[libvirt-users] connecting host and guest vm using a dummy nic

Discussion:

daggs

2018-04-27 22:39:10 UTC

Greetings all,

I have a host machine that runs a router within a vm.
I want to allow a connection between the host and the guest so the host can connect to the lan provided by the router vm.
I've created a dummy interface with these commands:
$ ip link add ens99-dummy type dummy
$ ip link set ens99-dummy address 52:54:00:1f:d0:ff

this resulted with this output:
$ ifconfig ens99-dummy
ens99-dummy Link encap:Ethernet HWaddr 52:54:00:1F:D0:FF
inet6 addr: fe80::84b5:24ff:fe62:c16d/64 Scope:Link
UP BROADCAST RUNNING NOARP MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:899 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:321727 (314.1 KiB)

in my libvirt's xml file I have this entry:
<interface type='direct'>
<mac address='52:54:00:0c:cb:3e'/>
<source dev='ens99-dummy' mode='private'/>
<target dev='macvtap0'/>
<model type='e1000'/>
<alias name='net0'/>
<address type='pci' domain='0x0000' bus='0x02' slot='0x01' function='0x0'/>
</interface>

after the vm is up, I see the adapter in the vm and the host has one more entry:
$ ifconfig macvtap0
macvtap0 Link encap:Ethernet HWaddr 52:54:00:0C:CB:3E
inet6 addr: fe80::5054:ff:fe0c:cb3e/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:889 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:500
RX bytes:0 (0.0 B) TX bytes:320523 (313.0 KiB)

but when I try to request ip via dhcp from both macvtap0 and ens99-dummy I don't get any.
any idea why?

Laine Stump

2018-04-30 17:31:51 UTC

Permalink

Post by daggs
Greetings all,
I have a host machine that runs a router within a vm.
I want to allow a connection between the host and the guest so the host can connect to the lan provided by the router vm.
$ ip link add ens99-dummy type dummy
$ ip link set ens99-dummy address 52:54:00:1f:d0:ff
$ ifconfig ens99-dummy
ens99-dummy Link encap:Ethernet HWaddr 52:54:00:1F:D0:FF
inet6 addr: fe80::84b5:24ff:fe62:c16d/64 Scope:Link
UP BROADCAST RUNNING NOARP MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:899 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:321727 (314.1 KiB)
<interface type='direct'>
<mac address='52:54:00:0c:cb:3e'/>
<source dev='ens99-dummy' mode='private'/>
<target dev='macvtap0'/>
<model type='e1000'/>
<alias name='net0'/>
<address type='pci' domain='0x0000' bus='0x02' slot='0x01' function='0x0'/>
</interface>

MY first question would be "Why are you using macvtap?" This
unnecessarily complicates the plumbing, leading to more potential places
where it could fail.

The 2nd question is - Have you looked at the dhcp server running on the
guest to verify that it is indeed listening for DHCP requests on the
guest-side interface associated with the macvtap interface, and that it
is receiving those requests and sending a reply?

Post by daggs
$ ifconfig macvtap0
macvtap0 Link encap:Ethernet HWaddr 52:54:00:0C:CB:3E
inet6 addr: fe80::5054:ff:fe0c:cb3e/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:889 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:500
RX bytes:0 (0.0 B) TX bytes:320523 (313.0 KiB)
but when I try to request ip via dhcp from both macvtap0 and ens99-dummy I don't get any.
any idea why?

Definitely you wouldn't be able to use the macvtap0 device, so if
anything you would want to be using ens99-dummy, but I'm not even
certain *that* would work, as I've never tried it.

