diff --git a/cmd/restic/cmd_forget.go b/cmd/restic/cmd_forget.go index 01848745c..c814472f7 100644 --- a/cmd/restic/cmd_forget.go +++ b/cmd/restic/cmd_forget.go @@ -16,10 +16,9 @@ var cmdForget = &cobra.Command{ Long: ` The "forget" command removes snapshots according to a policy. Please note that this command really only deletes the snapshot object in the repository, which -is a reference to data stored there. In order to remove this (now unreferenced) -data after 'forget' was run successfully, see the 'prune' command. -When using this command on special append-only repositories, please see the -documentation for security considerations. +is a reference to data stored there. In order to remove the unreferenced data +after "forget" was run successfully, see the "prune" command. Please also read +the documentation for "forget" to learn about important security considerations. EXIT STATUS =========== diff --git a/doc/030_preparing_a_new_repo.rst b/doc/030_preparing_a_new_repo.rst index 47bd7439d..d6fb830c6 100644 --- a/doc/030_preparing_a_new_repo.rst +++ b/doc/030_preparing_a_new_repo.rst @@ -657,9 +657,8 @@ credentials) is encrypted/decrypted locally, then sent/received via A more advanced version of this setup forbids specific hosts from removing files in a repository. See the `blog post by Simon Ruderich `_ -for details. When using ``restic forget`` on such a repository, additional -security considerations apply: please review the documentation on removing -backup snapshots. +for details and the documentation for the ``forget`` command to learn about +important security considerations. The rclone command may also be hard-coded in the SSH configuration or the user's public key, in this case it may be sufficient to just start the SSH diff --git a/doc/060_forget.rst b/doc/060_forget.rst index afa4f2c80..5bf87f024 100644 --- a/doc/060_forget.rst +++ b/doc/060_forget.rst @@ -14,14 +14,13 @@ Removing backup snapshots ######################### -All backup space is finite, so restic allows removing old snapshots. -This can be done either manually (by specifying a snapshot ID to remove) -or by using a policy that describes which snapshots to forget. For all -remove operations, two commands need to be called in sequence: -``forget`` to remove a snapshot and ``prune`` to remove the remaining -data that was only referenced by this snapshot from the repository. This can -be automated with the ``--prune`` option of the ``forget`` command, -which runs ``prune`` automatically if snapshots have been removed. +All backup space is finite, so restic allows removing old snapshots. This can +be done either manually (by specifying a snapshot ID to remove) or by using a +policy that describes which snapshots to forget. For all remove operations, two +commands need to be called in sequence: ``forget`` to remove snapshots, and +``prune`` to remove the remaining data that was referenced only by the removed +snapshots. The latter can be automated with the ``--prune`` option of ``forget``, +which runs ``prune`` automatically if any snapshots were actually removed. Pruning snapshots can be a time-consuming process, depending on the number of snapshots and data to process. During a prune operation, the @@ -164,8 +163,8 @@ The most important command-line parameter here is ``--dry-run`` which instructs restic to not remove anything but print which snapshots would be removed. -.. note:: If you use append-only backups, some best practices apply. - Please refer to the security considerations below. +.. note:: If you use an append-only repository, some best practices apply. + Please refer to the security considerations below for more information. When ``forget`` is run with a policy, restic loads the list of all snapshots, then groups these by host name and list of directories. The grouping @@ -175,27 +174,27 @@ snapshots separately. This is a safety feature. The ``forget`` command accepts the following parameters: -- ``--keep-last n`` never delete the ``n`` last (most recent) snapshots -- ``--keep-hourly n`` for the last ``n`` hours in which a snapshot was - made, keep only the last snapshot for each hour. +- ``--keep-last n`` keep the ``n`` last (most recent) snapshots. +- ``--keep-hourly n`` for the last ``n`` hours which have one or more + snapshots, keep only the most recent one for each hour. - ``--keep-daily n`` for the last ``n`` days which have one or more - snapshots, only keep the last one for that day. + snapshots, keep only the most recent one for that day. - ``--keep-weekly n`` for the last ``n`` weeks which have one or more - snapshots, only keep the last one for that week. + snapshots, keep only the most recent one for that week. - ``--keep-monthly n`` for the last ``n`` months which have one or more - snapshots, only keep the last one for that month. + snapshots, keep only the most recent one for that month. - ``--keep-yearly n`` for the last ``n`` years which have one or more - snapshots, only keep the last one for that year. + snapshots, keep only the most recent one for that year. - ``--keep-tag`` keep all snapshots which have all tags specified by this option (can be specified multiple times). -- ``--keep-within duration`` keep all snapshots which have been made within - the duration of the latest snapshot. ``duration`` needs to be a number of - years, months, days, and hours, e.g. ``2y5m7d3h`` will keep all snapshots - made in the two years, five months, seven days, and three hours before the - latest snapshot. +- ``--keep-within duration`` keep all snapshots having a timestamp within + the specified duration of the latest snapshot, where ``duration`` is a + number of years, months, days, and hours. E.g. ``2y5m7d3h`` will keep all + snapshots made in the two years, five months, seven days, and three hours + before the latest (most recent) snapshot. - ``--keep-within-hourly duration`` keep all hourly snapshots made within - specified duration of the latest snapshot. The duration is specified in - the same way as for ``--keep-within`` and the method for determining + the specified duration of the latest snapshot. The ``duration`` is specified + in the same way as for ``--keep-within`` and the method for determining hourly snapshots is the same as for ``--keep-hourly``. - ``--keep-within-daily duration`` keep all daily snapshots made within specified duration of the latest snapshot. @@ -206,11 +205,13 @@ The ``forget`` command accepts the following parameters: - ``--keep-within-yearly duration`` keep all yearly snapshots made within specified duration of the latest snapshot. -.. note:: All calendar related ``--keep-*`` options work on the natural time +.. note:: All calendar related options (``--keep-*``) work on the natural time boundaries and not relative to when you run the ``forget`` command. Weeks - are Monday 00:00 -> Sunday 23:59, days 00:00 to 23:59, hours :00 to :59, etc. - Snapshots seemingly made in the future (relative to when you run the - ``forget`` command) will be ignored and never removed. + are Monday 00:00 to Sunday 23:59, days 00:00 to 23:59, hours :00 to :59, etc. + +.. note:: All duration related options (``--keep-within`` and ``--keep-within-*``) + ignore snapshots with a timestamp in the future (relative to when the + ``forget`` command is run) and these snapshots will hence not be removed. .. note:: Specifying ``--keep-tag ''`` will match untagged snapshots only. @@ -317,12 +318,12 @@ four Sundays, but remove the rest: --------------------------------------------------------------- 8 snapshots -The result of the ``forget --keep-daily`` operation only partially depends on when it -is run: it will only count the days for which a snapshot exists, although -with a `time` lying in the future are ignored and never removed. This is a -safety feature: it prevents restic from removing snapshots when no new ones are -created. Otherwise, running ``forget --keep-daily 4`` on a Friday (without any -snapshot Monday to Thursday) would remove all snapshots! +The result of the ``forget --keep-daily`` operation only partially depends on +when it is run; it will only count the days for which a snapshot exists, +although snapshots with a `time` lying in the future are ignored and never +removed. This is a safety feature: it prevents restic from removing snapshots +when no new ones are created. Otherwise, running ``forget --keep-daily 4`` on +a Friday (without any snapshot Monday to Thursday) would remove all snapshots! Another example: Suppose you make daily backups for 100 years. Then ``forget --keep-daily 7 --keep-weekly 5 --keep-monthly 12 --keep-yearly 75`` @@ -345,34 +346,46 @@ could specify: Security considerations in append-only mode =========================================== -To prevent data from being deleted by a compromised backup client (for example -due to a ransomware infection), a so-called append-only mode can be used. This -requires the server to deny delete and overwrite operations, which is not -possible on many standard back-ends. Software such as `rest-server`_ or -`rclone`_ can be used instead or in addition. +To prevent a compromised backup client from deleting its backups (for example +due to a ransomware infection), a repository service/backend can serve the +repository in a so-called append-only mode. This means that the repository can +only be written to and read from, while delete and overwrite operations are +denied. Restic's `rest-server`_ features the append-only mode, but few other +standard backends do. To support append-only with such a backend, one can use +`rclone`_ as a complement in between the backup client and the backend service. .. _rest-server: https://github.com/restic/rest-server/ -.. _rclone: https://rclone.org/ +.. _rclone: https://rclone.org/commands/rclone_serve_restic/ -To recover disk space from obsolete snapshots, ``forget`` and ``prune`` must be -run on a repository with full read-write access. If an attacker can do this, -the protection offered by append-only mode is void. However, even if only the -trusted client runs the ``forget`` command, it can be possible for the attacker -to add snapshots in such a pattern that all legitimate snapshots are removed. +To remove snapshots and recover the corresponding disk space, the ``forget`` +and ``prune`` commands must have full read, write and delete access to the +repository. If an attacker has this, the protection offered by append-only +mode is naturally void. -If the ``forget`` policy is to keep three weekly snapshots, the attacker can -add an empty backup for each of the last three weeks with a `time` slightly -newer than the existing snapshots (but still within the target week). The next -time the repository administrator (or cron job) runs the ``forget`` policy, the -legitimate snapshots will be removed. Even without pruning, recovering data -would be messy and some metadata will be lost. +However, even with append-only mode active, an attacker who is able to add +additional and empty or otherwise useless snapshots to the repository can +potentially cause a situation where a trusted client running ``forget`` with +certain ``--keep-*`` options might unknowingly remove legitimate snapshots, +leaving only the attackers useless snapshots. -To avoid this, policies applied to append-only repositories should use the -``--keep-within`` option. If the system time is set correctly when ``forget`` -runs, this will allow you to notice problems with the backup or the compromised -host. This is, of course, limited to the specified duration: if -``restic forget --keep-within 7d`` is run 8 days after the last good snapshot, -then the attacker can still use that opportunity to remove all good snapshots. +For example, if the ``forget`` policy is to keep three weekly snapshots, and +the attacker adds an empty snapshot for each of the last three weeks, all with +a timestamp (see the ``backup`` command's ``-`time`` option) slightly more +recent than the existing snapshots (but still within the target week), then the +next time the repository