source: project/release/4/ugarit/trunk/README.txt @ 25525

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1# Introduction
2
3Ugarit is a backup/archival system based around content-addressible storage.
4
5This allows it to upload incremental backups to a remote server or a
6local filesystem such as an NFS share or a removable hard disk, yet
7have the archive instantly able to produce a full snapshot on demand
8rather than needing to download a full snapshot plus all the
9incrementals since. The content-addressible storage technique means
10that the incrementals can be applied to a snapshot on various kinds of
11storage without needing intelligence in the storage itself - so the
12snapshots can live within Amazon S3 or on a removable hard disk.
13
14Also, the same storage can be shared between multiple systems that all
15back up to it - and the incremental upload algorithm will mean that
16any files shared between the servers will only need to be uploaded
17once. If you back up a complete server, than go and back up another
18that is running the same distribution, then all the files in `/bin`
19and so on that are already in the storage will not need to be backed
20up again; the system will automatically spot that they're already
21there, and not upload them again.
22
23## So what's that mean in practice?
24
25You can run Ugarit to back up any number of filesystems to a shared
26archive, and on every backup, Ugarit will only upload files or parts
27of files that aren't already in the archive - be they from the
28previous snapshot, earlier snapshots, snapshot of entirely unrelated
29filesystems, etc. Every time you do a snapshot, Ugarit builds an
30entire complete directory tree of the snapshot in the archive - but
31reusing any parts of files, files, or entire directories that already
32exist anywhere in the archive, and only uploading what doesn't already
33exist.
34
35The support for parts of files means that, in many cases, gigantic
36files like database tables and virtual disks for virtual machines will
37not need to be uploaded entirely every time they change, as the
38changed sections will be identified and uploaded.
39
40Because a complete directory tree exists in the archive for any
41snapshot, the extraction algorithm is incredibly simple - and,
42therefore, incredibly reliable and fast. Simple, reliable, and fast
43are just what you need when you're trying to reconstruct the
44filesystem of a live server.
45
46Also, it means that you can do lots of small snapshots. If you run a
47snapshot every hour, then only a megabyte or two might have changed in
48your filesystem, so you only upload a megabyte or two - yet you end up
49with a complete history of your filesystem at hourly intervals in the
50archive.
51
52Conventional backup systems usually either store a full backup then
53incrementals to their archives, meaning that doing a restore involves
54reading the full backup then reading every incremental since and
55applying them - so to do a restore, you have to download *every
56version* of the filesystem you've ever uploaded, or you have to do
57periodic full backups (even though most of your filesystem won't have
58changed since the last full backup) to reduce the number of
59incrementals required for a restore. Better results are had from
60systems that use a special backup server to look after the archive
61storage, which accept incremental backups and apply them to the
62snapshot they keep in order to maintain a most-recent snapshot that
63can be downloaded in a single run; but they then restrict you to using
64dedicated servers as your archive stores, ruling out cheaply scalable
65solutions like Amazon S3, or just backing up to a removable USB or
66eSATA disk you attach to your system whenever you do a backup. And
67dedicated backup servers are complex pieces of software; can you rely
68on something complex for the fundamental foundation of your data
69security system?
70
71## System Requirements
72
73Ugarit should run on any POSIX-compliant system that can run [Chicken
74Scheme](http://www.call-with-current-continuation.org/). It stores and
75restores all the file attributes reported by the `stat` system call -
76POSIX mode permissions, UID, GID, mtime, and optionally atime and
77ctime (although the ctime cannot be restored due to POSIX
78restrictions). Ugarit will store files, directories, device and
79character special files, symlinks, and FIFOs.
80
81Support for extended filesystem attributes - ACLs, alternative
82streams, forks and other metadata - is possible, due to the extensible
83directory entry format; support for such metadata will be added as
84required.
85
86Currently, only local filesystem-based archive storage backends are
87complete: these are suitable for backing up to a removable hard disk
88or a filesystem shared via NFS or other protocols. However, the
89backend can be accessed via an SSH tunnel, so a remote server you are
90able to install Ugarit on to run the backends can be used as a remote
91archive.
92
93However, the next backend to be implemented will be one for Amazon S3,
94and an SFTP backend for storing archives anywhere you can ssh
95to. Other backends will be implemented on demand; an archive can, in
96principle, be stored on anything that can store files by name, report
97on whether a file already exists, and efficiently download a file by
98name. This rules out magnetic tapes due to their requirement for
99sequential access.
100
101Although we need to trust that a backend won't lose data (for now), we
102don't need to trust the backend not to snoop on us, as Ugarit
103optionally encrypts everything sent to the archive.
104
105## Terminology
106
107A Ugarit backend is the software module that handles backend
108storage. An archive is an actual storage system storing actual data,
109accessed through the appropriate backend for that archive. The backend
110may run locally under Ugarit itself, or via an SSH tunnel, on a remote
111server where it is installed.
112
113For example, if you use the recommended "splitlog" filesystem backend,
114your archive might be `/mnt/bigdisk` on the server `prometheus`. The
115backend (which is compiled along with the other filesystem backends in
116the `backend-fs` binary) must be installed on `prometheus`, and Ugarit
117clients all over the place may then use it via ssh to
118`prometheus`. However, even with the filesystem backends, the actual
119storage might not be on `prometheus` where the backend runs -
120`/mnt/bigdisk` might be an NFS mount, or a mount from a storage-area
121network. This ability to delegate via SSH is particularly useful with
122the "cache" backend, which reduces latency by storing a cache of what
123blocks exist in a backend, thereby making it quicker to identify
124already-stored files; a cluster of servers all sharing the same
125archive might all use SSH tunnels to access an instance of the "cache"
126backend on one of them (using some local disk to store the cache),
127which proxies the actual archive storage to an archive on the other
128end of a high-latency Internet link, again via an SSH tunnel.
129
130## What's in an archive?
131
132An Ugarit archive contains a load of blocks, each up to a maximum size
133(usually 1MiB, although other backends might impose smaller
134limits). Each block is identified by the hash of its contents; this is
135how Ugarit avoids ever uploading the same data twice, by checking to
136see if the data to be uploaded already exists in the archive by
137looking up the hash. The contents of the blocks are compressed and
138then encrypted before upload.
