Re: [libvirt] Different mac address used in libvirtd
On Wed, Jun 20, 2012 at 07:01:17PM +0800, Yong Sheng Gong wrote:
Hi,
I found libvirtd will create vnet0 with a different mac other than the one defined in
domain file. The mac in Domain file will be used by qemu-kvm.
I want to know how libvirtd decides the mac for vnet0, Who can give me a hint about the
location of the libvirtd codes which does this work?
I cannot find how the libvirtd translates the mac from one defined in domain file to
vnet0's one in src/util/bridge.c: brSetInterfaceMac()
<interface type='bridge'>
<mac address='fa:16:3e:28:64:c2'/>
<source bridge='br100'/>
<target dev='vnet0'/>
<filterref filter='nova-instance-instance-00000002-fa163e2864c2'>
<parameter name='DHCPSERVER' value='10.0.1.1'/>
<parameter name='PROJNET' value='10.0.1.0'/>
<parameter name='PROJMASK' value='255.255.255.0'/>
<parameter name='IP' value='10.0.1.2'/>
</filterref>
<alias name='net0'/>
<address type='pci' domain='0x0000' bus='0x00'
slot='0x03' function='0x0'/>
</interface>
The XML mac address config, sets the MAC address for the *guest*
NIC, ie what the guest uses as its eth0.
[root@robinlinux eclipsecdt]# ifconfig vnet0
vnet0 Link encap:Ethernet HWaddr FE:16:3E:28:64:C2
inet6 addr: fe80::fc16:3eff:fe28:64c2/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:553 errors:0 dropped:0 overruns:0 frame:0
TX packets:1485 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:500
RX bytes:136360 (133.1 KiB) TX bytes:280590 (274.0 KiB)
vnet0 is the backend of the guest NIC, and its MAC addr
is more or less irrelevant to functioning of the guest
itself, since traffic does not originate on this NIC.
The only important thing is that this TAP device must
have a high value MAC address, to avoid the bridge
device using the TAP device's MAC as its own. Hence
when creating the TAP Device libvirt takes the guest
MAC addr and simply sets the top byte to 0xFE
Daniel
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