Berkeley Remote Login (rlogin)
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The head of the Berkeley remote protocol family is the remote login command, rlogin. As the name clearly implies, the purpose of this program is to allow a user on a UNIX host to log in to another host over a TCP/IP internetwork. Since Telnet is also often used for remote login, rlogin and Telnet are sometimes considered alternatives to each other for TCP/IP remote login. While they can be used in a very similar way, however, they are quite different in a few respects.
From a conceptual standpoint, Telnet is designed as a protocol to enable terminal/host communication. As I mentioned in the Telnet overview topic, the protocol was not designed specifically for the purpose of remote login. In contrast, rlogin was intended for that specific purpose, and this is reflected in its operation.
The protocol requires rlogin server software to be running on the host that is going to allow remote access; it is usually called rlogind (for rlogin daemon, the latter word being the standard UNIX term for a background server process). The server listens for incoming connection requests on TCP port 513. A user who wants to remotely log in to the server runs the rlogin command on his or her local host, and specifies the name of the server. The client makes a TCP connection to the server, and then sends to the server a string containing the following information:
The server processes this information and begins the login process. It will normally prompt the user for a password to log in to the remote host. Assuming the password is correct, the user will be logged in to the remote host and can use it as if he or she were locally connected.
From a practical standpoint, the rlogin command is much simpler than Telnet; it does not support Telnets full command structure, nor capabilities such as option negotiation. It does include a small set of commands, however. The client is able to send to the server one key piece of information: the current size of the terminal window in use. The server is able to tell the client to turn on or off flow control, request that the client send it the current window size, or flush pending output that the server has sent, up to a certain point in the data stream.
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