R-tools

R-tools are a set of Unix/Linux tools that allow basic unsecured administration of Unix/Linux systems by establishing a Remote Shell. Similar in nature to Telnet but much less popular, R-tools is considered by most IT professionals to be dangerous and obsolete. The much preferred way to do a remote shell is ssh.
R-tools connection.
R-tools connects over TCP port 513 with the command rlogin, r-tools is considered to be insecure because it can be configured to only require a hostname and username; transmissions using r-tools are not encrypted.