139
140Every file uploaded is, unless it's small enough to fit in a single
141block, chopped into blocks, and each block uploaded. This way, the
142entire contents of your filesystem can be uploaded - or, at least,
143only the parts of it that aren't already there! The blocks are then
144tied together to create a snapshot by uploading blocks full of the
145hashes of the data blocks, and directory blocks are uploaded listing
146the names and attributes of files in directories, along with the
147hashes of the blocks that contain the files' contents. Even the blocks
148that contain lists of hashes of other blocks are subject to checking
149for pre-existence in the archive; if only a few MiB of your
150hundred-GiB filesystem has changed, then even the index blocks and
151directory blocks are re-used from previous snapshots.
152
153Once uploaded, a block in the archive is never again changed. After
154all, if its contents changed, its hash would change, so it would no
155longer be the same block! However, every block has a reference count,
156tracking the number of index blocks that refer to it. This means that
157the archive knows which blocks are shared between multiple snapshots
158(or shared *within* a snapshot - if a filesystem has more than one
159copy of the same file, still only one copy is uploaded), so that if a
160given snapshot is deleted, then the blocks that only that snapshot is
161using can be deleted to free up space, without corrupting other
162snapshots by deleting blocks they share. Keep in mind, however, that
163not all storage backends may support this - there are certain
164advantages to being an append-only archive. For a start, you can't
165delete something by accident! The supplied fs backend supports
166deletion, while the splitlog backend does not yet. However, the actual
167snapshot deletion command hasn't been implemented yet either, so it's
168a moot point for now...
169
170Finally, the archive contains objects called tags. Unlike the blocks,
171the tags contents can change, and they have meaningful names rather
172than being identified by hash. Tags identify the top-level blocks of
173snapshots within the system, from which (by following the chain of
174hashes down through the index blocks) the entire contents of a
175snapshot may be found. Unless you happen to have recorded the hash of
176a snapshot somewhere, the tags are where you find snapshots from when
177you want to do a restore!
178
179Whenever a snapshot is taken, as soon as Ugarit has uploaded all the
180files, directories, and index blocks required, it looks up the tag you
181have identified as the target of the snapshot. If the tag already
182exists, then the snapshot it currently points to is recorded in the
183new snapshot as the "previous snapshot"; then the snapshot header
184containing the previous snapshot hash, along with the date and time
185and any comments you provide for the snapshot, and is uploaded (as
186another block, identified by its hash). The tag is then updated to
187point to the new snapshot.
188
189This way, each tag actually identifies a chronological chain of
190snapshots. Normally, you would use a tag to identify a filesystem
191being backed up; you'd keep snapshotting the filesystem to the same
192tag, resulting in all the snapshots of that filesystem hanging from
193the tag. But if you wanted to remember any particular snapshot
194(perhaps if it's the snapshot you take before a big upgrade or other
195risky operation), you can duplicate the tag, in effect 'forking' the
196chain of snapshots much like a branch in a version control system.
197
198# Using Ugarit
199
200## Installation
201
202Install [Chicken Scheme](http://www.call-with-current-continuation.org/) using their [installation instructions](http://chicken.wiki.br/Getting%20started#Installing%20Chicken).
203
204Ugarit can then be installed by typing (as root):
205
206    chicken-install ugarit
207
208See the [chicken-install manual](http://wiki.call-cc.org/manual/Extensions#chicken-install-reference) for details if you have any trouble, or wish to install into your home directory.
209
210## Setting up an archive
211
212Firstly, you need to know the archive identifier for the place you'll
213be storing your archives. This depends on your backend. The archive
214identifier is actually the command line used to invoke the backend for
215a particular archive; communication with the archive is via standard
216input and output, which is how it's easy to tunnel via ssh.
217
218### Local filesystem backends
219
220These backends use the local filesystem to store the archives. Of
221course, the "local filesystem" on a given server might be an NFS mount
222or mounted from a storage-area network.
223
224#### Logfile backend
225
226The logfile backend works much like the original Venti system. It's
227append-only - you won't be able to delete old snapshots from a logfile
228archive, even when I implement deletion. It stores the archive in two
229sets of files; one is a log of data blocks, split at a specified
230maximum size, and the other is the metadata: an sqlite database used
231to track the location of blocks in the log files, the contents of
232tags, and a count of the logs so a filename can be chosen for a new one.
233
234To set up a new logfile archive, just choose where to put the two
235parts. It would be nice to put the metadata file on a different
236physical disk to the logs directory, to reduce seeking. If you only
237have one disk, you can put the metadata file in the log directory
238("metadata" is a good name).
239
240You can then refer to it using the following archive identifier:
241
242      "backend-fs splitlog ...log directory... ...metadata file... max-logfile-size"
243
244For most platforms, a max-logfile-size of 900000000 (900 MB) should
245suffice. For now, don't go much bigger than that on 32-bit systems
246until Chicken's `file-position` function is fixed to work with files
247more than 1GB in size.
248
249#### Filesystem backend
250
251The filesystem backend creates archives by storing each block or tag
252in its own file, in a directory. To keep the objects-per-directory
253count down, it'll split the files into subdirectories. Because of
254this, it uses a stupendous number of inodes (more than the filesystem
255being backed up). Only use it if you don't mind that; splitlog is much
256more efficient.
257
258To set up a new filesystem-backend archive, just create an empty
259directory that Ugarit will have write access to when it runs. It will
260probably run as root in order to be able to access the contents of
261files that aren't world-readable (although that's up to you), so be
262careful of NFS mounts that have `maproot=nobody` set!
263
264You can then refer to it using the following archive identifier:
265
266      "backend-fs fs ...path to directory..."
267
268### Proxying backends
269
270These backends wrap another archive identifier which the actual
271storage task is delegated to, but add some value along the way.
272
273### SSH tunnelling
274
275It's easy to access an archive stored on a remote server. The caveat
276is that the backend then needs to be installed on the remote server!
277Since archives are accessed by running the supplied command, and then
278talking to them via stdin and stdout, the archive identified needs
279only be:
280
281      "ssh ...hostname... '...remote archive identifier...'"
282
283### Cache backend
284
285The cache backend is used to cache a list of what blocks exist in the
286proxied backend, so that it can answer queries as to the existance of
287a block rapidly, even when the proxied backend is on the end of a
288high-latency link (eg, the Internet). This should speed up snapshots,
289as existing files are identified by asking the backend if the archive
290already has them.
291
292The cache backend works by storing the cache in a local sqlite
293file. Given a place for it to store that file, usage is simple:
294
295      "backend-cache ...path to cachefile... '...proxied archive identifier...'"
