Applications: Remote Login (TELNET,Rlogin) In this chapter we shall explore internetworking by examining high level internet services and protocols that support them. These services form an integral part of TCP/IP. High Level services provide increased communication functionality,and allow users and programs to interact with automated services on remote machines and with remote users.

Remote Interactive Computing A remote system can be contacted using the client-server model. For this a specially designed server is required and there is a possibility of the remote machine getting swamped with server processes. We can avoid this and provide more generality by allowing a user to establish a login session on a remote machine and then execute commands.

TELNET Protocol Telnet allows a user to log into a computer across an internet. Telnet establishes a TCP connection and then passes keystrokes from the users keyboard directly to the remote computer. The client software asks the user to specify domain name or IP address of the remote machine to be contacted.

TELNET Services Telnet offers three basic services. 1. It defines a network virtual terminal that provides a standard interface to remote systems. 2. Telnet includes a mechanism that allows the client and server to negotiate options. 3. Telnet treats both ends of the connection symmetrically.

TELNET Operation

Pros And Cons Of TELNET Server As An Application Program Advantage It makes modification and control of the server easier than if the code were embedded in the operating system. Disadvantage Inefficiency - Each keystroke travels from client keyboard - client OS - client program -server program-server OS and back to client program.

Accommodating Heterogeneity TELNET interoperate between as many systems as possible. TELNET must accommodate the details of heterogeneous computers and operating systems. For example: –In some systems lines of text are terminated by the ASCII carriage control character (CR). –Some require ASCII linefeed (LF) character. –Some require the two-character sequence of CR-LF.

Accommodating Heterogeneity TELNET defines how data and command sequences are sent across the internet known as network virtual terminal (NVT). Fig illustrates. –The Client Software translates keystrokes and command sequences from the user’s terminal in to NVT and sends them to the server. –Server software translates incoming data and commands from NVT format into the format the remote system requires.

Accommodating Heterogeneity For data returning, the remote server translates from the remote machine’s format to NVT, and the local client translates from NVT to the local machine’s format. All communication involves 8-bit bytes. At startup, NVT uses the standard 7-bit USASCII representation for data. USASCII character set includes 95 characters that have “printable” graphics and 33 “control” codes.

Accommodating Heterogeneity The NVT standard defines interpretations for control characters as shown in Fig NVT also defines the standard line termination to be a two-character sequence CR-LF. –TELNET client map the end-of-line of local terminal into CR-LF for transmission. –TELNET server translates CR-LF into the appropriate end-of-line character sequence for the remote machine.

Passing Commands That Control the Remote Side UNIX systems reserve character generated by CONTROL-C as the interrupt key. Depressing CONTROL-C key causes UNIX to terminate the executing program. TELNET NVT accommodates control functions by defining how they are passed from the client to the server. NVT defines a conceptual “interrupt” key that requests program termination.

Passing Commands That Control the Remote Side Fig lists the control functions NVT recognizes. The Client receives control functions from a user in addition to normal data and passes them to the server system where they must be interpreted.

Passing Commands That Control the Remote Side TELNET encodes the control functions as escape sequence across the TCP connection. An escape sequence uses a reserved octet to indicate that a control code octet follows. In Telnet, the reserved octet that starts an escape sequence is known as the interpret as command IAC) octet. Fig 25.5 lists the possible commands and the decimal encoding used for each.

Passing Commands That Control the Remote Side Signals generated by conceptual keys on an NVT key-board each have a corresponding command.

Forcing The Server To Read A Control Function TELNET cannot rely on the conventional data stream alone to carry control sequences between client and server. TELNET uses an out of band signal to solve this problem. TCP implements out of band signaling with the urgent data mechanism. TELNET appends a reserved octet called data mark..

Forcing The Server To Read A Control Function TELNET causes TCP to signal the server by sending a segment with URGENT DATA bit set. Segments carrying urgent data bypass flow control and reach the server immediately. The server reads and discards all data until if finds the data mark. The server returns to normal processing when it encounters the data mark.

