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TELNET and RloginTELNET and Rlogin

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Outline:TELNET and Rlogin

• Remote Interactive Applications:– TELNET

– Protocol

– Pseudo Terminal

– Network Virtual Terminal

• Accommodating Heterogeneity between Different Systems.

• Controlling the Remote Device

• TELNET Options

• Terminal Types

• Remote Commands in UNIX

• Application Sessions

• Tools Available on the Internet:– Traceroute

– Ping

– Tracecon

– Finger

– Whois

– Netfind

– Ph

– Lookup

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Remote Interactive Applications:

• Client / Server Applications:– Interactive remote computing applications

allow users to connect to and control remote machines as if they were actually there.

– All operating system commands and functions can be utilized when remotely logging in.

– Restrictions only exist when displaying video output to a remote terminal (video port redirection).

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Remote Interactive Applications:

• Users have access to all the commands available on the remote system without needing to provide specialized servers.

• Interactive software tends to be more complex since things should be handled in real-time (control character sequences, echo input to the local screen, etc.)

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Remote Interactive Applications:

• Clients usually override the local interpretation of all keys except the “Escape” key.

• This is to allow a user to truly control a remote machine as if they were sitting in front of it.

• Problems do arise, however, when client applications fail to map all control commands to the appropriate remote commands.

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TELNET Protocol:

• TELNET - A simple (standard) remote terminal protocol which:– Passes keystrokes.– Carries output to the client.– Provides a transparent service (appears as if we

are directly attached to the destination machine).

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TELNET Protocol:

• TELNET provides three basic services:– Defines the Network Virtual Terminal (NVT)

standard interface to remote systems (clients are built to the interface).

– Provides a set of standard options (7-bit or 8-bit ASCII character sets) and option negotiation.

– Treats both ends of the connection symmetrically (either program can become a client).

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TELNET:Pseudo Terminal

• Pseudo Terminal:– Is an entry point in the operating system (O.S.)

that allows a running program (like the TELNET server) to transfer characters to the O.S. as if they came from a terminal.

– The O.S. is required to support this, if TELNET is to be an application-level service.

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Image source: Internetworking with TCP/IP vol I (1995 Prentice Hall, Douglas Comer)

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TELNET:Network Virtual Terminal

• Network Virtual Terminal (NVT):– Is defined by TELNET to accommodate

heterogeneity among computer systems.– It handles different computers and operating

systems by defining how data and command sequences are sent across the Internet.

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TELNET:Network Virtual Terminal

• The network virtual terminal specifies that Telnet must communicate using 8-bit bytes (1 octet).

• It uses the standard 7-bit US-ASCII representation of data and reserves bytes with the high-order bit set for command sequences.

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user’sI/O

deviceClient Server Server’s

System

TCP connection across internet

Client System format used NVT format used Server System format used

Network Virtual Terminal (NVT):

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Accommodating Heterogeneitybetween Different Systems:

• Since the NVT accommodates different systems and terminal types, it must also treat common control sequences in a generalized fashion.

• To accommodate line termination in a standard way, the NVT uses a combination of the two common characters (Carriage Return and Line Feed).

• Therefore, it uses a two character sequence of CR-LF for line termination.

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Accommodating Heterogeneitybetween Different Systems:

• The NVT accommodates control functions by defining how they are passed from client to server.

• It defines a conceptual “interrupt” key that requests termination of the most recent command.

• When an “interrupt” sequence character is found, the previous command is terminated.

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NVT Interpretation ofUS-ASCII Control Characters.

ASCII Control Code Decimal Value Assigned Meaning

NUL 0 No operation

BEL 7 Bell sound

BS 8 Move left one position

LF 10 Move down (vertically)

VT 11 Move down to next verticaltab

FF 12 Move to top of next page

CR 13 Move to left margin oncurrent line.

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Controlling the Remote Device:

• NVT keeps control commands separate from normal ASCII character sets, for two reasons:– Greater flexibility - transfer all ASCII character

sequences as well as all possible control functions.

– Avoids signal ambiguity - client specifies the signals, so there is no confusion on how to treat input characters.

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Control Functions used by NVT:

Signal MeaningIP Interrupt Process (terminate)

AO Abort Output (flush buffer)

AYT Are You There? (server test)

EC Erase Character (delete previous)

EL Erase Line (delete current line)

SYNCH Synchronize(clear data path until TCP urgent data point)

BRK Break (break key)

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Controlling the Remote Device:

• The IAC (Interpret as Command) octet:– Is a reserved byte of information which

indicates a control code octet follows in the data stream.

– It signifies the beginning of an “escape sequence”

– It is denoted by decimal code 255.– When IAC is meant to appear as data, the

sender doubles it (IAC-IAC).

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Telnet Encoded Commands:

Command Decimal Encoding MeaningDON’T 254 Deny option request

DO 253 Approval of option request

WON’T 252 Refuse option request

WILL 251 Option agreement

GA 249 “Go Ahead” signal

IP 244 “Interrupt Process” signal

BRK 243 “Break” signal

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Out-of-Band Signalingin TELNET:

• TELNET cannot rely on conventional data streams alone for control sequences, because misbehaving applications that need controlling might inadvertently block the data stream.

• To truly handle control sequences, TELNET uses Out-of-Band signaling where the URG bit is set in the TCP segment used to send it.

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TELNET Options:

• TELNET option negotiation is done between the client and server:– Half or Full Duplex mode.– The remote machine determines the user’s

terminal type.– The “I think you are using a VT100 terminal”

message is common.

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TELNET Options:

• Telnet normally uses a 7-bit data stream 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.– The IAC control command must always be

doubled because of In-band signaling.