296
297The cache file will be automatically created if it doesn't already
298exist, so make sure there's write access to the containing directory.
299
300 - WARNING - WARNING - WARNING - WARNING - WARNING - WARNING -
301
302If you use a cache on an archive shared between servers, make sure
303that you either:
304
305 * Never delete things from the archive
306
307or
308
309 * Make sure all access to the archive is via the same cache
310
311If a block is deleted from an archive, and a cache on that archive is
312not aware of the deletion (as it did not go "through" the caching
313proxy), then the cache will record that the block exists in the
314archive when it does not. This will mean that if a snapshot is made
315through the cache that would use that block, then it will be assumed
316that the block already exists in the archive when it does
317not. Therefore, the block will not be uploaded, and a dangling
318reference will result!
319
320Some setups which *are* safe:
321
322 * A single server using an archive via a cache, not sharing it with
323   anyone else.
324
325 * A pool of servers using an archive via the same cache.
326
327 * A pool of servers using an archive via one or more caches, and
328   maybe some not via the cache, where nothing is ever deleted from
329   the archive.
330
331 * A pool of servers using an archive via one cache, and maybe some
332   not via the cache, where deletions are only performed on servers
333   using the cache, so the cache is always aware.
334
335## Writing a ugarit.conf
336
337`ugarit.conf` should look something like this:
338
339      (storage <archive identifier>)
340      (hash tiger "<salt>")
341      [double-check]
342      [(compression [deflate|lzma])]
343      [(encryption aes <key>)]
344      [(file-cache "<path>")]
345      [(rule ...)]
346
347The hash line chooses a hash algorithm. Currently Tiger-192 (`tiger`),
348SHA-256 (`sha256`), SHA-384 (`sha384`) and SHA-512 (`sha512`) are
349supported; if you omit the line then Tiger will still be used, but it
350will be a simple hash of the block with the block type appended, which
351reveals to attackers what blocks you have (as the hash is of the
352unencrypted block, and the hash is not encrypted). This is useful for
353development and testing or for use with trusted archives, but not
354advised for use with archives that attackers may snoop at. Providing a
355salt string produces a hash function that hashes the block, the type
356of block, and the salt string, producing hashes that attackers who can
357snoop the archive cannot use to find known blocks (see the "Security
358model" section below for more details). Whichever hash function you
359use, you will need to install the required Chicken egg with one of the
360following commands:
361
362    chicken-install -s tiger-hash  # for tiger
363    chicken-install -s sha2        # for the SHA hashes
364
365`double-check`, if present, causes Ugarit to perform extra internal
366consistency checks during backups, which will detect bugs but may slow
367things down.
368
369`lzma` is the recommended compression option for low-bandwidth
370backends or when space is tight, but it's very slow to compress;
371deflate or no compression at all are better for fast local
372archives. To have no compression at all, just remove the `(compression
373...)` line entirely. Likewise, to use compression, you need to install
374a Chicken egg:
375
376       chicken-install -s z3       # for deflate
377       chicken-install -s lzma     # for lzma
378
379Likewise, the `(encryption ...)` line may be omitted to have no
380encryption; the only currently supported algorithm is aes (in CBC
381mode) with a key given in hex, as a passphrase (hashed to get a key),
382or a passphrase read from the terminal on every run. The key may be
38316, 24, or 32 bytes for 128-bit, 192-bit or 256-bit AES. To specify a
384hex key, just supply it as a string, like so:
385
386      (encryption aes "00112233445566778899AABBCCDDEEFF")
387
388...for 128-bit AES,
389
390      (encryption aes "00112233445566778899AABBCCDDEEFF0011223344556677")
391
392...for 192-bit AES, or
393
394      (encryption aes "00112233445566778899AABBCCDDEEFF00112233445566778899AABBCCDDEEFF")
395
396...for 256-bit AES.
397
398Alternatively, you can provide a passphrase, and specify how large a
399key you want it turned into, like so:
400
401      (encryption aes ([16|24|32] "We three kings of Orient are, one in a taxi one in a car, one on a scooter honking his hooter and smoking a fat cigar. Oh, star of wonder, star of light; star with royal dynamite"))
402
403Finally, the extra-paranoid can request that Ugarit prompt for a
404passphrase on every run and hash it into a key of the specified
405length, like so:
406
407      (encryption aes ([16|24|32] prompt))
408
409(note the lack of quotes around `prompt`, distinguishing it from a passphrase)
410
411Please read the "Security model" section below for details on the
412implications of different encryption setups.
413
414Again, as it is an optional feature, to use encryption, you must
415install the appropriate Chicken egg:
416
417       chicken-install -s aes
418
419A file cache, if enabled, significantly speeds up subsequent snapshots
420of a filesystem tree. The file cache is a file (which Ugarit will
421create if it doesn't already exist) mapping filenames to
422(mtime,size,hash) tuples; as it scans the filesystem, if it finds a
423file in the cache and the mtime and size have not changed, it will
424assume it is already archived under the specified hash. This saves it
425from having to read the entire file to hash it and then check if the
426hash is present in the archive. In other words, if only a few files
427have changed since the last snapshot, then snapshotting a directory
428tree becomes an O(N) operation, where N is the number of files, rather
429than an O(M) operation, where M is the total size of files involved.
430
431For example:
432
433      (storage "ssh ugarit@spiderman 'backend-fs splitlog /mnt/ugarit-data /mnt/ugarit-metadata/metadata 900000000'")
434      (hash tiger "Giung0ahKahsh9ahphu5EiGhAhth4eeyDahs2aiWAlohr6raYeequ8uiUr3Oojoh")
435      (encryption aes (32 "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"))
436      (compression lzma)
437      (file-cache "/var/ugarit/cache")
438
439Be careful to put a set of parentheses around each configuration
440entry. White space isn't significant, so feel free to indent things
441and wrap them over lines if you want.
442
443Keep copies of this file safe - you'll need it to do extractions!
444Print a copy out and lock it in your fire safe! Ok, currently, you
445might be able to recreate it if you remember where you put the
446storage, but encryption keys are harder to remember.
447
448## Your first backup
449
450Think of a tag to identify the filesystem you're backing up. If it's
451`/home` on the server `gandalf`, you might call it `gandalf-home`. If
452it's the entire filesystem of the server `bilbo`, you might just call
453it `bilbo`.
