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@@ -48,6 +48,7 @@ Details about sshd usage: `sshd(8) <https://www.openbsd.org/cgi-bin/man.cgi/Open
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SSH Configuration
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~~~~~~~~~~~~~~~~~
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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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@@ -74,3 +75,16 @@ This will cause the server to send a keep alive to the client every 10 seconds.
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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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+
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+When the client is untrusted, it is a good idea to set the backup
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+user's shell to a simple implementation (``/bin/sh`` is only an example and may or may
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+not be such a simple implementation)::
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+
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+ chsh -s /bin/sh BORGUSER
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+
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+Because the configured shell is used by `openssh <https://www.openssh.com/>`_
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+to execute the command configured through the ``authorized_keys`` file
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+using ``"$SHELL" -c "$COMMAND"``,
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+setting a minimal shell implementation reduces the attack surface
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+compared to when a feature-rich and complex shell implementation is
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+used.
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Max-Julian Pogner