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You can take a disk file, format it as an ext2, ext3, or reiser filesystem, and then mount it, just like a physical drive. It's then possible to read and write files to this newly-mounted device. You can also copy the complete filesystem, since it is just a file, to another computer. If security is an issue, read on. This article will show you how to encrypt the filesystem and mount it with ACL (Access Control Lists), which gives you rights beyond the traditional read (r), write (w), and execute (x) permissions for the three user groups "file", "owner", and "other". Copyright notice: All reader-contributed material on freshmeat.net is the property and responsibility of its author; for reprint rights, please contact the author directly. This is an excellent way to investigate different filesystems without having to reformat a physical drive, which means you avoid the hassle of moving all your data. This method is quick -- very quick compared to preparing a physical device. You can then read and write files to the mounted device, but what is truly great about this technique is that you can explore different filesystems such as reiserfs, ext3, or ext2 without having to purchase an additional physical drive. Since the same file can be mounted on more than one mount point, you can investigate sync rates. Creating a filesystem in this manner allows you to set a hard limit on the amount of space used, which, of course, will be equal to the file size. This can be an advantage if you need to move this information to other servers. Since the contents cannot grow beyond the file, you can easily keep track of how much space is being used. First, you want to create a 20MB file by executing the following command:
$ dd if=/dev/zero of=disk-image count=40960
40960+0 records in
40960+0 records out
You created a 20 MB file because, by default, dd uses a block size of 512 bytes. That makes the size: 40960*512=20971520.
$ ls -l disk-image
-rw-rw-r-- 1 chirico chirico 20971520 Sep 3 14:24 disk-image
Next, to format this as an ext3 filesystem, you just execute the following command:
$ /sbin/mkfs -t ext3 -q disk-image
mke2fs 1.32 (09-Nov-2002)
disk-image is not a block special device.
Proceed anyway? (y,n) y
You are asked whether to proceed because this is a file, and not a block device. That is OK. We will mount this as a loopback device so that this file will simulate a block device. Next, you need to create a directory that will serve as a mount point for the loopback device.
$ mkdir fs
You are now one step away from the last step. You just want to find out what the next available loopback device number is. Normally, loopback devices start at zero (/dev/loop0) and work their way up (/dev/loop1, /dev/loop2, ... /dev/loopn). An easy way for you to find out what loopback devices are being used is to look into /proc/mounts, since the mount command may not give you what you need.
$ cat /proc/mounts
rootfs / rootfs rw 0 0
/dev/root / ext3 rw 0 0
/proc /proc proc rw,nodiratime 0 0
none /sys sysfs rw 0 0
/dev/sda1 /boot ext3 rw 0 0
none /dev/pts devpts rw 0 0
/proc/bus/usb /proc/bus/usb usbdevfs rw 0 0
none /dev/shm tmpfs rw 0 0
On my computer, I have no loopback devices mounted, so I'm OK to start with zero. You must do the next command as root, or with an account that has superuser privileges.
# mount -o loop=/dev/loop0 disk-image fs
That's it. You just mounted the file as a device. Now take a look at /proc/mounts, you will see this is using /dev/loop0.
$ cat /proc/mounts
rootfs / rootfs rw 0 0
/dev/root / ext3 rw 0 0
/proc /proc proc rw,nodiratime 0 0
none /sys sysfs rw 0 0
/dev/sda1 /boot ext3 rw 0 0
none /dev/pts devpts rw 0 0
/proc/bus/usb /proc/bus/usb usbdevfs rw 0 0
none /dev/shm tmpfs rw 0 0
/dev/loop0 /home/chirico/junk/fs ext3 rw 0 0
You can now create new files, write to them, read them, and do everything you normally would do on a disk drive. First, I'll give access to the chirico account.
# chown -R chirico.chirico /home/chirico/junk/fs
Now, under the chirico account, it is possible to create files.