454
455Then from your shell, run (as root):
456
457      # ugarit snapshot <ugarit.conf> [-c] [-a] <tag> <path to root of filesystem>
458
459For example, if we have a `ugarit.conf` in the current directory:
460
461      # ugarit snapshot ugarit.conf -c localhost-etc /etc
462
463Specify the `-c` flag if you want to store ctimes in the archive;
464since it's impossible to restore ctimes when extracting from an
465archive, doing this is useful only for informational purposes, so it's
466not done by default. Similarly, atimes aren't stored in the archive
467unless you specify `-a`, because otherwise, there will be a lot of
468directory blocks uploaded on every snapshot, as the atime of every
469file will have been changed by the previous snapshot - so with `-a`
470specified, on every snapshot, every directory in your filesystem will
471be uploaded! Ugarit will happily restore atimes if they are found in
472an archive; their storage is made optional simply because uploading
473them is costly and rarely useful.
474
475## Exploring the archive
476
477Now you have a backup, you can explore the contents of the
478archive. This need not be done as root, as long as you can read
479`ugarit.conf`; however, if you want to extract files, run it as root
480so the uids and gids can be set.
481
482      $ ugarit explore <ugarit.conf>
483
484This will put you into an interactive shell exploring a virtual
485filesystem. The root directory contains an entry for every tag; if you
486type `ls` you should see your tag listed, and within that tag, you'll
487find a list of snapshots, in descending date order, with a special
488entry `current` for the most recent snapshot. Within a snapshot,
489you'll find the root directory of your snapshot, and will be able to
490`cd` into subdirectories, and so on:
491
492      > ls
493      Test <tag>
494      > cd Test
495      /Test> ls
496      2009-01-24 10:28:16 <snapshot>
497      2009-01-24 10:28:16 <snapshot>
498      current <snapshot>
499      /Test> cd current
500      /Test/current> ls
501      README.txt <file>
502      LICENCE.txt <symlink>
503      subdir <dir>
504      .svn <dir>
505      FIFO <fifo>
506      chardev <character-device>
507      blockdev <block-device>
508      /Test/current> ls -ll LICENCE.txt
509      lrwxr-xr-x 1000 100 2009-01-15 03:02:49 LICENCE.txt -> subdir/LICENCE.txt
510      target: subdir/LICENCE.txt
511      ctime: 1231988569.0
512
513As well as exploring around, you can also extract files or directories
514(or entire snapshots) by using the `get` command. Ugarit will do its
515best to restore the metadata of files, subject to the rights of the
516user you run it as.
517
518Type `help` to get help in the interactive shell.
519
520## Duplicating tags
521
522As mentioned above, you can duplicate a tag, creating two tags that
523refer to the same snapshot and its history but that can then have
524their own subsequent history of snapshots applied to each
525independently, with the following command:
526
527      $ ugarit fork <ugarit.conf> <existing tag> <new tag>
528
529## `.ugarit` files
530
531By default, Ugarit will archive everything it finds in the filesystem
532tree you tell it to snapshot. However, this might not always be
533desired; so we provide the facility to override this with `.ugarit`
534files, or global rules in your `.conf` file.
535
536Note: The syntax of these files is provisional, as I want to
537experiment with usability, as the current syntax is ugly. So please
538don't be surprised if the format changes in incompatible ways in
539subsequent versions!
540
541In quick summary, if you want to ignore all files or directories
542matching a glob in the current directory and below, put the following
543in a `.ugarit` file in that directory:
544
545      (* (glob "*~") exclude)
546
547You can write quite complex expressions as well as just globs. The
548full set of rules is:
549
550* `(glob "`*pattern*`")` matches files and directories whose names
551  match the glob pattern
552
553* `(name "`*name*`")` matches files and directories with exactly that
554  name (useful for files called `*`...)
555
556* `(modified-within ` *number* ` seconds)` matches files and
557  directories modified within the given number of seconds
558
559* `(modified-within ` *number* ` minutes)` matches files and
560  directories modified within the given number of minutes
561
562* `(modified-within ` *number* ` hours)` matches files and directories
563  modified within the given number of hours
564
565* `(modified-within ` *number* ` days)` matches files and directories
566  modified within the given number of days
567
568* `(not ` *rule*`)` matches files and directories that do not match
569  the given rule
570
571* `(and ` *rule* *rule...*`)` matches files and directories that match
572  all the given rules
573
574* `(or ` *rule* *rule...*`)` matches files and directories that match
575  any of the given rules
576
577Also, you can override a previous exclusion with an explicit include
578in a lower-level directory:
579
580    (* (glob "*~") include)
581
582You can bind rules to specific directories, rather than to "this
583directory and all beneath it", by specifying an absolute or relative
584path instead of the `*`:
585
586    ("/etc" (name "passwd") exclude)
587
588If you use a relative path, it's taken relative to the directory of
589the `.ugarit` file.
590
591You can also put some rules in your `.conf` file, although relative
592paths are illegal there, by adding lines of this form to the file:
593
594    (rule * (glob "*~") exclude)
595
596# Questions and Answers
597
598## What happens if a snapshot is interrupted?
599
600Nothing! Whatever blocks have been uploaded will be uploaded, but the
601snapshot is only added to the tag once the entire filesystem has been
602snapshotted. So just start the snapshot again. Any files that have
603already be uploaded will then not need to be uploaded again, so the
604second snapshot should proceed quickly to the point where it failed
605before, and continue from there.
606
607Unless the archive ends up with a partially-uploaded corrupted block
608due to being interrupted during upload, you'll be fine. The filesystem
609backend has been written to avoid this by writing the block to a file
610with the wrong name, then renaming it to the correct name when it's
611entirely uploaded.
612
613## Should I share a single large archive between all my filesystems?
614
615I think so. Using a single large archive means that blocks shared
616between servers - eg, software installed from packages and that sort
617of thing - will only ever need to be uploaded once, saving storage
618space and upload bandwidth. However, do not share an archive between
619servers that do not mutually trust each other, as they can all update
620the same tags, so can meddle with each other's snapshots - and read
621each other's snapshots.
622
623# Security model
624
625I have designed and implemented Ugarit to be able to handle cases
626where the actual archive storage is not entirely trusted.
627
628However, security involves tradeoffs, and Ugarit is configurable in
629ways that affect its resistance to different kinds of attacks. Here I
630will list different kinds of attack and explain how Ugarit can deal
631with them, and how you need to configure it to gain that
632protection.
633
634## Archive snoopers
635
636This might be somebody who can intercept Ugarit's communication with
637the archive at any point, or who can read the archive itself at their
638leisure.
