[libvirt] RFC: Implement virDomainGetIPAddress()

Michal Novotny minovotn at redhat.com
Fri Jul 15 08:37:54 UTC 2011

On 07/15/2011 10:28 AM, Matthias Bolte wrote:
> 2011/7/15 Michal Novotny <minovotn at redhat.com>:
>> On 07/14/2011 05:42 PM, Matthias Bolte wrote:
>>> 2011/7/14 Michal Novotny <minovotn at redhat.com>:
>>>> Hi guys,
>>>> some time ago I've been investigating the options to get the guest's IP
>>>> address information without having to connect to guest's VNC window or
>>>> console. It was for one project I've been working on and I found that
>>>> the solution lies in the procfs, precisely in the /proc/{PID}/net/arp...
>>>> The format is as follows:
>>>> $ cat /proc/{PID}/net/arp
>>>> IP address       HW type     Flags       HW address            Mask
>>>> Device
>>>>   0x1         0x2         52:54:00:35:76:e6     *
>>>> virbr0
>>>> where the HW address matches the MAC address associated to the guest's
>>>> NIC. Implementing such an API shouldn't be a big problem however I know
>>>> that there's some option to run libvirt on Windows machines. It should
>>>> be just the client so it shouldn't really matter however I'd like to ask
>>>> you whether it's really not an issue.
>>> Windows or not is irrelevant here as the IP address lookup cannot be
>>> implemented in a general way/place, but will have to be implemented by
>>> the hypervisor/network drivers.
>>>> The function should return a string of the guest's IP address as read
>>>> from the procfs or return a NULL value if there's no IP address
>>>> associated with the guest.
>>>> If the multiple NICs are being used by the guest then the function
>>>> should return either the IP address matching the MAC address passed to
>>>> the function or the first IP address if omitted.
>>>> The prototype should be:
>>>> char *virDomainGetIPAddress(virDomainPtr domain, char *devmac);
>>> First of all you're missing the unsigned int flags parameter.
>>> Also did you consider that the MAC to IP(v4|v6) mapping isn't
>>> necessarily a 1:1 mapping, but the signature of your function requires
>>> this?
>> Well, for this I considered the IPv4 address only.
>>> I took a look at this and wonder what's the difference between
>>> /proc/{PID}/net/arp and /proc/net/arp. Also as it's ARP it'll only
>>> work for IPv4.
>> Honestly I was unable to see it in /proc/net/arp. I saw just some ARP
>> records but not related to the qemu-kvm process I tried on my Fedora-14
>> box but I was able to see those records in /proc/{PID}/net/arp and
>> therefore I was looking to the /proc/{PID}/net/arp and why I mentioned
>> this instead.
>> Considering the fact this will be working just for IPv4 there should be
>> some prototype like:
>> char *virDomainGetIPAddress(virDomainPtr domain, char *devmac, unsigned int flags);
> Even if you're going to restrict it to IPv4 for now this signature
> won't do, as you can have multiple IPv4 addresses assigned to the same
> MAC address. How do you want to return multiple IP addresses? As a
> comma separated list as string and the caller has to parse it?
Wait a minute there please. The MAC address should be unique in the
system so user should be using just one IP address per one MAC address.
Since MAC address should be unique in the system then the IP address
assigned to this MAC address should be just one, shouldn't it ?
Nevertheless I think you know more about networking options than I do so
when I consider the scenario you wrote me about I don't like the idea of
leaving the parsing to the caller and for the scenario of multiple IP
addresses in the return value I recommend a new prototype:

char **virDomainGetIPAddress(virDomainPtr domain, char *devmac, unsigned int *count, unsigned int flags);

where count will be the output parameter with the number of elements in
the return value. It should be used like:

virDomainPtr domain = ...;
char *macaddr = "11:22:33:44:55:66";
char **ips = NULL;
int count = -1;

ips = virDomainGetIPAddress(domain, macaddr, &count, 0);

for (i = 0; i < count; i++) {
  ... ips[i] ...
  ... free(ips[i]) ...

The return value allocation should be done by the function itself and
the called should free the result (ips in this case).

Why did you mention the comma-separated list of IP addresses? I think
the solution with comma-separated list is not clean at all and the
solution I recommend now is much cleaner, don't you think?


Michal Novotny <minovotn at redhat.com>, RHCE, Red Hat
Virtualization | libvirt-php bindings | php-virt-control.org

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