You've provided no visibility into the configuration of the guest OS in
your virtual machine, but in general if you want a simple connection
between the host and guest that has *no other connections*, you can do
this with a bare tap device:

<interface type='ethernet'>
<model type='e1000'/>
</interface>

This will show up as a single device on the host and a single device in
the guest, not connected to a bridge or a macvtap device or anything,
and will not need any other "ip link blah blah" type setup on the host.
Simplifying your setup in this manner may make it easier to find the
source of your problem.

daggs

2018-04-30 19:16:22 UTC

Permalink

Greetings Laine,

Sent: Monday, April 30, 2018 at 8:31 PM
Subject: Re: [libvirt-users] connecting host and guest vm using a dummy nic

MY first question would be "Why are you using macvtap?" This
unnecessarily complicates the plumbing, leading to more potential places
where it could fail.

misconfiguration, question is, what should be the exact config for this.

The 2nd question is - Have you looked at the dhcp server running on the
guest to verify that it is indeed listening for DHCP requests on the
guest-side interface associated with the macvtap interface, and that it
is receiving those requests and sending a reply?

will test it. as said before, I'm not sure what should be the proper config in libvirt.

Definitely you wouldn't be able to use the macvtap0 device, so if
anything you would want to be using ens99-dummy, but I'm not even
certain *that* would work, as I've never tried it.

so use ens99-dummy directly?

You've provided no visibility into the configuration of the guest OS in
your virtual machine, but in general if you want a simple connection
between the host and guest that has *no other connections*, you can do
<interface type='ethernet'>
<model type='e1000'/>
</interface>
This will show up as a single device on the host and a single device in
the guest, not connected to a bridge or a macvtap device or anything,
and will not need any other "ip link blah blah" type setup on the host.
Simplifying your setup in this manner may make it easier to find the
source of your problem.

ok, here is the entire xml file:

<domain type='kvm'>
<name>router</name>
<uuid>ed8eabe2-ced3-4224-aa12-60fb31dd3fd4</uuid>
<memory unit='KiB'>1048576</memory>
<currentMemory unit='KiB'>1048576</currentMemory>
<vcpu placement='static'>2</vcpu>
<os>
<type arch='x86_64' machine='pc-q35-2.8'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
</features>
<cpu mode='host-passthrough'/>
<clock offset='utc'>
<timer name='rtc' tickpolicy='catchup'/>
<timer name='pit' tickpolicy='delay'/>
<timer name='hpet' present='no'/>
</clock>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<pm>
<suspend-to-mem enabled='no'/>
<suspend-to-disk enabled='no'/>
</pm>
<devices>
<emulator>/usr/bin/kvm</emulator>
<disk type='file' device='disk'>
<driver name='qemu' type='raw'/>
<source file='/home/router/lede-x86-64-combined-ext4.img'/>
<target dev='vda' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x05' slot='0x00' function='0x0'/>
</disk>
<controller type='usb' index='0' model='ich9-ehci1'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1d' function='0x7'/>
</controller>
<controller type='usb' index='0' model='ich9-uhci1'>
<master startport='0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1d' function='0x0' multifunction='on'/>
