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I have a motherboard with onboard NIC (single Ethernet NIC in the computer); also USB port is constantly occupied with Wifi dongle.

in my /etc/udev/rules.d/70-persistent-net.rules, found this:

# PCI device 0x10ec:0x8168 (r8169) SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="6c:f0:49:e8:22:cd", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"

# PCI device 0x10ec:0x8168 (r8169) SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="00:1e:68:dc:96:52", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth1"

Currently, eth0 is detected. If I reboot, it may become eth1 (don't sure if mac will change, I wont reboot this PC today).

# ifconfig 
eth0      Link encap:Ethernet  HWaddr 00:1e:68:dc:96:52  

Sometimes, eth0 is not detected and eth1 comes up. Why? Can it be some BIOS bug with setting onboard MAC when cold-booted improperly?

Currently the ruleset is changed to force name to eth0 (for both macs), should the next reboots reveal if it helps..

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That is very odd. It sounds like your MAC address is flipping between two very different values, which is neither random nor bit-flipped. Weird. –  sybreon Dec 20 '10 at 10:53
    
Can you clarify how many NICs the pc has in it? One or two? You say "Onboard NIC, single NIC". –  Glen M Dec 20 '10 at 11:03
    
Can you also explain if you have a wireless card or other network=type adapter? Possibly IRDA? –  Glen M Dec 20 '10 at 11:04
    
@Glen M, fixed: "onboard NIC (single Ethernet NIC in the computer)". USB Wifi device is constantly inserted to port so doubt that it can affect reboots. But.. –  kagali-san Dec 20 '10 at 12:30
    
What's the MAC address on the USB wifi device? Can you send the output of "ifconfig -a"? What's the MAC address of the USB NIC? –  Glen M Dec 20 '10 at 16:46
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