Telnet Options TELNET option negotiation is done between the client and server: Usually a data stream passes a 7-bit data and uses the highest-order (8th bit) to pass control information. Using option negotiation, TELNET can also use an 8-bit data character set. In the case of 8-bit data sets: - Both ends must agree to use 8-bit data. This is done by exchanging ``option code sequences''. - TELNET defines a special byte, the Interpret As Command (IAC) with the value TELNET uses in-band signaling and IAC must be doubled. - When IAC is received, the following byte(s) is interpreted as a TELNET Command. Either end of the connection (client or server) can request options.

Commonly Used TELNET Options Variety of options can be negotiated between a TELNET client and server using commands at any stage during the connection for separate RFCs. - Transmit Binary: Change transmission to Binary. - Echo: Usually enabled by the server, to indicate that the server will echo every character it receives. - Suppress-GA: Original TELNET implementation defaulted to “half duplex" operation. Modern links normally allow bidirectional operation(Full duplex) and the "suppress go ahead" option is enabled.

Commonly used Telnet Options - Status: Request for status of a TELNET option from a remote site. - Timing-Mark: Request Timing mark be inserted in return stream to synchronize two ends of connection. - Terminal-Type: Exchange information about the make and model of a terminal being used. - End-of-Record: Terminate data sent with EOR code. - Limemode: Complete lines are assembled at each end and transmitted in one "go".

TELNET Option Negotiation Options are agreed by a process of negotiation which results in the client and server having a common view of various extra capabilities that affect the interchange and the operation of applications. Either end of a TELNET dialogue can enable or disable an option either locally or remotely. The initiator sends a 3 byte command of the form IAC,, The response is of the same form.

TELNET Option Negotiation Operation is one of : Description Code Meaning WILL 251 Sender wants to enable the option. DO 252 Sender wants the receiver to enable option. WONT 253 Sender wants to disable the option. DONT 254 Sender wants the receiver to disable option. Associated with each of the these there are various possible responses : - If the Sender sends a request WILL X and if the Receiver Responds DO X, it implies that, the sender would like to use a certain facility if the receiver can handle it. Receiver agrees and the Option is now in effect. - If the Sender sends WILL X and if the Receiver Responds DON’T Receiver says it cannot support the option. Option is not in effect.

TELNET Option Negotiation - Similarly DO X requests that the receiving party begin using option X, and WILL X or WON’T X means the receiver would start using option X and the option is in effect or the receiver would not use option X and so the option is not in effect. - Negotiation requires 3 bytes. - For example if the sender wants the other end to suppress go-ahead it would send the byte sequence 255(IAC),251(WILL),3 The final byte of the three byte sequence identifies the required action.

Rlogin (BSD UNIX) Rlogin is the 4.3 BSD UNIX remote login service that supports trusted hosts (automatic authorization). Rlogin allow a user to log in on another host via a network. If not explicitly denoted, rlogin attempts to login a user on the destination machine using the same user name and password as the source. Thus it makes it possible for a user to have login name X on one machine and Y on another, and still be able to remotely login from one of the machines to the other without typing a password each time.

Rlogin (BSD UNIX) Rsh (remote shell): - Invokes a command interpreter on the remote UNIX machine. Passes any command line arguments to the command interpreter, skipping the login step completely. The format of a command invocation using rsh is: rsh machine command - For example typing, rsh merlin ps on any one of the machines in the Purdue Univ. executes the ps command on the machine merlin, with UNIX’s standard input and standard output connected across the network to the user’s keyboard and display. The user sees the output as if he or she were logged into machine merlin.

Rlogin (BSD UNIX) - rsh does not ask for a password, so it can be used in programs as well as from the keyboard. - This may be a security issue at certain sites, so administrators may not allow executing programs to utilize this command. rlogin understands UNIX notions of standard input, standard output, and standard error as well as terminal control functions (understands both local and remote computing environments). For this reason, it communicates better than general purpose remote login protocols (like TELNET).

Summary TELNET provided by most TCP/IP implementations works between hosts using different operating systems. Rlogin from BSD UNIX designed to work between UNIX systems only. TELNET has many options. Rlogin provides only one option. Option negotiation not required. In contrast to TELNET, rlogin allows system managers and users more flexibility to establish equivalence of accounts on multiple machines, but it is not as widely available as TELNET.

Practical Example Telnet from host with IP address ( ) to Router with IP Address ( ). Capturing frames using Sniffer. Sniffer is a software tool used by network administrators for testing, analyzing network traffic and design.