rmt - remote magtape protocol module
Rmt is a program used by the remote dump(8),
restore(8) or tar(1) programs 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 magnetic tapes, performs the commands, then responds with a
status indication. 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
arguments, 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 decimal number suitable for passing to open(2).
If a device had already been opened, it is closed before a new open is
performed.
- 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 encountered. 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
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
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
specified 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
successful. 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 MTIOCGET
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 operation was successful, the numerical
value of the structure member is returned in decimal. The following sub
commands are supported:
- 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 member 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 position.
- 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.
All responses are of the form described above.
People should be discouraged from using this for a remote file
access protocol.
The dump/restore backup suit was ported to Linux's Second
Extended File System by Remy Card <card@Linux.EU.Org>. He maintained
the initial 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>.
The dump/restore backup suit is available from
<https://dump.sourceforge.io>
The rmt command appeared in 4.2BSD.