[libvirt-users] Network interface name of guest via python API

[Dennis Jacobfeuerborn]

Hi,
I'm trying to get some basic data from running guests on a libvirt HV
but one thing I'm missing is the name of the network interfaces of the
guests. I can get the MAC address from the XML but there seems to be no
function to actually get the name of the interface.

The only function that comes close is this one:
https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainInterfaceAddresses

This function requires a parameter that I don't really have enough
information about.

What is the best way to enumerate a guests interfaces including the name?

Regards,
Dennis

[Laine Stump]

Post by Dennis Jacobfeuerborn
Hi,
I'm trying to get some basic data from running guests on a libvirt HV
but one thing I'm missing is the name of the network interfaces of the
guests. I can get the MAC address from the XML but there seems to be no
function to actually get the name of the interface.
https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainInterfaceAddresses
This function requires a parameter that I don't really have enough
information about.

Which parameter is that? flags? That seems fairly thoroughly explained
in the documentation you pointed at.

Post by Dennis Jacobfeuerborn
What is the best way to enumerate a guests interfaces including the name?

Assuming you mean the name of the interface *as it appears to the
guest*... Since the host can't directly know the names of devices on the
guest, the only way you can learn this is with cooperation from the OS
running on the guest. One way of getting that cooperation is to make
sure the guest is running the qemu guest agent (and that libvirt's
domain config has the proper serial channel configured), then
virDomainInterfaceAddresses() with the
VIR_DOMAIN_INTERFACE_ADDRESSES_SRC_AGENT flag will return entries that
contain the interface name as it appears to the guest (the other modes
of virDomainInterfaceAddresses will show the names of the tap devices on
the *host*, not of the interfaces in the guest).

You can check if your guests are running the qemu guest agent (and try
out the virDomainInterfaceAddresses() API at the same time) with this
shell command (run as root on the host):

virsh domifaddr $guestname --source agent

Here is info in the libvirt wiki about installing / configuring the qemu
guest agent:

https://wiki.libvirt.org/page/Qemu_guest_agent

(I just checked on a Fedora 27 guest that I previously defined with
virt-manager to use spice for video, I found that virt-manager had
automatically added the necessary serial channel device
("org.qemu.guest_agent.0"), and that the Linux kernel in the guest
automatically noticed that and setup a guest agent in the guest. It may
(will) take more work to get the guest agent working for other guest
OSes, e.g. Windows.)

[Dennis Jacobfeuerborn]

Post by Laine Stump

Post by Dennis Jacobfeuerborn
Hi,
I'm trying to get some basic data from running guests on a libvirt HV
but one thing I'm missing is the name of the network interfaces of the
guests. I can get the MAC address from the XML but there seems to be no
function to actually get the name of the interface.
https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainInterfaceAddresses
This function requires a parameter that I don't really have enough
information about.

Which parameter is that? flags? That seems fairly thoroughly explained
in the documentation you pointed at.

I'm actually talking about the "source" parameter.

Post by Laine Stump

Post by Dennis Jacobfeuerborn
What is the best way to enumerate a guests interfaces including the name?

Assuming you mean the name of the interface *as it appears to the
guest*... Since the host can't directly know the names of devices on the
guest, the only way you can learn this is with cooperation from the OS
running on the guest. One way of getting that cooperation is to make
sure the guest is running the qemu guest agent (and that libvirt's
domain config has the proper serial channel configured), then
virDomainInterfaceAddresses() with the
VIR_DOMAIN_INTERFACE_ADDRESSES_SRC_AGENT flag will return entries that
contain the interface name as it appears to the guest (the other modes
of virDomainInterfaceAddresses will show the names of the tap devices on
the *host*, not of the interfaces in the guest).
You can check if your guests are running the qemu guest agent (and try
out the virDomainInterfaceAddresses() API at the same time) with this
virsh domifaddr $guestname --source agent
Here is info in the libvirt wiki about installing / configuring the qemu
https://wiki.libvirt.org/page/Qemu_guest_agent
(I just checked on a Fedora 27 guest that I previously defined with
virt-manager to use spice for video, I found that virt-manager had
automatically added the necessary serial channel device
("org.qemu.guest_agent.0"), and that the Linux kernel in the guest
automatically noticed that and setup a guest agent in the guest. It may
(will) take more work to get the guest agent working for other guest
OSes, e.g. Windows.)

Assuming that the guest agent is running is not really an option as I
don't really have control over how the guests are installed.
What I'm really interested in is a way to uniquely identify a network
interface.
Let's say I want gather statistics for an interface using
virDomainInterfaceStats() for a guest with multiple interfaces how do I
ensure that I always get the proper stats for the individual interfaces?
If there was an ID for each interface that would be easy but that
doesn't seem to exist and now that I realize that the name apparently is
created on startup of the guest that name might not be a stable
identifier either.
I guess I can (ab)use the MAC address but is this really guaranteed to
exist for each Interface? What about SDN-like setups that don't use
Layer-2 networking at all and set the MAC to something like
"00:00:00:00:00:00"?

Regards,
Dennis