NAME

       rmt - remote magtape protocol module

SYNOPSIS

       rmt

DESCRIPTION

       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.

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 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>.

AVAILABILITY

       The dump/restore backup suit is available from <https://dump.sourceforge.io>

HISTORY

       The rmt command appeared in 4.2BSD.