dns-lookup problem on eth1

[Jim Carter]

On Wed, 16 Apr 2003, Zuern, Frank wrote:
> Hello, I installed a wireless card beneath my normal network card on my
> laptop, running on SuSe 8.2
> both are running on the same 192.168 network and are configured via
> DHCP...

With either interface, you say the DHCP server gives you an IP address,
but only on eth0 (wired) do you get a default route.  The DHCP server is
supposed to pass out your netmask and default route.  Here are a few things
to check:

I think you said that only one interface was up at a time, although you are
testing in an environment where both wired and wireless are available.  If
that were not true, things could get confusing, although it should be
possible to make the multiple interface situation work smoothly, if you put
some effort into configuring it.

The DHCP server for wireless, is it configured to give the correct netmask?
The correct default route?  The netmask and default route that end up on
your machine, do they actually differ from what the DHCP server said to
use?  It's possible that both the wired and wireless DHCP server tell your
machine to use the same default route, meaning that you're accusing your
laptop of a crime that it did not commit.  By the way, the default route
would generally be to an IP address, e.g. "route add default gw
psnet-gw-4.math.ucla.edu", and the routing code infers which interface can
be used to reach that IP address.

It's unlikely but possible that your DHCP client could have a compulsion to
leave the default route alone.  On dhcpcd it's the -G switch; I don't know
what switch it might be on dhclient or pump.  When eth1 is up, do "ps" and
verify that dhcpcd does not have the -G switch.

Hmmm, why would "netstat -r -n" show a default route through eth0 when eth0
was down at the time?  Or did I misread one of your messages?  I'm pretty
sure that when an interface goes down, all routes through that interface
are removed automatically.

Let's assume this network topology:

0.0.0.0 -- 10.0.0.1 ------------------------
Internet   Router    |               10.0.0.3
                    10.0.0.2         Router ----------------
                    Wired laptop     10.0.1.1     |
                                                 10.0.1.3 ------10.0.1.2
                                                 Access point   Laptop

I'm assuming a netmask of 255.255.255.0 on all machines except for the
default routes themselves.  If the DHCP server tells 10.0.1.2 (wireless
laptop) that the default route is 10.0.0.1 (main router), the laptop will
fail to set this default route, because it cannot reach 10.0.0.1.  It needs
to use the sub-router 10.0.1.1 as its default route, and it can't figure
that out for itself -- the DHCP server has to tell it.  I don't know if you
have this sort of subnet arrangement, but if you do, it would explain
exactly your symptoms.

Hope this helps!

James F. Carter          Voice 310 825 2897    FAX 310 206 6673
UCLA-Mathnet;  6115 MSA; 405 Hilgard Ave.; Los Angeles, CA, USA  90095-1555
Email: jimc at math.ucla.edu    http://www.math.ucla.edu/~jimc (q.v. for PGP key)