| BORG-SERVE(1) | borg backup tool | BORG-SERVE(1) |
borg-serve - Start in server mode. This command is usually not used manually.
borg [common options] serve [options]
This command starts a repository server process. This command is usually not used manually.
See borg-common(1) for common options of Borg commands.
borg serve has special support for ssh forced commands (see authorized_keys example below): it will detect that you use such a forced command and extract the value of the --restrict-to-path option(s).
It will then parse the original command that came from the client, makes sure that it is also borg serve and enforce path restriction(s) as given by the forced command. That way, other options given by the client (like --info or --umask) are preserved (and are not fixed by the forced command).
Environment variables (such as BORG_HOSTNAME_IS_UNIQUE) contained in the original command sent by the client are not interpreted, but ignored. If BORG_XXX environment variables should be set on the borg serve side, then these must be set in system-specific locations like /etc/environment or in the forced command itself (example below).
# Allow an SSH keypair to only run borg, and only have access to /path/to/repo. # Use key options to disable unneeded and potentially dangerous SSH functionality. # This will help to secure an automated remote backup system. $ cat ~/.ssh/authorized_keys command="borg serve --restrict-to-path /path/to/repo",restrict ssh-rsa AAAAB3[...] # Set a BORG_XXX environment variable on the "borg serve" side $ cat ~/.ssh/authorized_keys command="export BORG_XXX=value; borg serve [...]",restrict ssh-rsa [...]
NOTE:
If you're using openssh-server < 7.2, however, you have to explicitly specify the ssh features to restrict and cannot simply use the restrict option as it 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.
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.
The Borg Collective
| 2021-03-22 |