</controller>
<controller type='usb' index='0' model='ich9-uhci2'>
<master startport='2'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1d' function='0x1'/>
</controller>
<controller type='usb' index='0' model='ich9-uhci3'>
<master startport='4'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1d' function='0x2'/>
</controller>
<controller type='sata' index='0'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1f' function='0x2'/>
</controller>
<controller type='pci' index='0' model='pcie-root'/>
<controller type='pci' index='1' model='dmi-to-pci-bridge'>
<model name='i82801b11-bridge'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1e' function='0x0'/>
</controller>
<controller type='pci' index='2' model='pci-bridge'>
<model name='pci-bridge'/>
<target chassisNr='2'/>
<address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
</controller>
<controller type='pci' index='3' model='pcie-root-port'>
<model name='ioh3420'/>
<target chassis='3' port='0x8'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x0' multifunction='on'/>
</controller>
<controller type='pci' index='4' model='pcie-root-port'>
<model name='ioh3420'/>
<target chassis='4' port='0x9'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
</controller>
<controller type='pci' index='5' model='pcie-root-port'>
<model name='ioh3420'/>
<target chassis='5' port='0xa'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
</controller>
<controller type='pci' index='6' model='pcie-root-port'>
<model name='ioh3420'/>
<target chassis='6' port='0xb'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x3'/>
</controller>
<controller type='pci' index='7' model='pcie-root-port'>
<model name='ioh3420'/>
<target chassis='7' port='0xc'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x4'/>
</controller>
<controller type='pci' index='8' model='pcie-root-port'>
<model name='ioh3420'/>
<target chassis='8' port='0xd'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x5'/>
</controller>
<interface type='direct'>
<mac address='52:54:00:e7:90:bc'/>
<source dev='ens99-dummy' mode='vepa'/>
<model type='rtl8139'/>
<address type='pci' domain='0x0000' bus='0x02' slot='0x01' function='0x0'/>
</interface>
<serial type='pty'>
<target type='isa-serial' port='0'/>
</serial>
<console type='pty'>
<target type='serial' port='0'/>
</console>
<input type='mouse' bus='ps2'/>
<input type='keyboard' bus='ps2'/>
<hostdev mode='subsystem' type='usb' managed='yes'>
<source>
<vendor id='0x148f'/>
<product id='0x5572'/>
</source>
<address type='usb' bus='0' port='1'/>
</hostdev>
<hostdev mode='subsystem' type='pci' managed='yes'>
<source>
<address domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
</source>
<address type='pci' domain='0x0000' bus='0x03' slot='0x00' function='0x0'/>
</hostdev>
<hostdev mode='subsystem' type='pci' managed='yes'>
<source>
<address domain='0x0000' bus='0x03' slot='0x00' function='0x0'/>
</source>
<address type='pci' domain='0x0000' bus='0x06' slot='0x00' function='0x0'/>
</hostdev>
<hostdev mode='subsystem' type='pci' managed='yes'>
<source>
<address domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>
</source>
<address type='pci' domain='0x0000' bus='0x07' slot='0x00' function='0x0'/>
</hostdev>
<memballoon model='virtio'>
<address type='pci' domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>
</memballoon>
</devices>
</domain>

