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I am running 15.10 on a Lenovo ThinkPad R61 and have been running Ubuntu since 8.04 on this machine. Since upgrading to 15.04 I have serious trouble with mobile broadband connections via USB modems.

While nm-applet says that a connection was established, there is actually no connection. One needs to repeat the 'connect to...' action several times until eventually a working connection becomes established. Once the connection is established, it remains stable as long as the current user is in an active session.

I have no clue where to start looking for system messages that could help further. On booting I occasionally see 'Internal Errors detected' messages mostly showing crashes of a package. Right now it says the urfkill package has crashed. Could there be a connection to the network connection issues?

I am happy to provide you any logfile output that could help. However, I did not want to flood my posting with potentially useless logfile copies. What can be done to enable mobile broadband to connect reliable and on the first trial?

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I was lucky enough to get access to a Linux Pro during Chemnitzer Linuxtage 2016 where an open workshop took place to support folks with their Linux problems. After two hours of fiddling he finally identified the reason for my LTE stick issue and solved it. I am happy to share the solution with you. So hopefully it helps others, too.

Obviously, Ubuntu network manager sometimes fails to add standard route 0.0.0.0 for device ppp0 (the LTE stick in my case). This causes name server issues and prevents your system from reaching any external resources, while nm-applet still displays the 'connected' icon.

Problem gets solved by manually adding this route 0.0.0.0 via shell script when ppp0 gets connected. The script goes to /etc/network/if-up.d/ and looks like this:

#!/bin/bash

if [ "$IFACE" == "ppp0" ]; then
  /sbin/ip route add to 0/0 via 0.0.0.0 dev ppp0 metric 750
fi

That's it.

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