Hello
I experience a strange thing on a linux V2.6.38 card. I installed and configured a wifi dongle wlan0 (WPA) and also connect the card via eth0.
If I ping using eth0 alone the gateway it works.
If I ping the gateway with either eth0 or wlan0 both connected it works
If I ping the gateway with wlan0 alone (from the boot) it DOESN'T work
If I ping the gate way wlan0, both connected and then I unplug wildly eth0 it also works.

How is this possible ?
Why the wlan0 doesn't work at boot and then works when eth0 is once plugged ?
In the same way, why wlan0 is then not also pingable from the outside ?

Thank you for your answers
below my interface file :

auto eth0
#iface eth0 inet dhcp
iface eth0 inet static
address 192.168.0.242
netmask 255.255.255.0
gateway 192.168.0.1
dns-nameservers 212.27.40.240 212.27.40.241

auto wlan0
iface wlan0 inet static
gateway 192.168.0.1
dns-nameservers 212.27.40.240 212.27.40.241
network 192.168.0.0
broadcast 192.168.0.255
netmask 255.255.255.0
wpa-driver wext
wpa-ssid Erium
wpa-ap-scan 1
wpa-scan-ssid 1
wpa-proto WPA RSN
wpa-pairwise TKIP CCMP
wpa-group TKIP CCMP
wpa-key-mgmt WPA-PSK
wpa-psk my.crypted.wpa-psk

It's only working with eth0 plugged in. You have wlan0 set up to static, rather than DHCP, and it does not have an IP Address assigned to it.

Ok thank you for your reply : your right : I forget to copy the adress but it was set.
I discovered that it works with wlan0 only when I "ifconfig eth0 down".
So it now works but I don't know why and I would like to understand.

Probably a routing issue.

# route -n
$ netstat -r

Or some newer way of doing things I have yet to learn. You can only have one default. Or at least that's how it used to work. If you don't set the device on the route add statement it will likely default to eth0 if both exist and both eth0 and wlan0 are up.

# route add default gw 192.168.0.1 dev wlan0

Or something like that.