what I don't get is how to connect the interface above to the dummy one I've created.

Laine Stump

2018-05-01 14:30:59 UTC

Permalink

Post by daggs
Greetings Laine,

Sent: Monday, April 30, 2018 at 8:31 PM
Subject: Re: [libvirt-users] connecting host and guest vm using a dummy nic

MY first question would be "Why are you using macvtap?" This
unnecessarily complicates the plumbing, leading to more potential places
where it could fail.

misconfiguration, question is, what should be the exact config for this.

will test it. as said before, I'm not sure what should be the proper config in libvirt.

Definitely you wouldn't be able to use the macvtap0 device, so if
anything you would want to be using ens99-dummy, but I'm not even
certain *that* would work, as I've never tried it.

so use ens99-dummy directly?

My suggestion is to scrap the idea of using macvtap and a dummy link
entirely, and just use a plain tap device. I can't think of any reason
why you would want to use macvtap rather than tap in this case

Post by daggs

(I actually meant the network configuration within the guest OS, not the
libvirt config of the virtual machine it's running on)

Post by daggs
<domain type='kvm'>
<name>router</name>
<uuid>ed8eabe2-ced3-4224-aa12-60fb31dd3fd4</uuid>
<memory unit='KiB'>1048576</memory>
<currentMemory unit='KiB'>1048576</currentMemory>
<vcpu placement='static'>2</vcpu>
<os>
<type arch='x86_64' machine='pc-q35-2.8'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
</features>
<cpu mode='host-passthrough'/>
<clock offset='utc'>
<timer name='rtc' tickpolicy='catchup'/>
<timer name='pit' tickpolicy='delay'/>
<timer name='hpet' present='no'/>
</clock>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<pm>
<suspend-to-mem enabled='no'/>
<suspend-to-disk enabled='no'/>
</pm>
<devices>
<emulator>/usr/bin/kvm</emulator>
<disk type='file' device='disk'>
<driver name='qemu' type='raw'/>
<source file='/home/router/lede-x86-64-combined-ext4.img'/>
<target dev='vda' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x05' slot='0x00' function='0x0'/>
</disk>
<controller type='usb' index='0' model='ich9-ehci1'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1d' function='0x7'/>
</controller>
<controller type='usb' index='0' model='ich9-uhci1'>
<master startport='0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1d' function='0x0' multifunction='on'/>
</controller>
<controller type='usb' index='0' model='ich9-uhci2'>
<master startport='2'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1d' function='0x1'/>
</controller>
<controller type='usb' index='0' model='ich9-uhci3'>
<master startport='4'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1d' function='0x2'/>
</controller>
<controller type='sata' index='0'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1f' function='0x2'/>
</controller>
<controller type='pci' index='0' model='pcie-root'/>
<controller type='pci' index='1' model='dmi-to-pci-bridge'>
<model name='i82801b11-bridge'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x1e' function='0x0'/>
</controller>
<controller type='pci' index='2' model='pci-bridge'>
<model name='pci-bridge'/>
<target chassisNr='2'/>
<address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
</controller>
<controller type='pci' index='3' model='pcie-root-port'>
<model name='ioh3420'/>
<target chassis='3' port='0x8'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x0' multifunction='on'/>
</controller>
<controller type='pci' index='4' model='pcie-root-port'>
<model name='ioh3420'/>
<target chassis='4' port='0x9'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
</controller>
<controller type='pci' index='5' model='pcie-root-port'>
<model name='ioh3420'/>
<target chassis='5' port='0xa'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
</controller>
<controller type='pci' index='6' model='pcie-root-port'>
<model name='ioh3420'/>
<target chassis='6' port='0xb'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x3'/>
</controller>
<controller type='pci' index='7' model='pcie-root-port'>
<model name='ioh3420'/>
<target chassis='7' port='0xc'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x4'/>
</controller>
<controller type='pci' index='8' model='pcie-root-port'>
<model name='ioh3420'/>
<target chassis='8' port='0xd'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x5'/>
</controller>
<interface type='direct'>
<mac address='52:54:00:e7:90:bc'/>
<source dev='ens99-dummy' mode='vepa'/>
<model type='rtl8139'/>
<address type='pci' domain='0x0000' bus='0x02' slot='0x01' function='0x0'/>
</interface>
<serial type='pty'>
<target type='isa-serial' port='0'/>
</serial>
<console type='pty'>
<target type='serial' port='0'/>
</console>
<input type='mouse' bus='ps2'/>
<input type='keyboard' bus='ps2'/>
<hostdev mode='subsystem' type='usb' managed='yes'>
<source>
<vendor id='0x148f'/>
<product id='0x5572'/>
</source>
<address type='usb' bus='0' port='1'/>
</hostdev>
<hostdev mode='subsystem' type='pci' managed='yes'>
<source>
<address domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
</source>
<address type='pci' domain='0x0000' bus='0x03' slot='0x00' function='0x0'/>
</hostdev>
<hostdev mode='subsystem' type='pci' managed='yes'>
<source>
<address domain='0x0000' bus='0x03' slot='0x00' function='0x0'/>
</source>
<address type='pci' domain='0x0000' bus='0x06' slot='0x00' function='0x0'/>
</hostdev>
<hostdev mode='subsystem' type='pci' managed='yes'>
<source>
<address domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>
</source>
<address type='pci' domain='0x0000' bus='0x07' slot='0x00' function='0x0'/>
</hostdev>
<memballoon model='virtio'>
<address type='pci' domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>
</memballoon>
</devices>
</domain>
what I don't get is how to connect the interface above to the dummy one I've created.

