[libvirt-users] question about how to set rng device on vm

[Yalan Zhang]

Hi Amos,

I'm a libvirt QE, and I can not understand the setting on libvirt.org for
rng device.
Could you please help to explain a little?
(The xml in  https://libvirt.org/formatdomain.html#elementsRng)
<devices>
  <rng model='virtio'>
    <rate period="2000" bytes="1234"/>
    <backend model='random'>/dev/random</backend>
    <!-- OR -->
    <backend model='egd' type='udp'>
      *<source mode='bind' service='1234'/>*
*      <source mode='connect' host='1.2.3.4' service='1234'/>*
    </backend>
  </rng>
</devices>

How did it work with source mode='bind' and source mode='connect' together?
which process on guest or host will act as server part, which for client
part?

One detail example:
start a vm with below device, and no egd running on host:
 <rng model='virtio'>
      <backend model='egd' type='udp'>
        <source mode='bind' service='1234'/>
        <source mode='connect' host='127.0.0.1' service='1234'/>
      </backend>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x09'
function='0x0'/>
    </rng>

qemu command line:
-chardev udp,id=charrng0,host=127.0.0.1,port=1234,localaddr=,localport=1234
-object rng-egd,id=objrng0,chardev=charrng0 -device
virtio-rng-pci,rng=objrng0,id=rng0,bus=pci.0,addr=0x9


In my understanding the purpose of the rng device on guest is to provide
guest a hardware RNG device /dev/hwrng which obtain seeds from the host.
The source can be /dev/random on host, then the xml will be:
<rng model='virtio'>
      <backend model='random'>/dev/random</backend>
    </rng>

can be hardware on host:
<rng model='virtio'>
      <backend model='random'>/dev/hwrng</backend>
    </rng>

can be edg daemon running on host:
   <rng model='virtio'>
      <backend model='egd' type='tcp'>
        <source mode='connect' host='127.0.0.1' service='1234'/>
      </backend>
    </rng>
(on host, there should be a egd daemon running on tcp 127.0.0.1:1234
 # egd.pl --debug-client --nofork localhost:1234)

Thank you very much and look forward for your response!


-------
Best Regards,
Yalan Zhang
IRC: yalzhang
Internal phone: 8389413
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