Cannot modify '/etc/hosts' as administrator

Hi all,

I know there're a lot of discussions and solutions to this question, but my case is very strange. I have tried all the methods I've seen on the internet, including

1. sudo nano /private/etc/hosts
2. sudo vi /private/etc/hosts
3. sudo /Applications/TextEdit.app/Contents/MacOS/TextEdit /etc/hosts (I encountered an error by running this command: TextEdit[5872:3e07] NSTrackPersistentURLs: LSSharedFileListInsertItemURL() failed at inserting URL file://localhost/private/etc/hosts (/private/etc/hosts) )
4. Manually change permissions of private and etc foloder through Finder
5. Use the system preference pane 'Hosts'
6. Use Gas Mask

All methods I used above have problem overwriting the file (1, 2 said my hosts file is read only and I'm not permitted to overwrite, 5,6 didn't make a change on hosts file after I edited and saved). The account I'm using is an admin account, so I should have the permission to overwrite the hosts file. I used to be able to edit the hosts file and this problem only comes out early this year. I doubt if my hosts file is somehow damaged? Has someone here experienced it and is able to help me figure it out? I would really appreciate it.