That's easy. Don't - see my suggestion above about using a tap device
instead of the macvtap+dummy combination (actually, I had thought the
"<source dev='ens99-dummy' ..." should connect the two, but according to
your experiments that doesn't result in a working setup, so...)

daggs

2018-05-01 16:12:22 UTC

Permalink

Greetings Laine,

Sent: Tuesday, May 01, 2018 at 5:30 PM
Subject: Re: [libvirt-users] connecting host and guest vm using a dummy nic

Post by daggs
Greetings Laine,

Sent: Monday, April 30, 2018 at 8:31 PM
Subject: Re: [libvirt-users] connecting host and guest vm using a dummy nic

MY first question would be "Why are you using macvtap?" This
unnecessarily complicates the plumbing, leading to more potential places
where it could fail.

misconfiguration, question is, what should be the exact config for this.

will test it. as said before, I'm not sure what should be the proper config in libvirt.

Definitely you wouldn't be able to use the macvtap0 device, so if
anything you would want to be using ens99-dummy, but I'm not even
certain *that* would work, as I've never tried it.

so use ens99-dummy directly?

Post by daggs

(I actually meant the network configuration within the guest OS, not the
libvirt config of the virtual machine it's running on)

I want to see if I understood you correctly, I add:
<interface type='ethernet'>
<model type='e1000'/>
</interface>
to the vm's xml, this will result in a new nic in the vm which and in the host which I can use by the host to request dhcp from the router within the vm?

Thanks,

Dagg.

Laine Stump

2018-05-02 14:56:51 UTC

Permalink

Post by Laine Stump
<interface type='ethernet'>
<model type='e1000'/>
</interface>
to the vm's xml, this will result in a new nic in the vm which and in the host which I can use by the host to request dhcp from the router within the vm?

Yes. As long as the emulated device in the guest is listening for DHCP
on that interface.

Also, note that (as long as your libvirt is at least version 2.1.0) if
you don't want to bother doing dhcp on the host and guest, you can
assign just a static IP address for the host side of that interface in
the libvirt XML like this:

<interface type='ethernet'>
<source>
<ip address='192.168.254.2'/>
</source>
...

You can even specify routes to be added automatically to the host when
that guest is started, e.g.:

<interface type='ethernet'>
<source>
<ip address='192.168.254.2'/>
<route family='ipv4' address='0.0.0.0' prefix='0'
gateway='192.168.254.1' metric='1'/>
</source>
...

(this assumes that on the guest side, the OS has configured the emulated
ethernet device to have address 192.168.254.1). This can be useful if
you want the host to use one path for traffic when the guest isn't
running, but switch when the guest *is* running. (In this case, if you
can set the default route on the host to have a metric of 2 or higher,
then the default route that goes through the guest would be preferred
when present).

(NB: if you read the documentation for configuring interfaces at
https://libvirt.org/formatdomain.html#ipconfig you'll see references on
how to configure the IP address of the *guest* in libvirt's xml. Note
that this *does not work* for qemu, but only for lxc (and possibly xen,
I don't recall right now). This is a limitation of the way that qemu/kvm
works (the fact that all guest-side IP info is configured in the guest
OS, and libvirt has no reasonable to get to it).)

daggs

2018-05-02 16:05:49 UTC

Permalink

Greetings Laine,

Sent: Wednesday, May 02, 2018 at 5:56 PM
Subject: Re: [libvirt-users] connecting host and guest vm using a dummy nic

Yes. As long as the emulated device in the guest is listening for DHCP
on that interface.
Also, note that (as long as your libvirt is at least version 2.1.0) if
you don't want to bother doing dhcp on the host and guest, you can
assign just a static IP address for the host side of that interface in
<interface type='ethernet'>
<source>
<ip address='192.168.254.2'/>
</source>
...
You can even specify routes to be added automatically to the host when
<interface type='ethernet'>
<source>
<ip address='192.168.254.2'/>
<route family='ipv4' address='0.0.0.0' prefix='0'
gateway='192.168.254.1' metric='1'/>
</source>
...
(this assumes that on the guest side, the OS has configured the emulated
ethernet device to have address 192.168.254.1). This can be useful if
you want the host to use one path for traffic when the guest isn't
running, but switch when the guest *is* running. (In this case, if you
can set the default route on the host to have a metric of 2 or higher,
then the default route that goes through the guest would be preferred
when present).
(NB: if you read the documentation for configuring interfaces at
https://libvirt.org/formatdomain.html#ipconfig you'll see references on
how to configure the IP address of the *guest* in libvirt's xml. Note
that this *does not work* for qemu, but only for lxc (and possibly xen,
I don't recall right now). This is a limitation of the way that qemu/kvm
works (the fact that all guest-side IP info is configured in the guest
OS, and libvirt has no reasonable to get to it).)

