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.TH BORG-SERVE 1 "2019-05-16" "" "borg backup tool"
.SH NAME
borg-serve \- Start in server mode. This command is usually not used manually.
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.SH SYNOPSIS
.sp
borg [common options] serve [options]
.SH DESCRIPTION
.sp
This command starts a repository server process. This command is usually not used manually.
.SH OPTIONS
.sp
See \fIborg\-common(1)\fP for common options of Borg commands.
.SS optional arguments
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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 directly point 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 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.
.TP
.B \-\-append\-only
only allow appending to repository segment files
.TP
.BI \-\-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.
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.SH EXAMPLES
.sp
borg serve has special support for ssh forced commands (see \fBauthorized_keys\fP
example below): it will detect that you use such a forced command and extract
the value of the \fB\-\-restrict\-to\-path\fP option(s).
.sp
It will then parse the original command that came from the client, makes sure
that it is also \fBborg serve\fP and enforce path restriction(s) as given by the
forced command. That way, other options given by the client (like \fB\-\-info\fP or
\fB\-\-umask\fP) are preserved (and are not fixed by the forced command).
.sp
Environment variables (such as BORG_HOSTNAME_IS_UNIQUE) 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
.nf
.ft C
# 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 [...]
.ft P
.fi
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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 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
\fBno\-port\-forwarding,no\-X11\-forwarding,no\-pty,no\-agent\-forwarding,no\-user\-rc\fP
in this case.
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.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
.nf
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Host backupserver
        ServerAliveInterval 10
        ServerAliveCountMax 30
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.fi
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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
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ClientAliveInterval 10
ClientAliveCountMax 30
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.fi
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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.
.SH SEE ALSO
.sp
\fIborg\-common(1)\fP
.SH AUTHOR
The Borg Collective
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