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Add a set of flags to rmt(8) to make it run in a restricted mode,
designed to work with rdump(8) to remote disk.

  -d <directory>   confines rmt to operate within a single directory.
  -r               enforces read-only mode.
  -w               enforces write-only mode.

This is quite usable with public ssh key setup, e.g. having the following in .ssh/authorized/keys:

  command="/etc/rmt -rd /backups/machine.example.conf",no-agent-forwarding,... ssh-rsa AAAAB3...

ok semarie@

.\"	$OpenBSD: rmt.8,v 1.13 2015/09/20 10:05:48 halex Exp $
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.\"     from: @(#)rmt.8	6.5 (Berkeley) 3/16/91
.\"
.Dd $Mdocdate: September 20 2015 $
.Dt RMT 8
.Os
.Sh NAME
.Nm rmt
.Nd remote magtape protocol module
.Sh SYNOPSIS
.Nm
.Op Fl r | w
.Op Fl d Ar directory
.Sh DESCRIPTION
.Nm
is a program used by the remote dump and restore programs
through an interprocess communication connection.
Traditionally it is used for manipulating a magnetic tape drive but it may
be used for regular file access as well.
.Nm
is normally started up with an
.Xr rcmd 3
or
.Xr rcmdsh 3
call.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl d Ar directory
Confine file access to
.Ar directory .
Forward slashes in filenames are disallowed and symlinks are not followed.
.It Fl r
Read-only mode, suitable for use with
.Xr rrestore 8 .
.It Fl w
File write mode, suitable for use with
.Xr rdump 8
for dumping to regular files.
Creates missing files and refuses to open existing ones.
The file permission bits are set to readonly.
.El
.Pp
The
.Nm
program accepts requests specific to the manipulation of
magnetic tapes, performs the commands, then responds with
a status indication.
All responses are in ASCII and in one of two forms.
Successful commands have responses of:
.Pp
.D1 Sy A Ns Ar number Ns \en
.Pp
.Ar number
is an ASCII representation of a decimal number.
Unsuccessful commands are responded to with:
.Bd -filled -offset indent
.Sm off
.Sy E Ar error-number No \en Ar error-message No \en
.Sm on
.Ed
.Pp
.Ar error-number
is one of the possible error
numbers described in
.Xr intro 2
and
.Ar error-message
is the corresponding error string as printed
from a call to
.Xr perror 3 .
The protocol is comprised of the
following commands, which are sent as indicated - no spaces are supplied
between the command and its arguments, or between its arguments, and
.Ql \en
indicates that a newline should be supplied:
.Bl -tag -width Ds
.Sm off
.It Xo Ic \&O Ar device
.No \en Ar mode No \en
.Xc
.Sm on
Open the specified
.Ar device
using the indicated
.Ar mode .
.Ar device
is a full pathname and
.Ar mode
is an ASCII representation of a decimal
number suitable for passing to
.Xr open 2 .
If a device had already been opened, it is
closed before a new open is performed.
.It Ic C Ns Ar device Ns \en
Close the currently open device.
The
.Ar device
specified is ignored.
.Sm off
.It Xo Ic L
.Ar offset No \en
.Ar whence No \en
.Xc
.Sm on
Perform an
.Xr lseek 2
operation using the specified parameters.
The response value is that returned from the
.Xr lseek 2
call.
.It Ic W Ns Ar count Ns \en
Write data onto the open device.
.Nm
reads
.Ar count
bytes from the connection, aborting if
a premature end-of-file is encountered.
The response value is that returned from
the
.Xr write 2
call.
.It Ic R Ns Ar count Ns \en
Read
.Ar count
bytes of data from the open device.
If
.Ar count
exceeds the size of the data buffer (10 kilobytes), it is
truncated to the data buffer size.
.Nm
then performs the requested
.Xr read 2
and responds with
.Sy A Ns Ar count-read Ns \en
if the read was
successful; otherwise an error in the
standard format is returned.
If the read was successful, the data read is then sent.
.Sm off
.It Xo Ic I Ar operation
.No \en Ar count No \en
.Xc
.Sm on
Perform an
.Dv MTIOCOP
.Xr ioctl 2
command using the specified parameters.
The parameters are interpreted as the
ASCII representations of the decimal values
to place in the
.Ar mt_op
and
.Ar mt_count
fields of the structure used in the
.Xr ioctl 2
call.
The return value is the
.Ar count
parameter when the operation is successful.
.It Ic S
Return the status of the open device, as
obtained with a
.Dv MTIOCGET
.Xr ioctl 2
call.
If the operation was successful, an
.Dq ack
is sent with the size of the status buffer, then the status buffer is
sent (in binary).
.El
.Pp
Any other command causes
.Nm
to exit.
.Sh DIAGNOSTICS
All responses are of the form described above.
.Sh SEE ALSO
.Xr rcmd 3 ,
.Xr rcmdsh 3 ,
.Xr mtio 4 ,
.Xr rdump 8 ,
.Xr rrestore 8
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.2 .
.Sh BUGS
People tempted to use this for a remote file access protocol
are discouraged.