rmt-tar

tar <operation> -f [user@]host:<remote_device> [options] [files...]

Examples:
tar -cvf backup@remoteserver:/dev/rmt0 /home/user/data
tar -xvf remoteserver:/dev/rmt0

-f [user@]host:<remote_device>
    Specifies the remote tape device. The user@ part is optional and defines the username for connecting to the remote host. host is the remote machine's hostname or IP address. <remote_device> is the path to the tape device (e.g., /dev/rmt0) on the remote system.

TAR_RSH_COMMAND or RSH_COMMAND (environment variable)
    Allows specifying an alternative remote shell program to be used for establishing the connection to the remote host instead of the default rsh. For example, setting TAR_RSH_COMMAND=ssh instructs tar to use ssh for secure communication.

The term rmt-tar does not refer to a standalone Linux command named "rmt-tar". Instead, it describes the functionality where the standard tar utility interacts with remote tape devices using the rmt (remote tape) protocol.

The rmt protocol allows a local tar command to perform operations (like reading, writing, or rewinding) on a tape device located on a remote machine. When tar is invoked with a remote device specified in the format [user@]host:/path/to/device, it typically establishes a connection to the remote host (historically via rsh, now more commonly via ssh) and executes the rmt daemon there. This rmt daemon acts as a server, performing the requested tape operations on behalf of the local tar and transferring data across the network.

This mechanism is crucial for managing backups and archives on centralized tape libraries or remote servers without physically accessing the tape drive, making it a powerful tool in distributed environments, especially in older Unix and Linux setups.

Security Concerns: Historically, rmt-tar operations often relied on rsh, which is highly insecure due to unencrypted data transfer and authentication via `.rhosts` files. Modern usage should always prefer ssh for secure transport.

Remote `rmt` Daemon: The remote host must have the rmt daemon (typically /usr/libexec/rmt or /usr/lib/rmt) installed and accessible via the chosen remote shell (rsh or ssh).

Performance: Network latency and bandwidth limitations can significantly impact the speed of remote tape operations, especially for large archives.

Firewall Configuration: Appropriate firewall rules must be in place on both the local and remote systems to allow the chosen remote shell protocol (e.g., port 22 for ssh) to establish a connection.

Error Diagnosis: Debugging issues with remote tape operations can be challenging, as errors might originate on the remote host and be relayed back through the rmt protocol.

When tar needs to access a remote device (e.g., host:/dev/rmt0), it doesn't directly connect to the tape drive. Instead, it invokes a remote shell (like ssh or rsh) on the target host. This remote shell then executes the rmt program on the remote machine. The rmt program acts as a proxy: it accepts commands (like 'open device', 'read block', 'write block', 'close device') from the local tar via the network connection, performs the actual tape I/O operations on the remote device, and returns the results or data blocks back to the local tar. This client-server architecture abstracts the tape hardware and allows for network-transparent tape access.

To use rmt-tar securely with ssh, ensure that ssh is properly configured for passwordless authentication between the local and remote machines. This usually involves setting up SSH keys (e.g., using ssh-keygen and ssh-copy-id). Additionally, the remote ssh server must be configured to allow execution of the rmt command. The TAR_RSH_COMMAND environment variable should be set to ssh on the client side to ensure tar uses ssh instead of rsh.

The rmt protocol emerged in early Unix environments to facilitate remote access to tape drives for backup and restoration tasks. It allowed administrators to centralize tape libraries and manage backups from various client machines across a network. Initially, connections were typically established using the rsh utility. However, with the rise of network security concerns, the unencrypted nature of rsh became a significant vulnerability. Consequently, modern Unix and Linux distributions largely transitioned to using ssh as the preferred secure transport layer for rmt operations, ensuring encrypted communication and stronger authentication methods.

tar(1), rmt(8), ssh(1), rsh(1)