Now that we have our fancy new VPS and are allowed to create multiple user accounts, I’ve run into a problem with basic linux permissions that you really only find when you have multiple users working in the same space. In my case, we need multiple users to have access to all of our online property web roots. I started by using chown to force the entire web root under the ownership of www-data:www-data and adding everyone who needed access to the secondary www-data group. This works fine until people start making changes. Each new file they write becomes owned by only them and their primary group.

For example, stevecrozz creates a new file in the web root and it ends up looking like this:

-rw-r--r--  1 stevecrozz stevecrozz    0 2009-02-14 01:50 stuff

Of course this means that no one else will be able to make changes to that file. After a little digging in the man pages I found the set-group-id flag which forces new files to be owned by the group of the containing folder. That gets me much closer, but not quite there.

$ rm -Rf test
$ mkdir test
$ sudo chown www-data:www-data test
$ sudo chmod 2775 test
$ touch test/somefile
$ ls -lAh test
total 0
-rw-r--r-- 1 stevecrozz www-data 0 2009-02-14 02:07 somefile

The group ownership of the parent folder is preserved, but the file still isn’t writeable by another member of the group. This is where I ran into the limits of basic unix-style permissions. Luckily, as I found, there’s a more complete acl-style setup ready to go. In ubuntu, you can install the tools to make it work with (oddly enough) “apt-get install acl”.

Your filesystem needs to be mounted with acl support, so edit your /etc/fstab and find the line for the filesystem you want to alter. and add acl in there. I changed the line:

/dev/sda1       /           ext3    defaults,errors=remount-ro,noatime    0 1


/dev/sda1       /           ext3    defaults,acl,errors=remount-ro,noatime    0 1

Remount it with “sudo mount -o remount,acl /” instead of rebooting.

Now you’re ready to go with much more powerful access control. Now you can go crazy. In this example I’ll make all the directories in my web root world readable and group writeable including every new file and directory.

$ cd /var/www
$ sudo find -type d -exec chmod 0775 {} \;
$ sudo find -type f -exec chmod 0664 {} \;
$ sudo setfacl -R -m d:u::rwx,d:g::rwx,d:o:--- .
$ ls -lAh
total 40K
drwxrwsr-x+  9 www-data www-data 4.0K 2009-02-14 01:28
drwxrwsr-x+ 11 www-data www-data 4.0K 2009-02-14 01:20
drwxrwsr-x+  3 www-data www-data 4.0K 2009-02-14 01:42
drwxrwsr-x+ 13 www-data www-data 4.0K 2009-02-14 01:14
drwxrwsr-x+  3 www-data www-data 4.0K 2009-02-14 00:47
$ getfacl
# file:
# owner: www-data
# group: www-data
$ touch
$ mkdir
$ ls -lAh
drwxrwsr-x+ 2 stevecrozz www-data 4.0K 2009-02-14 02:32 somedir
-rw-rw-r--  1 stevecrozz www-data    0 2009-02-14 02:31 somefile

You can see that new files created are owned by the www-data group and have the group write flag set. New folders are added in the same way plus they have the access control flags of the parent. If anyone needs access to write to the web root, I just add that person to the www-data group. Those pluses on the end of the permission strings mean that we’ve got acls set which we can examine with getfacl, and also that I am awesome.