borg/docs/man/borg-serve.1

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.TH "BORG-SERVE" "1" "2025-04-28" "" "borg backup tool"
.SH NAME
borg-serve \- Start in server mode. This command is usually not used manually.
.SH SYNOPSIS
.sp
borg [common options] serve [options]
.SH DESCRIPTION
.sp
This command starts a repository server process.
.sp
borg serve can currently support:
.INDENT 0.0
.IP \(bu 2
Getting automatically started via ssh when the borg client uses a  <ssh://> \&...
remote repository. In this mode, \fIborg serve\fP will live until that ssh connection
gets terminated.
.IP \(bu 2
Getting started by some other means (not by the borg client) as a long\-running socket
server to be used for borg clients using a socket://... repository (see the \fI\-\-socket\fP
option if you do not want to use the default path for the socket and pid file).
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.SH OPTIONS
.sp
See \fIborg\-common(1)\fP for common options of Borg commands.
.SS options
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.TP
.BI \-\-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\(aqt need to point directly to a repository.
.TP
.BI \-\-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 \fB\-\-restrict\-to\-path\fP sub\-directories are not accessible; PATH needs to point directly 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.
.UNINDENT
.SH EXAMPLES
.sp
\fBborg serve\fP has special support for ssh forced commands (see \fBauthorized_keys\fP
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:
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.IP \(bu 2
Options that control the log level and debug topics printed
such as \fB\-\-verbose\fP, \fB\-\-info\fP, \fB\-\-debug\fP, \fB\-\-debug\-topic\fP, etc.
.IP \(bu 2
\fB\-\-lock\-wait\fP to allow the client to control how long to wait before
giving up and aborting the operation when another process is holding a lock.
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.sp
Environment variables (such as BORG_XXX) contained in the original
command sent by the client are \fInot\fP interpreted, but ignored. If BORG_XXX environment
variables should be set on the \fBborg serve\fP side, then these must be set in system\-specific
locations like \fB/etc/environment\fP or in the forced command itself (example below).
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.sp
.EX
# Allow an SSH keypair to run only 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=\(dqborg serve \-\-restrict\-to\-path /path/to/repo\(dq,restrict ssh\-rsa AAAAB3[...]

# Set a BORG_XXX environment variable on the \(dqborg serve\(dq side
$ cat ~/.ssh/authorized_keys
command=\(dqexport BORG_XXX=value; borg serve [...]\(dq,restrict ssh\-rsa [...]
.EE
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.sp
\fBNOTE:\fP
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The examples above use the \fBrestrict\fP directive. This does automatically
block potential dangerous ssh features, even when they are added in a future
update. Thus, this option should be preferred.
.sp
If you\(aqre using openssh\-server < 7.2, however, you have to specify explicitly
the ssh features to restrict and cannot simply use the restrict option as it
has been introduced in v7.2. We recommend to use
\fBno\-port\-forwarding,no\-X11\-forwarding,no\-pty,no\-agent\-forwarding,no\-user\-rc\fP
in this case.
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.sp
Details about sshd usage: sshd(8) <https://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man8/sshd.8>

.SS SSH Configuration
.sp
\fBborg serve\fP\(aqs pipes (\fBstdin\fP/\fBstdout\fP/\fBstderr\fP) are connected to the \fBsshd\fP process on the server side. In the event that the SSH connection between \fBborg serve\fP and the client is disconnected or stuck abnormally (for example, due to a network outage), it can take a long time for \fBsshd\fP to notice the client is disconnected. In the meantime, \fBsshd\fP continues running, and as a result so does the \fBborg serve\fP process holding the lock on the repository. This can cause subsequent \fBborg\fP operations on the remote repository to fail with the error: \fBFailed to create/acquire the lock\fP\&.
.sp
In order to avoid this, it is recommended to perform the following additional SSH configuration:
.sp
Either in the client side\(aqs \fB~/.ssh/config\fP file, or in the client\(aqs \fB/etc/ssh/ssh_config\fP file:
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.sp
.EX
Host backupserver
        ServerAliveInterval 10
        ServerAliveCountMax 30
.EE
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.sp
Replacing \fBbackupserver\fP with the hostname, FQDN or IP address of the borg server.
.sp
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.
.sp
On the server side\(aqs \fBsshd\fP configuration file (typically \fB/etc/ssh/sshd_config\fP):
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.sp
.EX
ClientAliveInterval 10
ClientAliveCountMax 30
.EE
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.sp
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\(aqs sshd process will be terminated, causing the \fBborg serve\fP process to terminate gracefully and release the lock on the repository.
.sp
If you then run borg commands with \fB\-\-lock\-wait 600\fP, 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.
.sp
You may, of course, modify the timeout values demonstrated above to values that suit your environment and use case.
.sp
When the client is untrusted, it is a good idea to set the backup
user\(aqs shell to a simple implementation (\fB/bin/sh\fP is only an example and may or may
not be such a simple implementation):
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.sp
.EX
chsh \-s /bin/sh BORGUSER
.EE
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.sp
Because the configured shell is used by openssh <https://www.openssh.com/>

to execute the command configured through the \fBauthorized_keys\fP file
using \fB\(dq$SHELL\(dq \-c \(dq$COMMAND\(dq\fP,
setting a minimal shell implementation reduces the attack surface
compared to when a feature\-rich and complex shell implementation is
used.
.SH SEE ALSO
.sp
\fIborg\-common(1)\fP
.SH AUTHOR
The Borg Collective
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