I configured my arm Debian/Wheezy box with two ethernet connections, one through the lan port and the other through a wifi dongle. The configuration is the same as what described here for Raspberry Pi and it worked fine there. However, when I configured my server on Pogo Mobile, it also has Debian/Wheezy installed as RPi, I have very difficult time to make it work.
First, if I connected the ethernet cable and made the wlan0 up, I could ssh into the server on either ip address. However, when I unplugged the ethernet cable, I could not ssh into the wifi ip address anymore. Then I found that if I disabled eth0 by "ifdown eth0," the wifi ip address will still work even after unplugging the ethernet cable. So I added "ifdown eth0" to rc.local before exit. Sure enough, I can restart the device with the ethernet cable unplugged and the wifi still works.
Now came the second problem, I found that, even though the device could reach other devices on the same local net, it couldn't ping entities on internet outside local net. By checking using "route -n", I found the the gateway was reset after doing "ifdown eth0". It would work again if I did a "route add default gw 192.169.x.x(Gatewayip)", so I added this line to rc.local also. After that, everything works fine for now.
However, there is another strange thing going on. When I check the available ip addresses by using Advanced IP Scanner, I can see that the ip address of the ethernet port still showed up and I can ssh in using that ip, even though "ifconfig" showed only lo and wlan0 but no eth0.
The fix I did was quite a Kludge. What I want is like in RPi, when both are connected, I can ssh into it and connect to internet on either ip address. If only one connection is up, I would be able to connect to it using the connected one. However, I don't know too much about Linux networking to figure this out. Hope someone knowledgeable can help.
Thanks in advance.