639
640### Reading your data
641
642If you enable encryption, then all the blocks sent to the archive are
643encrypted using a secret key stored in your Ugarit configuration
644file. As long as that configuration file is kept safe, and the AES
645algorithm is secure, then attackers who can snoop the archive cannot
646decode your data blocks. Enabling compression will also help, as the
647blocks are compressed before encrypting, which is thought to make
648cryptographic analysis harder.
649
650Recommendations: Use compression and encryption when there is a risk
651of archive snooping. Keep your Ugarit configuration file safe using
652UNIX file permissions (make it readable only by root), and maybe store
653it on a removable device that's only plugged in when
654required. Alternatively, use the "prompt" passphrase option, and be
655prompted for a passphrase every time you run Ugarit, so it isn't
656stored on disk anywhere.
657
658### Looking for known hashes
659
660A block is identified by the hash of its content (before compression
661and encryption). If an attacker was trying to find people who own a
662particular file (perhaps a piece of subversive literature), they could
663search Ugarit archives for its hash.
664
665However, Ugarit has the option to "key" the hash with a "salt" stored
666in the Ugarit configuration file. This means that the hashes used are
667actually a hash of the block's contents *and* the salt you supply. If
668you do this with a random salt that you keep secret, then attackers
669can't check your archive for known content just by comparing the hashes.
670
671Recommendations: Provide a secret string to your hash function in your
672Ugarit configuration file. Keep the Ugarit configuration file safe, as
673per the advice in the previous point.
674
675## Archive modifiers
676
677These folks can modify Ugarit's writes into the archive, its reads
678back from the archive, or can modify the archive itself at their leisure.
679
680Modifying an encrypted block without knowing the encryption key can at
681worst be a denial of service, corrupting the block in an unknown
682way. An attacker who knows the encryption key could replace a block
683with valid-seeming but incorrect content. In the worst case, this
684could exploit a bug in the decompression engine, causing a crash or
685even an exploit of the Ugarit process itself (thereby gaining the
686powers of a process inspector, as documented below). We can but hope
687that the decompression engine is robust. Exploits of the decryption
688engine, or other parts of Ugarit, are less likely due to the nature of
689the operations performed upon them.
690
691However, if a block is modified, then when Ugarit reads it back, the
692hash will no longer match the hash Ugarit requested, which will be
693detected and an error reported. The hash is checked after
694decryption and decompression, so this check does not protect us
695against exploits of the decompression engine.
696
697This protection is only afforded when the hash Ugarit asks for is not
698tampered with. Most hashes are obtained from within other blocks,
699which are therefore safe unless that block has been tampered with; the
700nature of the hash tree conveys the trust in the hashes up to the
701root. The root hashes are stored in the archive as "tags", which an
702archive modifier could alter at will. Therefore, the tags cannot be
703trusted if somebody might modify the archive. This is why Ugarit
704prints out the snapshot hash and the root directory hash after
705performing a snapshot, so you can record them securely outside of the
706archive.
707
708The most likely threat posed by archive modifiers is that they could
709simply corrupt or delete all of your archive, without needing to know
710any encryption keys.
711
712Recommendations: Secure your archives against modifiers, by whatever
713means possible. If archive modifiers are still a potential threat,
714write down a log of your root directory hashes from each snapshot, and keep
715it safe. When extracting your backups, use the `ls -ll` command in the
716interface to check the "contents" hash of your snapshots, and check
717they match the root directory hash you expect.
718
719## Process inspectors
720
721These folks can attach debuggers or similar tools to running
722processes, such as Ugarit itself.
723
724Ugarit backend processes only see encrypted data, so people who can
725attach to that process gain the powers of archive snoopers and
726modifiers, and the same conditions apply.
727
728People who can attach to the Ugarit process itself, however, will see
729the original unencrypted content of your filesystem, and will have
730full access to the encryption keys and hashing keys stored in your
731Ugarit configuration. When Ugarit is running with sufficient
732permissions to restore backups, they will be able to intercept and
733modify the data as it comes out, and probably gain total write access
734to your entire filesystem in the process.
735
736Recommendations: Ensure that Ugarit does not run under the same user
737ID as untrusted software. In many cases it will need to run as root in
738order to gain unfettered access to read the filesystems it is backing
739up, or to restore the ownership of files. However, when all the files
740it backs up are world-readable, it could run as an untrusted user for
741backups, and where file ownership is trivially reconstructible, it can
742do restores as a limited user, too.
743
744## Attackers in the source filesystem
745
746These folks create files that Ugarit will back up one day. By having
747write access to your filesystem, they already have some level of
748power, and standard Unix security practices such as storage quotas
749should be used to control them. They may be people with logins on your
750box, or more subtly, people who can cause servers to writes files;
751somebody who sends an email to your mailserver will probably cause
752that message to be written to queue files, as will people who can
753upload files via any means.
754
755Such attackers might use up your available storage by creating large
756files. This creates a problem in the actual filesystem, but that
757problem can be fixed by deleting the files. If those files get
758archived into Ugarit, then they are a part of that snapshot. If you
759are using a backend that supports deletion, then (when I implement
760snapshot deletion in the user interface) you could delete that entire
761snapshot to recover the wasted space, but that is a rather serious
762operation.
763
764More insidiously, such attackers might attempt to abuse a hash
765collision in order to fool the archive. If they have a way of creating
766a file that, for instance, has the same hash as your shadow password
767file, then Ugarit will think that it already has that file when it
768attempts to snapshot it, and store a reference to the existing
769file. If that snapshot is restored, then they will receive a copy of
770your shadow password file. Similarly, if they can predict a future
771hash of your shadow password file, and create a shadow password file
772of their own (perhaps one giving them a root account with a known
773password) with that hash, they can then wait for the real shadow
774password file to have that hash. If the system is later restored from
775that snapshot, then their chosen content will appear in the shadow
776password file. However, doing this requires a very fundamental break
777of the hash function being used.
778
779Recommendations: Think carefully about who has write access to your
780filesystems, directly or indirectly via a network service that stores
781received data to disk. Enforce quotas where appropriate, and consider
782not backing up "queue directories" where untrusted content might
783appear; migrate incoming content that passes acceptance tests to an
784area that is backed up. If necessary, the queue might be backed up to
785a non-snapshotting system, such as rsyncing to another server, so that
786any excessive files that appear in there are removed from the backup
787in due course, while still affording protection.
