NAME

borg-serve - Start in server mode. This command is usually not used manually.

SYNOPSIS

borg [common options] serve [options]

DESCRIPTION

This command starts a repository server process. This command is usually not used manually.

OPTIONS

See borg-common(1) for common options of Borg commands.

options

--restrict-to-path PATH: restrict repository access to PATH. Can be specified multiple times to allow the client access to several directories. Access to all sub-directories is granted implicitly; PATH doesn't need to directly point to a repository.
--restrict-to-repository PATH: restrict repository access. Only the repository located at PATH (no sub-directories are considered) is accessible. Can be specified multiple times to allow the client access to several repositories. Unlike --restrict-to-path sub-directories are not accessible; PATH needs to directly point at a repository location. PATH may be an empty directory or the last element of PATH may not exist, in which case the client may initialize a repository there.
--append-only: only allow appending to repository segment files. Note that this only affects the low level structure of the repository, and running delete or prune will still be allowed. See append_only_mode in Additional Notes for more details.
--storage-quota QUOTA: Override storage quota of the repository (e.g. 5G, 1.5T). When a new repository is initialized, sets the storage quota on the new repository as well. Default: no quota.

EXAMPLES

borg serve has special support for SSH forced commands (see authorized_keys example below): if the environment variable SSH_ORIGINAL_COMMAND is set it will ignore some options given on the command line and use the values from the variable instead. This only applies to a carefully controlled allowlist of safe options. This list currently contains:

Options that control the log level and debug topics printed such as --verbose, --info, --debug, --debug-topic, etc.
--lock-wait to allow the client to control how long to wait before giving up and aborting the operation when another process is holding a lock.

Environment variables (such as BORG_XXX) contained in the original command sent by the client are not interpreted; they are ignored. If BORG_XXX environment variables need to 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 key pair to only run borg, and only have access to /path/to/repo.
# Use key options to disable unneeded and potentially dangerous SSH functionality.
# This helps 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="BORG_XXX=value borg serve [...]",restrict ssh-rsa [...]

NOTE:

The examples above use the restrict directive and assume a POSIX-compliant shell set as the user's login shell. This automatically blocks potentially dangerous SSH features, even when they are added in a future update. Thus, this option should be preferred.

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 was introduced in v7.2. We recommend using no-port-forwarding,no-X11-forwarding,no-pty,no-agent-forwarding,no-user-rc in this case.

Details about sshd usage: sshd(8) <https://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man8/sshd.8>

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

Replace 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 keepalive 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 following 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.

When the client is untrusted, it is a good idea to set the backup user's shell to a simple implementation (/bin/sh is only an example and may or may not be such a simple implementation):

chsh -s /bin/sh BORGUSER

Because the configured shell is used by openssh <https://www.openssh.com/>

to execute the command configured through the authorized_keys file using "$SHELL" -c "$COMMAND", setting a minimal shell implementation reduces the attack surface compared to when a feature-rich and complex shell implementation is used.

AUTHOR

The Borg Collective