$ cd /home/chirico/fs
$ mkdir one two three
$ ls -l
total 15
drwx------ 2 chirico chirico 12288 Sep 3 14:28 lost+found
drwxrwxr-x 2 chirico chirico 1024 Sep 3 14:34 one
drwxrwxr-x 2 chirico chirico 1024 Sep 3 14:34 three
drwxrwxr-x 2 chirico chirico 1024 Sep 3 14:34 two
$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda2 17G 11G 4.6G 71% /
/dev/sda1 99M 83M 11M 89% /boot
none 62M 0 62M 0% /dev/shm
/home/chirico/junk/disk-image
20M 1.1M 18M 6% /home/chirico/junk/fs
If you need to umount the filesystem, as root, just issue the umount command. If you need to free the loopback device, execute the losetup command with the -d option. You can execute both commands as follows:
# umount /home/chirico/junk/fs
# losetup -d /dev/loop0
Using RWX -- The Old Way To CollaborateBefore we get started with ACL, how would you set up rights on the filesystem so that users could create and save documents that others could modify? For instance, let's say that users chirico and sporkey are collaborating on a project together. Well, you have to add everyone to the same group. You would execute commands like these:.
# groupadd sharefs
# chown -R root.sharefs /home/chirico/junk/fs
# chmod 2775 /home/chirico/junk/fs
# usermod -G sharefs sporkey
# usermod -G sharefs chirico
Note that if these changes do not take effect for your users (for
example, if they were logged in when you executed the commands), they'll
have to log out and log in again or execute the " More importantly, even though the old way worked for you, at some point, new users may need to be added to the project. What if some of these users only need a subset of the rights? For instance, you have developers, testers, managers, and a few special people. There are limits to what the rwx type rights can do. ACL solves a lot of these problems. ACL, Reiserfs, and AES Encryption: The 2.6 KernelFor the next steps, I will assume that you are running Red Hat Fedora Core 2. If not, reference the 2.6 kernel upgrade section below. Four things will be covered in this section:
Your installation of Fedora Core 2, by default, will be configured for loop, cryptoloop, and aes, but it is highly unlikely that you will have all of these modules loaded. So, execute the following commands to load these modules (you will need to do this as root):
# modprobe loop
# modprobe cryptoloop
# modprobe aes
Next, create a directory to store the files. The Reiser filesystem will require more space than the ext3 filesystem.
# mkdir /home/diskimg
# cd /home/diskimg
Instead of creating the file zeroed out, like you did with the ext3 filesystem, this one is going to contain random bits, which may add a little extra security.
# dd if=/dev/urandom of=disk-aes count=102400
We need to encrypt the loop device, so you need to use losetup. You will be prompted for a password, which you will need to remember when you mount the device.
# losetup -e aes /dev/loop1 ./disk-aes
Password:
This step is new also. Instead of formating the file directly, you will format the loop device. The file stays encrypted. Again, you will be prompted to continue, so just enter "y".
# mkfs -t reiserfs /dev/loop1
mkfs.reiserfs 3.6.13 (2003 www.namesys.com)
A pair of credits:
Elena Gryaznova performed testing and benchmarking.
The Defense Advanced Research Projects Agency (DARPA, www.darpa.mil) is the
primary sponsor of Reiser4. DARPA does not endorse this project; it merely
sponsors it.
Guessing about desired format.. Kernel 2.6.8-1.521 is running.
Format 3.6 with standard journal
Count of blocks on the device: 12800
Number of blocks consumed by mkreiserfs formatting process: 8212
Blocksize: 4096
Hash function used to sort names: "r5"
Journal Size 8193 blocks (first block 18)
Journal Max transaction length 1024
inode generation number: 0
UUID: 435e3495-5e2e-489d-bf55-1b5f9a44b670
ATTENTION: YOU SHOULD REBOOT AFTER FDISK!
ALL DATA WILL BE LOST ON '/dev/loop1'!
Continue (y/n):y
Initializing journal - 0%....20%....40%....60%....80%....100%
Syncing..ok
Tell your friends to use a kernel based on 2.4.18 or later, and especially not a
kernel based on 2.4.9, when you use reiserFS. Have fun.
ReiserFS is successfully created on /dev/loop1.
Create the mount point /fs, and mount this device. Note that you will be entering the acl option as well. Plus, you will prompted for a password.
# mkdir /fs
# mount -o loop,encryption=aes,acl ./disk-aes /fs
Password:
Ok, now take a look at the mount command. It should show up as the Reiser filesystem, encrypted, using ACL. Note that it says loop2; it mounted it on /dev/loop2, which is one above what losetup specified, /dev/loop1.
