'd love to use that option,
> too!
>
> But I do think that not everything is lost. There is xenstore - this
> might allow you to read the configured ip (and much more) from within
> the domU, but I do not have any usable experience regarding this and it
> might require a bit of scripting (and maybe changing permissions in the
> dom0?)
> Maybe that is a usable hint for you.
>
> Cheers
> CRT
>
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> https://lists.xen.org/xen-users

Attachment: xenstore-vifip.sh
Description: application/shellscript

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