thanks alot!
like you've said, I've defined the interface in the xml, booted the device and updated /etc/network/interfaces to alloc dhcp to vnet0.
that resulted in the proper ip and I can see the router inside the vm.

one thing bothers me, ethtool vnet0 says the speed is 10Mb/s, asking google suggested several solutions.
I've tried changing the model to virtio but the speed stays the same, the vm router is running kernel 4.4.92, could that be the issue?

Thanks,

Dagg.

Laine Stump

2018-05-02 17:09:53 UTC

Permalink

Post by daggs
Greetings Laine,

Sent: Wednesday, May 02, 2018 at 5:56 PM
Subject: Re: [libvirt-users] connecting host and guest vm using a dummy nic

Yes. As long as the emulated device in the guest is listening for DHCP
on that interface.
Also, note that (as long as your libvirt is at least version 2.1.0) if
you don't want to bother doing dhcp on the host and guest, you can
assign just a static IP address for the host side of that interface in
<interface type='ethernet'>
<source>
<ip address='192.168.254.2'/>
</source>
...
You can even specify routes to be added automatically to the host when
<interface type='ethernet'>
<source>
<ip address='192.168.254.2'/>
<route family='ipv4' address='0.0.0.0' prefix='0'
gateway='192.168.254.1' metric='1'/>
</source>
...
(this assumes that on the guest side, the OS has configured the emulated
ethernet device to have address 192.168.254.1). This can be useful if
you want the host to use one path for traffic when the guest isn't
running, but switch when the guest *is* running. (In this case, if you
can set the default route on the host to have a metric of 2 or higher,
then the default route that goes through the guest would be preferred
when present).
(NB: if you read the documentation for configuring interfaces at
https://libvirt.org/formatdomain.html#ipconfig you'll see references on
how to configure the IP address of the *guest* in libvirt's xml. Note
that this *does not work* for qemu, but only for lxc (and possibly xen,
I don't recall right now). This is a limitation of the way that qemu/kvm
works (the fact that all guest-side IP info is configured in the guest
OS, and libvirt has no reasonable to get to it).)

BTW, if you'll be running other guests on your host, and may start them
in different orders, you should probably add "<target dev='vmrouter'/>
(or whatever name you like, 15 chars or less and *not* starting with
"vnet") to the <interface> config. IF you do that, you can be assured
that the name of the device on the host will always be consistently
named "vmrouter" (otherwise it just gets the lowest numbered available
"vnetX" device name each time it's started).

Post by daggs
one thing bothers me, ethtool vnet0 says the speed is 10Mb/s, asking google suggested several solutions.
I've tried changing the model to virtio but the speed stays the same, the vm router is running kernel 4.4.92, could that be the issue?

The reported speed should be inconsequential, since there is no wire and
no physical switch involved. It will transfer data as fast as the code
can be executed.

