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RMT(8) System Manager's Manual RMT(8)

rmtremote magtape protocol module

rmt [-r | -w] [-d directory]

rmt 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. rmt is normally started up with an rcmd(3) or rcmdsh(3) call.

The options are as follows:

Confine file access to directory. Forward slashes in filenames are disallowed and symlinks are not followed.
Read-only mode, suitable for use with rrestore(8).
File write mode, suitable for use with rdump(8) for dumping to regular files. Creates missing files and refuses to open existing ones. The file permission bits are set to readonly.

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 two forms. Successful commands have responses of:


number is an ASCII representation of a decimal number. Unsuccessful commands are responded to with:


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:

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(2) call.
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.
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 an 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(2) call. The return value is the count parameter when the operation is successful.
Return the status of the open device, as obtained with a MTIOCGET ioctl(2) 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).

Any other command causes rmt to exit.

All responses are of the form described above.

rcmd(3), rcmdsh(3), mtio(4), rdump(8), rrestore(8)

The rmt command appeared in 4.2BSD.

People tempted to use this for a remote file access protocol are discouraged.

September 20, 2015 OpenBSD-6.0