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TELNET Options:

• Either end of the connection (client or server) can request options.

• This interaction is symmetric (with respect to option negotiation), to allow reconfiguration of their interactions.

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Terminal Types:

• Many terminal types exist. Telnet and the NVT, through option negotiation, allow the client and server to agree on the specific terminal emulation to use.

• Terminal emulation types include:– VT100– VT220– ANSI

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Terminal Types:

• Extensions have been made to the Telnet protocol to provide terminal emulation of proprietary interfaces and displays.

• Support for IBM 3270 terminal displays was added through several RFCs (1041, 1576, 1646, 1647) and the tn3270 program.

• OSI application-layer protocols also exist to provide remote terminal connectivity.

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Remote Commands in UNIX:

• Rlogin (Remote Login):– 4.3 BSD UNIX remote login service that

supports trusted hosts (automatic authorization).

– If not explicitly denoted, Rlogin attempts to login a user on the destination machine using the same user name and password as the source.

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Remote Commands in UNIX:

• Rsh (remote shell):– Invokes a command interpreter on the remote

UNIX machine.– And passes any command line arguments to the

command interpreter, skipping the login step completely.

rsh machine command

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Remote Commands in 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..

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Remote Commands in UNIX:

• 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).

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Application Sessions:

• “Sessions” are used with remote login protocols and file transfer protocols (FTP).

• A session is established during connection and dictated by which side transmits at which time.

• Session control is embedded within TCP/IP application protocols (unlike OSI protocols where it resides in the ‘session layer’).

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Tools Available on the Internet:Traceroute

• Traceroute - A debugging tool used to send test packets to each hop found along an Internet path and determine the amount of time required for each.

• Traceroute at Paris:(http://hplyot.obspm.fr/cgi-bin/nph-traceroute?hertz.njit.edu)

• Traceroute at Brownell:(http://www.brownell.edu/cgi-bin/traceroute.bt?hertz.njit.edu)

• Traceroute at MIT:(http://bs.mit.edu:8001/cgi-bin/traceroute?hertz.njit.edu)

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Tools Available on the Internet:Ping

• Ping sends a packet to a remote or local host, requesting an echo:– If the echo is returned, the site is up.– If the echo is not returned, it can indicate that

the site is down or there is some other problem.• Ping at SMSI:

(http://www.smsi.com/cgi-bin/ping)

• Ping at UK:(http://js.stir.ac.uk/jsbin/wwping)

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Tools Available on the Internet:Tracecon

• Tracecon can give you a thorough summary of your TCP/IP connection:– This site will automatically determine what

your IP address is, and do diagnostic functions based on it.

– The diagnosis may show a different IP address if your site has a firewall installed.

• Network diagnosis from your IP address: (http://www.wiskit.com/cgi-bin/tracecon)

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Tools Available on the Internet:Finger• Finger - can help you to find an email address or

correct an undeliverable or looping message:– Finger hits a user’s account and displays the

“.plan” file (if available).• Finger Gateway at Indiana:

(http://www.cs.indiana.edu/finger/gateway)

• Finger Gateway at St. Andrews: (http://bute.st-andrews.ac.uk:8080/~www_sa/personal/finger.cgi)

• Finger Gateway at NASA: (http://eos.nasa.gov/cgi-bin/finger)

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Tools Available on the Internet:Whois

• Whois - provides information on registered computer network users, domains, and organizations:– It can help you to find email addresses and host

and domain names.• Whois Gateway at St. Louis:

(http://biogopher.wustl.edu:70/7/email/uwho_frm)

• Whois at Germany:(http://www.th-darmstadt.de/cgi-bin/whois.old)

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Tools Available on the Internet:Netfind

• Netfind - will provide email addresses and login ID information. It is similar to Finger.– You need both the user ID and host name to use

netfind.• Netfind Gateway at Internic:

(gopher://ds.internic.net:4320/7netfind%20dblookup)

• Netfind at Nova: (gopher://cs4sun.cs.ttu.edu:4320/7netfind%20dblookup)

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Tools Available on the Internet:Ph

• Ph - can provide information on email and snail mail addresses and phone numbers:– Searches are usually limited to local

information (or institutions listed by the on-line service below).

– When searching, try using both the full name and the last name.

• Ph at NCSA: (http://fiaker.ncsa.uiuc.edu:8080/cgi-bin/phfd)

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Tools Available on the Internet:Lookup

• Lookup service - provides a speedy and robust search engine, spanning the capabilities of many directory resources.– Membership is required: provide first name,

last name, and partial email address. • Membership page:

(http://www.lookup.com/lookup/member.html)

• Already a member: (http://www.lookup.com)

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QuestionsTELNET and Rlogin

• Give your reasoning why you feel there are so many different terminal types and control sequences.

• Could the Network Virtual Terminal simply be used by all terminal applications and avoid the need of conversion/types?

• What type of information does the Traceroute program provide to a network administrator?

• How do Telnet and Rlogin differ?• What purpose is served by the pseudo-terminal?

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Reference MaterialsTELNET and Rlogin• IEN 148 (RFC-764) TELNET PROTOCOL:

(http://www.cis.ohio-state.edu/htbin/ien/ien148.html)

• RFC 854: Telnet Protocol (http://www.cis.ohio-state.edu/htbin/rfc/rfc854.html)

• TCP/IP Illustrated, Vol 1: W. Richard Stevens.• Internet User Troubleshooting:

(http://www.iac.net/~othello/)

• RFC 1282: BSD Rlogin (http://www.cis.ohio-state.edu/htbin/rfc/rfc1282.html)

• RFC 1576: TN3270 Current Practices (http://www.cis.ohio-state.edu/htbin/rfc/rfc1576.html)