TELNET and Rlogin
LESSON 24

Outline:
TELNET and Rlogin

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).
  • 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.)
  • 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.

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).
• 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).

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.
    

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.
• 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.

Accommodating Heterogeneity between 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.
• 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.

NVT Interpretation of US-ASCII Control Characters.

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.

Control Functions used by NVT:

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).

Telnet Encoded Commands:

Out-of-Band Signaling in 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.

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.
• 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.
• 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.

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
• 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.

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.
• 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
• 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).

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").

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)
• raceroute 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)

Tools Available on the Internet:
Ping

• Ping sends a pcaket 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 problem.
• Ping at SMSI:

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

• Ping at UK:

  (http://js.stir.ac.uk/jsbin/wwping)

Tools Available on the Internet:
Tracecon

• Traceconcan give you a thorough summaray 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 adddress if your site has a firewall> installed.
• Network diagnosis from your IP address:

  (http://www.wiskit.com/cgi-bin/tracecon)

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)

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)

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)

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)

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)

Last Modification: (Sunday, August 25, 1996)

All work was written, produced, and is copyrighted by Daniel Z. Tabor Jr.
Page created by Daniel Z. Tabor Jr. Copyright ©1996 Illusion Industries Inc.