Whenever the eth0 interface is enabled (ie. ifup eth0), I am unable to connect to the internet. The modem dials and pppd starts, and "ppp0" shows up when you type "ifconfig", but trying to access any site returns "unknown host". All the settings seem exactly the same as when eth0 is down (same DNS addresses in /etc/resolv.conf, etc.), but it still doesnt work. As soon as I "ifdown eth0" and reconnect, all works. The funny thing is, if I load the network config tool (I use Yellow Dog Linux 2.2) and set up eth0 with the exact same parameters as always, I can connect and all works. Then, as soon as eth0 is disabled and re-enabled (like when the system is rebooted), the connection stops working again (until I ifdown eth0). This seems really strange and I have tried all sorts of things, but nothing seems to make any difference, other than not using eth0 at all (and I want to use it). Please help if you can!

I don't know if you find your ip address of the machine you are using being listed when you do a ifconfig, If you don't see it then you will have to try doing ifconfig up and you might wanna specify a route for the network connectivity later on.

I do get an ip address for both eth0 and ppp0, even when the connection is not working. Also, if I connect to the internet and then bring up eth0, the connection continues working until I reconnect. When bringing up eth0 when online the message "RTNETLINK answers: File exits", but I still get an ip address from ifconfig. I dont know if that is significant, but I can't think of anything else...

Sounds like a routing problem to me. When the ppp link is establish, the ppp uplink sends you an ip to use for your default route. Now this is linux, and as such doesn't like doing things without the consent of the user/admin. In other words, it might not set up the default route. It does this only: if you do _not_ already have a default route, _and_ the options to pppd include "defaultroute". So, first make sure that you don't already have a default route. When you're not connected with ppp, type "route -n". A default route is one that has "0.0.0.0" as destination. If you have one, get rid of it, then try ppp again. If you don't have one, check your ppp options. First check your dialer if it maybe has an option dealing with the default route. If not, check the file /etc/ppp/options. If you don't see "defaultroute" in there, put it in, and try ppp again. If it still doesn't work, check "route -n" when you're connected to ppp. If you see a default route (with a gateway ip from your provider), then the problem is somewhere else. Can you access hosts when you specify the ip instead of the hostname? If yes, your dns setup is screwy.

Ok, there is a destination of 0.0.0.0 when eth0 is started with ifup, but not after it is set up with the config tool. If I remove the 0.0.0.0 route with the route command, the internet connection works fine. If I type route -n when the connection is working I get a destination of 0.0.0.0, but it is assigned to the ppp0 interface, and all is well. However, I'm not sure how to stop ifup from adding the default route that I don't need. I suppose I could just get "route" to run at startup and remove the offending route, but surely there is a better way?

Yes there's definitely a better way. However, I don't know for sure, as I have no about about the details of your config tools. The route is most probably set by your ifup script, which in turn uses the config files from the config tool you mention. First step should be to check the config with the config tool, maybe you have set some option there accidently. If this doesn't get you anywhere, try to locate the config files, and check those. As last resort, if you're feeling adventureous, check the ifup script itself out. Try to figure out how it works and what it does, and from where it pulls its config info.