Ubuntu – How to share an optical drive in a way that fools the client into thinking it’s a local drive

Stefan led me down the right track. This will work on any system 12.04 and newer.

Your filesystem will also need extended attribute support. If you're using EXT4 you're fine, if you're using EXT3 you'll need to ensure your filesystem is mounted with the user_xattr attribute.

Here's how I set it up on the client machine, you don't need to do anything on the server side.

1. First, install the user space daemon: sudo apt-get install cachefilesd

2. Then turn it on by editing /etc/default/cachefilesd and changing the run line to RUN=yes

3. Ensure your NFS mount in /etc/fstab has an fsc option. Don't forget to remount it after you've made the changes. Here's my excerpt (modify to your needs):

   192.168.1.115:/home/jorge /home/jorge/Backup nfs fsc

4. Start the service: sudo /etc/init.d/cachefilesd start

Configuration of the cache is done in /etc/cachefilesd.conf, see the references for what the setting options are.

References:

• Linux Magazine
• /usr/share/doc/cachefilesd/howto.txt.gz
• /usr/share/doc/cachefilesd/README.gz
• How do I enable extended attributes on ext4?

Access control lists

The straight answer is access control lists (ACLs). Yeah, you can find a counterexample, but they're good enough in practice (unlike mere group writability which requires that users think about it all the time). What they do require is that the system administrator (root) define the groups, if you want files to be shared only by a named group (root can choose to delegate, for example by accepting groups from LDAP, but that's another story).

You do need participating users to have a umask of 022. If they create non-world-readable files routinely, this scheme won't work. But if they have a restrictive umask, it's presumably because they don't want to share files anyway.

Enabling ACLs

Ubuntu doesn't enable ACLs by default, so there's a one-time admin requirement. Edit /etc/fstab using your favorite editor, and change every line corresponding to a filesystem where you want to share files: add acl to the options. (Make sure not to change any other line, and not to use an editor that wraps long lines.) Here's an example line with the acl option added:

UUID=5e1ec7ed-face-dead-beef-c011ec7ab1e5  /  ext4  errors=remount-ro,acl  0 1

For the option to take effect the first time, use a command like the following (for each filesystem):

sudo mount -o remount,acl /

Install the ACL tools from the acl package.

Setting up the shared directory

To have files shared by the group mygroup:

setfacl -m group:mygroup:rwx /path/to/shared/root
setfacl -d -m group:mygroup:rwx /path/to/shared/root

If people create files and copy them to the shared directory, the files will be world-readable (because of the umask) and anyone in the group can add and remove files (because the group is group-writable). People can't edit each others' files, but that's a good thing or you'd run into editing conflicts straight away.

If you don't have a unix group, you can add users one by one:

setfacl -m user:bob:rwx /path/to/shared/root
setfacl -d -m user:bob:rwx /path/to/shared/root

Version control

If you do want people to be able to edit files in place, you also need something to prevent editing conflicts. That's version control.

You don't need any of this to share a git repository. You know there are solutions like gitosis, so use them.