administrator (or scheduled job) runs the ``forget`` +command with this policy, the legitimate snapshots will be removed (since the +policy will use the most recent snapshot within each week). Even without +running ``prune``, recovering data would be messy and some metadata lost. + +To avoid this, ``forget`` policies applied to append-only repositories should +use the ``--keep-within`` option, as this will keep not only the attacker's +snapshots but also the legitimate ones. Assuming the system time is correctly +set when ``forget`` runs, this will allow the administrator to notice problems +with the backup or the compromised host (e.g. by seeing more snapshots than +usual or snapshots with suspicious timestamps). This is, of course, limited to +the specified duration: if ``forget --keep-within 7d`` is run 8 days after the +last good snapshot, then the attacker can still use that opportunity to remove +all legitimate snapshots. Customize pruning ***************** diff --git a/doc/conf.py b/doc/conf.py index 97c728ac6..3c0af927b 100644 --- a/doc/conf.py +++ b/doc/conf.py @@ -35,7 +35,7 @@ master_doc = 'index' # General information about the project. project = 'restic' -copyright = '2021, restic authors' +copyright = '2018, restic authors' author = 'fd0' # The version info for the project you're documenting, acts as replacement for diff --git a/doc/design.rst b/doc/design.rst index 196ddaa66..e6a751be2 100644 --- a/doc/design.rst +++ b/doc/design.rst @@ -607,7 +607,7 @@ examples of things an adversary could achieve in various circumstances. An adversary with read access to your backup storage location could: - Attempt a brute force password guessing attack against a copy of the - repository (use strong passwords with sufficient entropy). + repository (please use strong passwords with sufficient entropy). - Infer which packs probably contain trees via file access patterns. - Infer the size of backups by using creation timestamps of repository objects. @@ -618,7 +618,7 @@ An adversary with network access could: - Determine from where you create your backups (i.e., the location where the requests originate). - Determine where you store your backups (i.e., which provider/target system). -- Infer the size of backups by using creation timestamps of repository objects. +- Infer the size of backups by observing network traffic. The following are examples of the implications associated with violating some of the aforementioned assumptions. @@ -629,11 +629,11 @@ system making backups could: - Render the entire backup process untrustworthy (e.g., intercept password, copy files, manipulate data). - Create snapshots (containing garbage data) which cover all modified files - and wait until a trusted host has used forget often enough to forget all + and wait until a trusted host has used ``forget`` often enough to remove all correct snapshots. -- Create a garbage snapshot for every existing snapshot with a slightly different - timestamp and wait until forget has run, thereby removing all correct - snapshots at once. +- Create a garbage snapshot for every existing snapshot with a slightly + different timestamp and wait until certain ``forget`` configurations has been + run, thereby removing all correct snapshots at once. An adversary with write access to your files at the storage location could: @@ -645,29 +645,26 @@ An adversary with write access to your files at the storage location could: the snapshot cannot be restored completely. Restic is not designed to detect this attack. -An adversary who compromises a host system with append-only access to the -backup repository could: +An adversary who compromises a host system with append-only (read+write allowed, +delete+overwrite denied) access to the backup repository could: -- Capture the password and decrypt backups from the past and in the future. - See the "leaked key" circumstance below. +- Capture the password and decrypt backups from the past and in the future + (see the "leaked key" example below for related information). - Render new backups untrustworthy *after* the host has been compromised (due to having complete control over new backups). An attacker cannot delete or manipulate old backups. As such, restoring old snapshots created *before* a host compromise remains possible. -- Potentially manipulate the ``restic forget`` command into deleting all +- Potentially manipulate the use of the ``forget`` command into deleting all legitimate snapshots, keeping only bogus snapshots added by the attacker. Ransomware might try this in order to leave only one option to get your data - back: paying the ransom. For safe use of ``restic forget``, see the - documentation on removing backup snapshots. + back: paying the ransom. For safe use of ``forget``, please see the + corresponding documentation on removing backup snapshots and append-only mode. -An adversary who has a leaked key for a repository could: - -- Decrypt existing and future backup data. If multiple hosts backup into the same - repository, an attacker will get access to the backup data of every host. - Since the local encryption key gives access to the master key, a password - change will not prevent this. Changing the master key can currently be done - using ``restic copy`` which moves the data into a new repository with a new - master key, or by making a completely new repository and new backup. - Re-encrypting all data without creating a new repository is tracked in - :issue:`1602`. +An adversary who has a leaked (decrypted) key for a repository could: +- Decrypt existing and future backup data. If multiple hosts backup into the + same repository, an attacker will get access to the backup data of every host. + Note that since the local encryption key gives access to the master key, a + password change will not prevent this. Changing the master key can currently + be done using the ``copy`` command, which moves the data into a new repository + with a new master key, or by making a completely new repository and new backup.