788
789# Future Directions
790
791Here's a list of planned developments, in approximate priority order:
792
793## General
794
795* More checks with `double-check` mode activated. Perhaps read blocks
796  back from the archive to check it matches the blocks sent, to detect
797  hash collisions. Maybe have levels of double-check-ness.
798
799* Everywhere I use (sql ...) to create an sqlite prepared statement,
800  don't. Create them all up-front and reuse the resulting statement
801  objects, it'll save memory and time. (done for backend-fs/splitlog
802  and backend/cache, file-cache still needs it).
803
804* Migrate the source repo to Fossil (when there's a
805  kitten-technologies.co.uk migration to Fossil), and update the egg
806  locations thingy.
807
808## Backends
809
810* Look at http://bugs.call-cc.org/ticket/492 - can this help?
811
812* Extend the backend protocol with a special "admin" command that
813  allows for arbitrary backend-specific operations, and write an
814  ugarit-backend-admin CLI tool to administer backends with it. The
815  input should be a single s-expression as a list, and the result
816  should be an alist which is displayed to the user in a friendly
817  manner, as "Key: Value\n" lines.
818
819* Extend the backend protocol with a `flush` command, such that
820  operations performed without a subsequent `flush` might not "stick" in
821  failure cases (make `close!` have an implicit `flush`, of
822  course). Use this to force an immediate commit in backends that use
823  sqlite, as well as the current practice of committing at close, tag
824  operations, and whenever it seems like a while has passed since the
825  last time. Make the frontend flush at crucial points.
826
827* Implement "info" admin commands for all backends, that list any
828  available stats, and at least the backend type and parameters.
829
830* Support for recreating the index and tags on a backend-splitlog if
831  they get corrupted, from the headers left in the log, as a "reindex"
832  admin command.
833
834* Support for flushing the cache on a backend-cache, via an admin
835  command.
836
837* Support for unlinking in backend-splitlog, by marking byte ranges as
838  unused in the metadata (and by touching the headers in the log so we
839  maintain the invariant that the metadata is a reconstructible cache)
840  and removing the entries for the unlinked blocks, perhaps provide an
841  option to attempt to re-use existing holes to put blocks in for
842  online reuse, and provide an offline compaction operation. Keep
843  stats in the index of how many byte ranges are unused, and how many
844  bytes unused, in each file, and report them in the info admin
845  interface, along with the option to compact any or all files.
846
847* Have read-only and unlinkable config flags in the backend-split
848  metadata file, settable via admin commands.
849
850* For people doing remote backups who want to not hog resources, write
851  a proxy backend that throttles bandwidth usage. Make it record the
852  time it last sent a request to the backend, and the number of bytes
853  read and written; then when a new request comes in, delay it until
854  at least the largest of (write bandwidth quota * bytes written) and
855  (read bandwidth quota * bytes read) seconds has passed since the
856  last request was sent. NOTE: Start the clock when SENDING, so the
857  time spent handling the request is already counting towards
858  bandwidth quotas, or it won't be fair.
859
860* Support for SFTP as a storage backend. Store one file per block, as
861  per `backend-fs`, but remotely. See
862  http://tools.ietf.org/html/draft-ietf-secsh-filexfer-13 for sftp
863  protocol specs; popen an `ssh -p sftp` connection to the server then
864  talk that simple binary protocol. Tada! Ideally make an sftp egg,
865  then a "ugarit-backend-sftp" egg to keep the dependencies optional.
866
867* Support for S3 as a storage backend. There is now an S3 egg! Make an
868  "ugarit-backend-s3" egg to keep the dependencies optional.
869
870* Support for replicated archives. This will involve a special storage
871  backend that can wrap any number of other archives, each tagged with
872  a trust percentage and read and write load weightings. Each block
873  will be uploaded to enough archives to make the total trust be at
874  least 100%, by randomly picking the archives weighted by their write
875  load weighting. A read-only archive automatically gets its write
876  load weighting set to zero, and a warning issued if it was
877  configured otherwise. A local cache will be kept of which backends
878  carry which blocks, and reads will be serviced by picking the
879  archive that carries it and has the highest read load weighting. If
880  that archive is unavailable or has lost the block, then they will be
881  tried in read load order; and if none of them have it, an exhaustive
882  search of all available archives will be performed before giving up,
883  and the cache updated with the results if the block is found. In
884  order to correctly handle archives that were unavailable during
885  this, we might need to log an "unknown" for that block key / archive
886  pair, rather than assuming the block is not there, and check it
887  later. Users
888  will be given an admin command to notify the backend of an archive
889  going missing forever, which will cause it to be removed from the
890  cache. Affected blocks should be examined and re-replicated if their
891  replication count is now too low. Another command should be
892  available to warn of impending deliberate removal, which will again
893  remove the archive from the cluster and re-replicate, the difference
894  being that the disappearing archive is usable for re-replicating
895  FROM, so this is a safe operation for blocks that are only on that
896  one archive. The individual physical archives
897  that we put replication on top of won't be "valid" archives unless
898  they are 100% replicated, as they'll contain references to blocks
899  that are on other archives. It might be a good idea to mark them as
900  such with a special tag to avoid people trying to restore directly
901  from them. A copy of the replication configuration could be stored
902  under a special tag to mark this fact, and to enable easy finding of
903  the proper replicated archive to work from. There should be a
904  configurable option to snapshot the cache to the archives whenever
905  the replicated archive is closed, too. The command line to the
906  backend, "backend-replicated", should point to an sqlite file for
907  the configuration and cache, and users should use admin commands to
908  add/remove/modify archives in the cluster.
909
910## Core
911
912* Log all WARNINGs produced during a snapshot job, and attach them to
913  the snapshot object as a text file.
914
915* Clarify what characters are legal in tag names sent to backends, and
916  what are legal in human-supplied tag names, and check that
917  human-supplied tag names match a regular expression. Leave space for
918  system-only tag names for storing archive metadata; suggest making a
919  hash sign illegal in tag names.
920
921* Clarify what characters are legal in block keys. Ugarit will only
922  issue hex characters for normal blocks, but may use other characters
923  for special metadata blocks; establish a contract of what backends
924  must support (a-z, A-Z, 0-9, hyphen?)
925
926* API documentation for the modules we export
927
928* Encrypt tags, with a hash inside to check it's decrypted
929  correctly. Add a special "#ugarit-archive-format" tag that records a
930  format version number, to note that this change has been
931  applied. Provide an upgrade tool. Don't do auto-upgrades, or
932  attackers will be able to drop in plaintext tags.
