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Add "SSH Configuration" section (#4493)

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Add "SSH Configuration" section

Add "SSH Configuration" section to "borg serve" documentation, to outline ssh/sshd configuration to prevent borg serve keeping a lock on a repo in the event the ssh connection is abnormally disconnected.

In response to issues #3988, #636 and #4485 (and probably others).
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Mike 2019-04-09 03:10:26 +08:00 • committed by TW
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docs/usage/serve.rst
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@ -39,3 +39,32 @@ locations like ``/etc/environment`` or in the forced command itself (example bel
has been introduced in v7.2. We recommend to use
``no-port-forwarding,no-X11-forwarding,no-pty,no-agent-forwarding,no-user-rc``
in this case.
SSH Configuration
~~~~~~~~~~~~~~~~~
``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``.
In order to avoid this, it is recommended to perform the following additional SSH configuration:
Either in the client side's ``~/.ssh/config`` file, or in the client's ``/etc/ssh/ssh_config`` file:
::
Host backupserver
ServerAliveInterval 10
ServerAliveCountMax 30
Replacing ``backupserver`` with the hostname, FQDN or IP address of the borg server.
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.
On the server side's ``sshd`` configuration file (typically ``/etc/ssh/sshd_config``):
::
ClientAliveInterval 10
ClientAliveCountMax 30
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.
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.
You may, of course, modify the timeout values demonstrated above to values that suit your environment and use case.