static IP setting

Adrian Inscribed thus:

Try giving one or the other interfaces its own address or use DHCP.

Not static on eth0 but with dhcp on both here is my working interfaces:

auto lo iface lo inet loopback auto eth0 allow-hotplug eth0 iface eth0 inet dhcp auto wlan0 allow-hotplug wlan0 iface wlan0 inet manual wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf wpa-ssid ********* wpa-psk *********** wpa-key-mgmt WPA-PSK wpa-group TKIP CCMP iface default inet dhcp

That appears to be consistent with what you asked for in /etc/network/interfaces: a dhcp addres for wlan0, a static address for eth0. What's the problem? Just connect to the dhcp address, which presumably you've configured your router to install in local DNS.

It's a bad idea to try to have the same address on both interfaces at once. If you want two separate static addresses, make them different ones. Otherwise you're relying on the switch fabric to do the right thing.

In message , Roger Bell_West writes

Thanks for the detailed reply.

What I want is to be able to use a static IP address whether I'm using WAN or ETH. Apart from when I was playing around this afternoon, I wouldn't have the dongle and a cable plugged in at the same time. To me, it makes sense to use the same IP address for both interfaces (as only one would be active at any time).

When I get a chance, I'll try setting the interfaces file with different IP addresses for the two interfaces.

Adrian

Then is is best to use DHCP and configure your DHCP server so that it serves the same static IP address for both MAC addresses (of the eth0 and the wlan) involved.

This configuration allows the Pi to set the address to only one interface at a time, and still you have automatic switching. I use it here, and it works fine.

Agreed.

The difficulty with using the same static address on two interfaces at once is that the ethernet transceiver can't reliably detect whether it has a cable plugged in or any ability to route packets. (Yes, I know about the link light. There are some interesting edge cases.) So in your configuration, both interfaces show as "up"; each packet might go out through either, because they both look equally valid.

You could I suppose have an init script that tries to bring up wlan0 and then falls back to eth0 if that fails, but the DHCP option will probably be less trouble.

Not on my Pis, but on a Debian Linux PC, which has an almost identical setup, I use the pre-up option in e/n/i to check if an interface has a link before trying to bring it up.

If the pre-up command returns False, then the interface isn't started, so my interfaces file is something like:

iface eth0 inet static pre-up /usr/local/eth0-check address blah-blah etc...

iface wlan0 inet static pre-up /usr/local/wlan0-check address blah-blah etc

eth0-check would use something like ethtool to check for a link and return true if one exists.

wlan0-check would check to see if eth0 is up and return false if it is.

(I actually do use DHCP, but this would work with static addresses too).

It won't switch from on to the other if one interface loses connection while up though.

Thanks for the suggestions.

I think I've now got the router set up to do this. A quick reset of the interfaces file and a reboot, and I'm back working on WiFi.

Adrian