933
934* Store a test block in the archive that is used to check the same
935  encryption and hash settings are used for an archive, consistently
936  (changing compression setting is supported, but changing encryption
937  or hash will lead to confusion). Encrypt the hash of the passphrase
938  and store it in the test block, which should have a name that cannot
939  clash with any actual hash (eg, use non-hex characters in its
940  name). When the block does not exist, create it; when it does exist,
941  check it against the current encryption and hashing settings to see
942  if it matches. When creating a new block, if the "prompt" passphrase
943  specification mechanism is in use, prompt again to confirm the
944  passphrase. If no encryption is in use, check the hash algorithm
945  doesn't change by storing the hash of a constant string,
946  unencrypted. To make brute-forcing the passphrase or hash-salt
947  harder, consider applying the hash a large number of times, to
948  increase the compute cost of checking it. Thanks to Andy Bennett for
949  this idea.
950
951* More `.ugarit` actions. Right now we just have exclude and include;
952  we might specify less-safe operations such as commands to run before
953  and after snapshotting certain subtrees, or filters (don't send this
954  SVN repository; instead send the output of `svnadmin dump`),
955  etc. Running arbitrary commands is a security risk if random users
956  write their own `.ugarit` files - so we'd need some trust-based
957  mechanism; they'd need to be explicitly enabled in `ugarit.conf`,
958  then a `.ugarit` option could disable all unsafe operations in a
959  subtree.
960
961* `.ugarit` rules for file sizes. In particular, a rule to exclude
962  files above a certain size. Thanks to Andy Bennett for this idea.
963
964* Support for FFS flags, Mac OS X extended filesystem attributes, NTFS
965  ACLs/streams, FAT attributes, etc... Ben says to look at Box Backup
966  for some code to do that sort of thing.
967
968* Implement lock-tag! etc. in backend-fs, as a precaution against two
969  concurrent snapshots racing over updating the tag, where concurrent
970  access to the archive is even possible.
971
972* Deletion support - letting you remove snapshots. Perhaps you might
973  want to remove all snapshots older than a given number of days on a
974  given tag. Or just remove X out of Y snapshots older than a given
975  number of days on a given tag. We have the core support for this;
976  just find a snapshot and `unlink-directory!` it, leaving a dangling
977  pointer from the snapshot, and write the snapshot handling code to
978  expect this. Again, check Box Backup for that.
979
980* Some kind of accounting for storage usage by snapshot. It'd be nice
981  to track, as we write a snapshot to the archive, how many bytes we
982  reuse and how many we back up. We can then store this in the
983  snapshot metadata, and so report them somewhere. The blocks uploaded
984  by a snapshot may well then be reused by other snapshots later on,
985  so it wouldn't be a true measure of 'unique storage', nor a measure
986  of what you'd reclaim by deleting that snapshot, but it'd be
987  interesting anyway.
988
989* Option, when backing up, to not cross mountpoints
990
991* Option, when backing up, to store inode number and mountpoint path
992  in directory entries, and then when extracting, keeping a dictionary
993  of this unique identifier to pathname, so that if a file to be
994  extracted is already in the dictionary and the hash is the same, a
995  hardlink can be created.
996
997* Archival mode as well as snapshot mode. Whereas a snapshot record
998  takes a filesystem tree and adds it to a chain of snapshots of the
999  same filesystem tree, archival mode takes a filesystem tree and
1000  inserts it into a search tree anchored on the specified tag,
1001  indexing it on a list of key+value properties supplied at archival
1002  time. An archive tag is represented in the virtual filesystem as a
1003  directory full of archive objects, each identified by their full
1004  hash; each archive object references the filesystem root as well as
1005  the key+value properties, and optionally a parent link like a
1006  snapshot, as an archive can be made that explicitly replaces an
1007  earlier one and should replace it in the index; there is also a
1008  virtual directory for each indexed property which contains a
1009  directory for each value of the property, full of symlinks to the
1010  archive objects, and subdirectories that allow multi-property
1011  searches on other properties. The index itself is stored as a B-Tree
1012  with a reasonably small block size; when it's updated, the modified
1013  index blocks are replaced, thereby gaining new hashes, so their
1014  parents need replacing, all the way up the tree until a new root
1015  block is created. The existing block unlink mechanism in the
1016  backends will reclaim storage for blocks that are superceded, if the
1017  backend supports it. When this is done, ugarit will offer the option
1018  of snapshotting to a snapshot tag, or archiving to an archive tag,
1019  or archiving to an archive tag while replacing a specified archive
1020  object (nominated by path within the tag), which causes it to be
1021  removed from the index (except from the directory listing all
1022  archives by hash), and the new archive object is inserted,
1023  referencing the old one as a parent.
1024
1025* Dump/restore format. On a dump, walk an arbitrary subtree of an
1026  archive, serialising objects. Do not put any hashes in the dump
1027  format - dump out entire files, and just identify objects with
1028  sequential numbers when forming the directory / snapshot trees. On a
1029  restore, read the same format and slide it into an archive (creating
1030  any required top-level snapshot objects if the dump doesn't start
1031  from a snapshot) and putting it onto a specified tag. The
1032  intension is that this format can be used to migrate your stuff
1033  between archives, perhaps to change to a better backend.
1034
1035## Front-end
1036
1037* Better error messages
1038
1039* Line editing in the "explore" CLI, ideally with tab completion
1040
1041* API mode: Works something like the backend API, except at the
1042  archive level. Supports all the important archive operations, plus
1043  access to sexpr stream writers and key stream writers,
1044  archive-node-fold, etc. Requested by andyjpb, perhaps I can write
1045  the framework for this and then let him add API functions as he desires.
1046
1047* Command-line support to extract the contents of a given path in the
1048  archive, rather than needing to use explore mode. Also the option to
1049  extract given just a block key (useful when reading from keys logged
1050  manually at snapshot time, or from a backend that has a tag log).
1051
1052* FUSE/9p support. Mount it as a read-only filesystem :-D Then
1053  consider adding Fossil-style writing to the `current` of a snapshot,
1054  with copy-on-write of blocks to a buffer area on the local disk,
1055  then the option to make a snapshot of `current`. Put these into
1056  separate "ugarit-frontend-9p" and "ugarit-frontend-fuse" eggs, to
1057  control the dependencies.