$ mount
/home/diskimg/disk-aes on /fs type reiserfs (rw,loop=/dev/loop2,encryption=aes,acl)
Exploring ACLWith ACL (Access Control Lists), you have finer control over access permissions. With the rwx permission scheme, you cannot easily change rights without creating new groups to handle the users. With ACL, you can set user permissions without creating a group, and individual users can add or remove access.
These rights are set with the # setfacl -R -m d:u:donkey:rwx,d:u:chirico:rwx,d:u:bozo2:rwx /fs Next, create a few directories as one of the users. The example below was done as the user chirico.
$ mkdir /fs/one
$ touch /fs/one/stuff
$ ls -l /fs/one/stuff
-rw-rw----+ 1 chirico chirico 0 Sep 3 17:48 /fs/one/stuff
Notice the plus sign in the last line. It tells us a little about who has access. So, as user chirico, the getfacl command can be executed:
$ getfacl /fs/one/stuff
getfacl: Removing leading '/' from absolute path names
# file: fs/one/stuff
# owner: chirico
# group: chirico
user::rw-
user:chirico:rwx #effective:rw-
user:donkey:rwx #effective:rw-
user:bozo2:rwx #effective:rw-
group::r-x #effective:r--
mask::rw-
other::---
We now see that donkey, chirico, and bozo2 have effective rights on this file. Chirico has enough rights to remove bozo2.
$ setfacl -x u:bozo2 /fs/one/stuff
$ getfacl /fs/one/stuff
getfacl: Removing leading '/' from absolute path names
# file: fs/one/stuff
# owner: chirico
# group: chirico
user::rw-
user:chirico:rwx
user:donkey:rwx
group::r-x
mask::rwx
other::---
This is just scratching the surface of what can be done with ACL. For more information, see some of the references below. 2.6 Kernel UpgradeThis article will get you started with the 2.6 kernel if you are currently running Red Hat 8 or 9. You may want to take a look at it to see what is involved. If you decide to upgrade, you will need to configure your kernel for the following:
CONFIG_BLK_DEV_LOOP
CONFIG_BLK_DEV_CRYPTOLOOP
CONFIG_CRYPTO_AES_586
This is done in the .config file, and you can download my config file here. Just look for kernel-2.6.8.1-i686-chirico-reiserfsacl.config in the tar.gz. In addition to upgrading the kernel, you will need the latest version of the Linux utilities. Currently, there is no need to patch this version. In the past, there was a patch, but this version worked fine for me. You will also need the Reiser tools. References
Other Articles by Mike Chirico
Author's bio: Mike Chirico, a father of triplets (all girls) lives outside of Philadelphia, PA, USA. He has worked with Linux since 1996, has a Masters in Computer Science and Mathematics from Villanova University, and has worked in computer-related jobs from Wall Street to the University of Pennsylvania. His hero is Paul Erdos, a brilliant number theorist who was known for his open collaboration with others. Mike's notes page is souptonuts. T-Shirts and Fame! We're eager to find people interested in writing articles on software-related topics. We're flexible on length, style, and topic, so long as you know what you're talking about and back up your opinions with facts. Anyone who writes an article gets a t-shirt from ThinkGeek in addition to 15 minutes of fame. If you think you'd like to try your hand at it, let jeff.covey@freshmeat.net know what you'd like to write about. [Comments are disabled]
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cryptoloop is obsolete Cryptoloop is obsolete, buggy and has security weankesses. You should use dm_crypt instead.
[»]
ACL permissions GUI Now we just need Gnome and Konqueror to manage ACL permissions.
[»]
dd blocksize dd has options to set the block size to values different from 512 bytes,
this reduces the need to fiddle with numbers. For the purpose of creating
loop-mountable files, the bs argument is what you want to use. The bs
argument accepts common suffixes (k, M, G) for large numbers, at least in
GNU's dd.
[»]
Re: dd blocksize How about the even more obvious:
[»]
Re: dd blocksize
[»]
OpenBSD Its good to know how to do that in Linux. I find this sort of thing useful
for keeping sensitive documents/code on my laptop. In OpenBSD the steps
are similar except, after creating the diskimage file:
[»]
unmount -- If you need to umount a file-system, you may be blocked from umounting, if someone else is on it. fuser will list the culprit users. # fuser -u /filesystemTo kill all processes accessing the file system /filesystem, run the following command: # fuser -km /filesystem
[»]
No need to check used loop devices. Simply using 'loop' in the options argument for mount will locate the first
available loop device and use it.
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