rmt
RMT(8) System management commands RMT(8)
NAME
rmt - remote magtape protocol module
SYNOPSIS
rmt
DESCRIPTION
Rmt is a program used by the remote dump(8), restore(8) or tar(1) pro-
grams in manipulating a magnetic tape drive through an interprocess
communication connection. Rmt is normally started up with an rexec(3)
or rcmd(3) call.
The rmt program accepts requests specific to the manipulation of mag-
netic tapes, performs the commands, then responds with a status indi-
cation. All responses are in ASCII and in one of the following two
forms.
Successful commands have responses of:
Anumber\n
where number is an ASCII representation of a decimal number.
Unsuccessful commands are responded to with:
Eerror-number\nerror-message\n
where error-number is one of the possible error numbers described in
intro(2) and error-message is the corresponding error string as
printed from a call to perror(3).
The protocol is comprised of the following commands, which are sent as
indicated - no spaces are supplied between the command and its argu-
ments, or between its arguments, and \n indicates that a newline
should be supplied:
Odevice\nmode\n
Open the specified device using the indicated mode. Device is
a full pathname and mode is an ASCII representation of a deci-
mal number suitable for passing to open(2). If a device had
already been opened, it is closed before a new open is per-
formed.
Cdevice\n
Close the currently open device. The device specified is
ignored.
Lwhence\noffset\n
Perform an lseek(2) operation using the specified parameters.
The response value is that returned from the lseek call.
Wcount\n
Write data onto the open device. Rmt reads count bytes from
the connection, aborting if a premature end-of-file is encoun-
tered. The response value is that returned from the write(2)
call.
Rcount\n
Read count bytes of data from the open device. If count exceeds
the size of the data buffer (10 kilobytes), it is truncated to
the data buffer size. Rmt then performs the requested read(2)
and responds with Acount-read\n 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.
Ioperation\ncount\n
Perform a MTIOCOP ioctl(2) command using the specified parame-
ters. The parameters are interpreted as the ASCII representa-
tions of the decimal values to place in the mt_op and mt_count
fields of the structure used in the ioctl call. The return
value is the count parameter when the operation is successful.
By issuing the I-1\n0\n command, a client will specify that he
is using the VERSION 1 protocol.
For a VERSION 0 client, the operation parameter is the platform
mt_op value (could be different if the client and the rmt
server are on two different platforms). For a VERSION 1 client,
the operation parameter is standardized as below:
0 Issue a MTWEOF command (write count end-of-file
records).
1 Issue a MTFSF command (forward space over count file
marks).
2 Issue a MTBSF command (backward space over count file
marks).
3 Issue a MTFSR command (forward space count inter-record
gaps).
4 Issue a MTBSR command (backward space count inter-record
gaps).
5 Issue a MTREW command (rewind).
6 Issue a MTOFFL command (rewind and put the drive
offline).
7 Issue a MTNOP command (no operation, set status only).
ioperation\ncount\n
Perform an extended MTIOCOP ioctl(2) command using the speci-
fied parameters. The parameters are interpreted as the ASCII
representations of the decimal values to place in the mt_op and
mt_count fields of the structure used in the ioctl call. The
return value is the count parameter when the operation is suc-
cessful. The possible operations are:
0 Issue a MTCACHE command (switch cache on).
1 Issue a MTNOCACHE command (switch cache off).
2 Issue a MTRETEN command (retension the tape).
3 Issue a MTERASE command (erase the entire tape).
4 Issue a MTEOM command (position to end of media).
5 Issue a MTNBSF command (backward space count files to
BOF).
S Return the status of the open device, as obtained with a MTI-
OCGET ioctl call. If the operation was successful, an “ack” is
sent with the size of the status buffer, then the status buffer
is sent (in binary, which is non-portable between different
platforms).
ssub-command
This is a replacement for the previous S command, portable
across different platforms. If the open device is a magnetic
tape, return members of the magnetic tape status structure, as
obtained with a MTIOCGET ioctl call. If the open device is not
a magnetic tape, an error is returned. If the MTIOCGET opera-
tion was successful, the numerical value of the structure mem-
ber is returned in decimal. The following sub commands are sup-
ported:
T return the content of the structure member mt_type which
contains the type of the magnetic tape device.
D return the content of the structure member mt_dsreg
which contains the "drive status register".
E return the content of the structure member mt_erreg
which contains the "error register". This structure mem-
ber must be retrieved first because it is cleared after
each MTIOCGET ioctl call.
R return the content of the structure member mt_resid
which contains the residual count of the last I/O.
F return the content of the structure member mt_fileno
which contains the file number of the current tape posi-
tion.
B return the content of the structure member mt_blkno
which contains the block number of the current tape
position.
f return the content of the structure member mt_flags
which contains MTF_ flags from the driver.
b return the content of the structure member mt_bf which
contains the optimum blocking factor.
Any other command causes rmt to exit.
DIAGNOSTICS
All responses are of the form described above.
SEE ALSO
rcmd(3), rexec(3), /usr/include/sys/mtio.h, rdump(8), rrestore(8)
BUGS
People should be discouraged from using this for a remote file access
protocol.
AUTHOR
The dump/restore backup suit was ported to Linux’s Second Extended
File System by Remy Card <card@Linux.EU.Org>. He maintained the ini-
tial versions of dump (up and including 0.4b4, released in january
1997).
Starting with 0.4b5, the new maintainer is Stelian Pop
<stelian@popies.net>.
AVAILABILITY
The dump/restore backup suit is available from <http://dump.source-
forge.net>
HISTORY
The rmt command appeared in 4.2BSD.
BSD version 0.4b39 of January 21, 2005 RMT(8)