1058
1059* Filesystem watching. Even with the hash-caching trick, a snapshot
1060  will still involve walking the entire directory tree and looking up
1061  every file in the hash cache. We can do better than that - some
1062  platforms provide an interface for receiving real-time notifications
1063  of changed or added files. Using this, we could allow ugarit to run
1064  in continuous mode, keeping a log of file notifications from the OS
1065  while it does an initial full snapshot. It can then wait for a
1066  specified period (one hour, perhaps?), accumulating names of files
1067  changed since it started, before then creating a new snapshot by
1068  uploading just the files it knows to have changed, while subsequent
1069  file change notifications go to a new list.
1070
1071## Testing
1072
1073* An option to verify a snapshot, walking every block in it checking
1074  there's no dangling references, and that everything matches its
1075  hash, without needing to put it into a filesystem, and applying any
1076  other sanity checks we can think of en route. Optionally compare it
1077  to an on-disk filesystem, while we're at it.
1078
1079* A unit test script around the `ugarit` command-line tool; the corpus
1080  should contain a mix of tiny and huge files and directories, awkward
1081  cases for sharing of blocks (many identical files in the same dir,
1082  etc), complex forms of file metadata, and so on. It should archive
1083  and restore the corpus several times over with each hash,
1084  compression, and encryption option.
1085
1086# Acknowledgements
1087
1088The original idea came from Venti, a content-addressed storage system
1089from Plan 9. Venti is usable directly by user applications, and is
1090also integrated with the Fossil filesystem to support snapshotting the
1091status of a Fossil filesystem. Fossil allows references to either be
1092to a block number on the Fossil partition or to a Venti key; so when a
1093filesystem has been snapshotted, all it now contains is a "root
1094directory" pointer into the Venti archive, and any files modified
1095therafter are copied-on-write into Fossil where they may be modified
1096until the next snapshot.
1097
1098We're nowhere near that exciting yet, but using FUSE, we might be able
1099to do something similar, which might be fun. However, Venti inspired
1100me when I read about it years ago; it showed me how elegant
1101content-addressed storage is. Finding out that the Git version control
1102system used the same basic tricks really just confirmed this for me.
1103
1104Also, I'd like to tip my hat to Duplicity. With the changing economics
1105of storage presented by services like Amazon S3 and rsync.net, I
1106looked to Duplicity as it provided both SFTP and S3 backends. However,
1107it worked in terms of full and incremental backups, a model that I
1108think made sense for magnetic tapes, but loses out to
1109content-addressed snapshots when you have random-access
1110media. Duplicity inspired me by its adoption of multiple backends, the
1111very backends I want to use, but I still hungered for a
1112content-addressed snapshot store.
1113
1114I'd also like to tip my hat to Box Backup. I've only used it a little,
1115because it requires a special server to manage the storage (and I want
1116to get my backups *off* of my servers), but it also inspires me with
1117directions I'd like to take Ugarit. It's much more aware of real-time
1118access to random-access storage than Duplicity, and has a very
1119interesting continuous background incremental backup mode, moving away
1120from the tape-based paradigm of backups as something you do on a
1121special day of the week, like some kind of religious observance. I
1122hope the author Ben, who is a good friend of mine, won't mind me
1123plundering his source code for details on how to request real-time
1124notification of changes from the filesystem, and how to read and write
1125extended attributes!
1126
1127Moving on from the world of backup, I'd like to thank the Chicken Team
1128for producing Chicken Scheme. Felix and the community at #chicken on
1129Freenode have particularly inspired me with their can-do attitudes to
1130combining programming-language elegance and pragmatic engineering -
1131two things many would think un-unitable enemies. Of course, they
1132didn't do it all themselves - R5RS Scheme and the SRFIs provided a
1133solid foundation to build on, and there's a cast of many more in the
1134Chicken community, working on other bits of Chicken or just egging
1135everyone on. And I can't not thank Henry Baker for writing the seminal
1136paper on the technique Chicken uses to implement full tail-calling
1137Scheme with cheap continuations on top of C; Henry already had my
1138admiration for his work on combining elegance and pragmatism in linear
1139logic. Why doesn't he return my calls? I even sent flowers.
1140
1141A special thanks should go to Christian Kellermann for porting Ugarit
1142to use Chicken 4 modules, too, which was otherwise a big bottleneck to
1143development, as I was stuck on Chicken 3 for some time! And to Andy
1144Bennett for many insightful conversations about future directions.
1145
1146Thanks to the early adopters who brought me useful feedback, too!
1147
1148And I'd like to thank my wife for putting up with me spending several
1149evenings and weekends and holiday days working on this thing...
1150
1151# Version history
1152
1153* 1.0.2: Made the file cache also commit periodically, rather than on
1154  every write, in order to improve performance.
1155
1156* 1.0.1: Consistency check on read blocks by default. Removed warning
1157  about deletions from backend-cache; we need a new mechanism to
1158  report warnings from backends to the user. Made backend-cache and
1159  backend-fs/splitlog commit periodically rather than after every
1160  insert, which should speed up snapshotting a lot, and reused the
1161  prepared statements rather than re-preparing them all the
1162  time. BUGFIX: splitlog backend now creates log files with
1163  "rw-------" rather than "rwx------" permissions; and all sqlite
1164  databases (splitlog metadata, cache file, and file-cache file) are
1165  created with "rw-------" rather then "rw-r--r--".
1166
1167* 1.0: Migrated from gdbm to sqlite for metadata storage, removing the
1168  GPL taint. Unit test suite. backend-cache made into a separate
1169  backend binary. Removed backend-log. BUGFIX: file caching uses mtime *and*
1170  size now, rather than just mtime. Error handling so we skip objects
1171  that we cannot do something with, and proceed to try the rest of the
1172  operation.
1173
1174* 0.8: decoupling backends from the core and into separate binaries,
1175  accessed via standard input and output, so they can be run over SSH
1176  tunnels and other such magic.
1177
1178* 0.7: file cache support, sorting of directories so they're archived
1179  in canonical order, autoloading of hash/encryption/compression
1180  modules so they're not required dependencies any more.
1181
1182* 0.6: .ugarit support.
1183
1184* 0.5: Keyed hashing so attackers can't tell what blocks you have,
1185  markers in logs so the index can be reconstructed, sha2 support, and
1186  passphrase support.
1187
1188* 0.4: AES encryption.
1189
1190* 0.3: Added splitlog backend, and fixed a .meta file typo.
1191
1192* 0.2: Initial public release.
1193
1194* 0.1: Internal development release.
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