Also, if virtio works, then *definitely* use that instead of e1000 - the
performance will be much better and overhead much lower.

daggs

2018-05-02 17:28:50 UTC

Permalink

Greetings Laine,

Sent: Wednesday, May 02, 2018 at 8:09 PM
Subject: Re: [libvirt-users] connecting host and guest vm using a dummy nic

Post by daggs
Greetings Laine,

Sent: Wednesday, May 02, 2018 at 5:56 PM
Subject: Re: [libvirt-users] connecting host and guest vm using a dummy nic

Yes. As long as the emulated device in the guest is listening for DHCP
on that interface.
Also, note that (as long as your libvirt is at least version 2.1.0) if
you don't want to bother doing dhcp on the host and guest, you can
assign just a static IP address for the host side of that interface in
<interface type='ethernet'>
<source>
<ip address='192.168.254.2'/>
</source>
...
You can even specify routes to be added automatically to the host when
<interface type='ethernet'>
<source>
<ip address='192.168.254.2'/>
<route family='ipv4' address='0.0.0.0' prefix='0'
gateway='192.168.254.1' metric='1'/>
</source>
...
(this assumes that on the guest side, the OS has configured the emulated
ethernet device to have address 192.168.254.1). This can be useful if
you want the host to use one path for traffic when the guest isn't
running, but switch when the guest *is* running. (In this case, if you
can set the default route on the host to have a metric of 2 or higher,
then the default route that goes through the guest would be preferred
when present).
(NB: if you read the documentation for configuring interfaces at
https://libvirt.org/formatdomain.html#ipconfig you'll see references on
how to configure the IP address of the *guest* in libvirt's xml. Note
that this *does not work* for qemu, but only for lxc (and possibly xen,
I don't recall right now). This is a limitation of the way that qemu/kvm
works (the fact that all guest-side IP info is configured in the guest
OS, and libvirt has no reasonable to get to it).)

I intend to run only one vm on this system, thanks for the tip.

The reported speed should be inconsequential, since there is no wire and
no physical switch involved. It will transfer data as fast as the code
can be executed.
Also, if virtio works, then *definitely* use that instead of e1000 - the
performance will be much better and overhead much lower.

so any network test will do?

Thanks,

Dagg.

Laine Stump

2018-05-02 22:13:54 UTC

Permalink

Post by daggs

Sent: Wednesday, May 02, 2018 at 8:09 PM
Also, if virtio works, then *definitely* use that instead of e1000 - the
performance will be much better and overhead much lower.

so any network test will do?

Do you mean to test and see if virtio-net is acceptable for your guest
OS? If the guest OS has the driver for virtio-net, then definitely you
should use it. I've never heard of a case of any of the drivers that
emulate real hardware performing anywhere near as well as virtio-net.

daggs

2018-05-03 06:39:27 UTC

Permalink

Greetings Laine,

Sent: Thursday, May 03, 2018 at 1:13 AM
Subject: Re: [libvirt-users] connecting host and guest vm using a dummy nic

Post by daggs

Sent: Wednesday, May 02, 2018 at 8:09 PM
Also, if virtio works, then *definitely* use that instead of e1000 - the
performance will be much better and overhead much lower.

so any network test will do?

no, I mean test the virtual nic speed, will ipref work?
also, in the guest os, the module attached to the virtual nic is virtio-pci, is that expected?

daggs

2018-05-03 10:29:34 UTC

Permalink

Greetings Laine,

Sent: Thursday, May 03, 2018 at 9:39 AM
Subject: Re: [libvirt-users] connecting host and guest vm using a dummy nic
Greetings Laine,

Sent: Thursday, May 03, 2018 at 1:13 AM
Subject: Re: [libvirt-users] connecting host and guest vm using a dummy nic

Post by daggs

Sent: Wednesday, May 02, 2018 at 8:09 PM
Also, if virtio works, then *definitely* use that instead of e1000 - the
performance will be much better and overhead much lower.

so any network test will do?

no, I mean test the virtual nic speed, will ipref work?
also, in the guest os, the module attached to the virtual nic is virtio-pci, is that expected?
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***@router:/# iperf -s
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 85.3 KByte (default)
------------------------------------------------------------
[ 4] local 10.1.0.138 port 5001 connected with 10.1.0.212 port 57590
[ ID] Interval Transfer Bandwidth
[ 4] 0.0-10.0 sec 33.1 GBytes 28.4 Gbits/sec

looks very good indeed. thanks for all the help.