Why does one interface interfere with another?

[Greg Rundlett (freephile)]

I have a system with two physical network interfaces; a cat45 ethernet port
and a wireless card - otherwise known as any normal computer.

I configured the wired interface (eth0) to be static by editing
/etc/network/interfaces (see bottom) and I let the network-manager applet
handle the networking system, making my wireless interface address governed
by DHCP.

When the wired port has no connection (because I connect the cable to
another system), the wireless gets all confused and doesn't connect.
 Sometimes it doesn't even show my wireless network (causing me to blame the
Netgear wireless router and reboot the wireless router.)  Trying a ping to
the router, it tries to go through the wired address.  This happens even if
there is NO wired connection from the time of boot.  I can instantly solve
the problem by manually downing the static interface (sudo ifdown eth0).
 But I don't understand why Ubuntu 10.4 is being so "dumb" or more likely,
why I'm being so dumb.

All I want to do is have a static interface so that I can reach my machine
at the same IP address under normal (wired) circumstances, but if I
disconnect that cable for some reason, I still expect the wireless to
function (either dynamic or static).

http://live.gnome.org/NetworkManager/SystemSettings touches on this problem
apparently by saying that I could set network-manager to ignore anything
defined in /etc/network/interfaces.  But, I don't want it ignored, I just
want it to work well :-)

I did more digging, and read through
https://help.ubuntu.com/10.04/internet/C/index.html (the fine
manual) Apparently the problem is that NetworkManager is trying to
automatically handle the eth0 interface at the same time that I've manually
defined it (eth0 shows up twice in nm-applet).  So, realizing this, I was
able to comment out the whole manual static setting in
/etc/network/interfaces and then supply those settings to the interface
defined through nm-applet, and restart nm-applet which had the effect that
it now only lists one "eth0"

I can see a lot of log output in /var/log/syslog (using gnome-system-log is
nice for that) which helps me (sometimes) understand what is going on.
 Sometimes it doesn't, like when you put "external" DNS servers ahead of the
router, the system will not connect -- so I had to put 192.168.1.1 first in
the list of NameServers even though it's just my NetGear router.

So, now that I guess I've solved my problem, does anyone know whether I'm
going to run into a problem by defining eth1 (wireless) to be the same
static address as eth0 when I connect eth0 with a CAT5 cable?

== /etc/network/interfaces  ==
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 192.168.1.2
netmask 255.255.255.0
gateway 192.168.1.1


Thanks,
Greg Rundlett
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