rmt - remote magtape protocol module
is a program used by the remote dump
(1) programs in manipulating a magnetic tape drive through an
interprocess communication connection. Rmt
is normally started up with
(3) or rcmd
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:
is an ASCII
representation of a decimal number.
Unsuccessful commands are responded to with:
is one of the possible error numbers described in
(2) and error-message
is the corresponding error string as
printed from a call to perror
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:
- 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.
- Close the currently open device. The device
specified is ignored.
- Perform an lseek(2) operation using the specified
parameters. The response value is that returned from the lseek
- 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
- 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.
- 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
- 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:
- Issue a MTWEOF command (write count
- Issue a MTFSF command (forward space over
count file marks).
- Issue a MTBSF command (backward space over
count file marks).
- Issue a MTFSR command (forward space count
- Issue a MTBSR command (backward space count
- Issue a MTREW command (rewind).
- Issue a MTOFFL command (rewind and put the drive
- Issue a MTNOP command (no operation, set status
- 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:
- Issue a MTCACHE command (switch cache on).
- Issue a MTNOCACHE command (switch cache off).
- Issue a MTRETEN command (retension the tape).
- Issue a MTERASE command (erase the entire
- Issue a MTEOM command (position to end of
- Issue a MTNBSF command (backward space count files
- 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
- 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:
- return the content of the structure member mt_type
which contains the type of the magnetic tape device.
- return the content of the structure member mt_dsreg
which contains the "drive status register".
- 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
- return the content of the structure member mt_resid
which contains the residual count of the last I/O.
- return the content of the structure member mt_fileno
which contains the file number of the current tape position.
- return the content of the structure member mt_blkno
which contains the block number of the current tape position.
- return the content of the structure member mt_flags
which contains MTF_ flags from the driver.
- return the content of the structure member mt_bf
which contains the optimum blocking factor.
Any other command causes rmt
All responses are of the form described above.
People should be discouraged from using this for a remote file access protocol.
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
backup suit is available from
command appeared in 4.2BSD.