Raspberry PI Wireless WLAN0 won't work without ETH0

There are plenty of other sites documenting how to initially configure wireless so I'm not going to repeat them.  This is specifically about a weird issue I had configuring the card for home.

No hardware issues, the latest Raspian build found the Edimax USB card with no issues.  My problem was the wlan0 interface would not work without eth0 also being active.

Further investigation led me to find the MAC address both the wlan0 & eth0 IP responded from was in fact the eth0 address.

Now this goes against everything I understand but in my case with the RPI ethernet cable plugged in:

Take note of the HWaddr on each card.

$ ifconfig
eth0      Link encap:Ethernet  HWaddr b8:27:eb:b0:0c:39  
          inet addr:192.168.99.75  Bcast:192.168.99.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1


wlan0     Link encap:Ethernet  HWaddr 80:1f:02:82:33:24  
          inet addr:192.168.99.78  Bcast:192.168.99.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

Then from another workstation, in this case I'm using NMAP:

$ sudo nmap -sn 192.168.99.75  **<< - ETH0**

Starting Nmap 6.25 ( http://nmap.org ) at 2013-02-03 10:19 GMT
Nmap scan report for 192.168.99.75
Host is up (0.020s latency).
MAC Address: B8:27:EB:B0:0C:39 (Raspberry Pi Foundation)
Nmap done: 1 IP address (1 host up) scanned in 0.09 seconds
Paul@lo-mbp-preg / $ sudo nmap -sn 192.168.99.78

$ sudo nmap -sn 192.168.99.78  **<< - WLAN0**

Starting Nmap 6.25 ( http://nmap.org ) at 2013-02-03 10:19 GMT
Nmap scan report for 192.168.99.78
Host is up (0.0044s latency).
MAC Address: B8:27:EB:B0:0C:39 (Raspberry Pi Foundation)
Nmap done: 1 IP address (1 host up) scanned in 0.07 seconds

You can see that the MAC Address/HWAddr for both ETH0 and WLAN0 is the same, and matches the ETH0 HWAddr from ifconfig.  So in my case the Wireless was not working and all traffic was passing via ETH0

I never actually found 'the reason' for this.  In the process of debugging it started working reliably.  Which I hate.  But this config is now working :

**/etc/network/interfaces**

auto lo

iface lo inet loopback
iface eth0 inet dhcp

allow-hotplug wlan0
iface wlan0 inet manual
wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf
iface default inet dhcp

**/etc/wpa_supplicant/wpa_supplicant.conf**

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
ctrl_interface_group=root
update_config=1
ap_scan=1

network={
 ssid="<ssid>"
 psk=<key>
}
network={
 ssid="<ssid>"
 scan_ssid=1
 key_mgmt=WPA-EAP
 pairwise=CCMP TKIP
 group=CCMP TKIP
 eap=PEAP
 identity="<uid>"
 password="<password>"
 #ca_cert="/etc/cert/ca.pem"
 phase1="peapver=1"
 #phase1="fast_provisioning=1"
 phase2="MSCHAPV2"
 pac_file="/etc/wpa_supplicant/pac"
}