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74 lines
4.2 KiB
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74 lines
4.2 KiB
ReStructuredText
.. include:: serve.rst.inc
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Examples
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~~~~~~~~
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``borg serve`` has special support for ssh forced commands (see ``authorized_keys``
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example below): if the environment variable SSH_ORIGINAL_COMMAND is set it will
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ignore some options given on the command line and use the values from the
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variable instead. This only applies to a carefully controlled allowlist of safe
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options. This list currently contains:
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- Options that control the log level and debug topics printed
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such as ``--verbose``, ``--info``, ``--debug``, ``--debug-topic``, etc.
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- ``--lock-wait`` to allow the client to control how long to wait before
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giving up and aborting the operation when another process is holding a lock.
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Environment variables (such as BORG_XXX) contained in the original
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command sent by the client are *not* interpreted, but ignored. If BORG_XXX environment
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variables should be set on the ``borg serve`` side, then these must be set in system-specific
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locations like ``/etc/environment`` or in the forced command itself (example below).
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::
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# Allow an SSH keypair to run only borg, and only have access to /path/to/repo.
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# Use key options to disable unneeded and potentially dangerous SSH functionality.
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# This will help to secure an automated remote backup system.
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$ cat ~/.ssh/authorized_keys
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command="borg serve --restrict-to-path /path/to/repo",restrict ssh-rsa AAAAB3[...]
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# Set a BORG_XXX environment variable on the "borg serve" side
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$ cat ~/.ssh/authorized_keys
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command="export BORG_XXX=value; borg serve [...]",restrict ssh-rsa [...]
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.. note::
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The examples above use the ``restrict`` directive. This does automatically
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block potential dangerous ssh features, even when they are added in a future
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update. Thus, this option should be preferred.
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If you're using openssh-server < 7.2, however, you have to specify explicitly
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the ssh features to restrict and cannot simply use the restrict option as it
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has been introduced in v7.2. We recommend to use
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``no-port-forwarding,no-X11-forwarding,no-pty,no-agent-forwarding,no-user-rc``
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in this case.
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Details about sshd usage: `sshd(8) <https://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man8/sshd.8>`_
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SSH Configuration
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~~~~~~~~~~~~~~~~~
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``borg serve``'s pipes (``stdin``/``stdout``/``stderr``) are connected to the ``sshd`` process on the server side. In the event that the SSH connection between ``borg serve`` and the client is disconnected or stuck abnormally (for example, due to a network outage), it can take a long time for ``sshd`` to notice the client is disconnected. In the meantime, ``sshd`` continues running, and as a result so does the ``borg serve`` process holding the lock on the repository. This can cause subsequent ``borg`` operations on the remote repository to fail with the error: ``Failed to create/acquire the lock``.
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In order to avoid this, it is recommended to perform the following additional SSH configuration:
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Either in the client side's ``~/.ssh/config`` file, or in the client's ``/etc/ssh/ssh_config`` file:
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::
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Host backupserver
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ServerAliveInterval 10
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ServerAliveCountMax 30
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Replacing ``backupserver`` with the hostname, FQDN or IP address of the borg server.
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This will cause the client to send a keepalive to the server every 10 seconds. If 30 consecutive keepalives are sent without a response (a time of 300 seconds), the ssh client process will be terminated, causing the borg process to terminate gracefully.
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On the server side's ``sshd`` configuration file (typically ``/etc/ssh/sshd_config``):
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::
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ClientAliveInterval 10
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ClientAliveCountMax 30
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This will cause the server to send a keep alive to the client every 10 seconds. If 30 consecutive keepalives are sent without a response (a time of 300 seconds), the server's sshd process will be terminated, causing the ``borg serve`` process to terminate gracefully and release the lock on the repository.
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If you then run borg commands with ``--lock-wait 600``, this gives sufficient time for the borg serve processes to terminate after the SSH connection is torn down after the 300 second wait for the keepalives to fail.
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You may, of course, modify the timeout values demonstrated above to values that suit your environment and use case.
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