How to Update crouton in China

After the latest update of Chrome OS, the audio output had some problem of the ubuntu precise installed on my C720 via crouton. After searching online for advice, I think the problem is related with driver of the audio device on my C720 and the best solution is to update crouton. To perform the update of crouton should be easy unless you are in China, which unfortunately is my case. Even though I have VPN and clean DNS to get out of GFW, it is not enough to complete the update. I spent three days to get my crouton working. And the following is the tips for who also may encounter similar problems.

First, you need to add a line 173.194.72.82 chromium.googlesource.com in your host file in the directory /usr/local/chroot/precise/etc/, so that in one step of the update, your computer can get connected to that repository to download necessary files. I also tried the ip address given in this discussion, but it does not work. And moreover, even with the correct ip address, you still need a VPN, at least in my case, to get connection. It seems for that ip address, not only DNS pollution but also ip block are used by GFW. And due to some unknown mechanism, the connection to chromium.googlesource.com is sometimes via SSL, and sometimes via http. The connection via SSL will fail, then you just need to restart the update process and hope for http connection this time.

Second, you had better switch to ubuntu us archive repository instead of some other repository so that all the packages needed for update are latest version. In my case, I also temporarily comment out all the third party software repositories in /etc/apt/. Otherwise, your update process may fail due to disconnection to some third part repository.

The update process should have no more problems after the first two settings, although I also met some other problems after update and spent some time on them. But I think they are not related with the update process.