TELNET and Rlogin

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TELNET and Rlogin
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OutlineTELNET 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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TELNETPseudo 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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TELNETNetwork 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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TELNETNetwork 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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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.
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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
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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
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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 users 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 InternetTraceroute

• 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/c
  gi-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.edu8001/cgi-bi
  n/traceroute?hertz.njit.edu)

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Tools Available on the InternetPing

• 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/p
  ing)
• Ping at UK (http//js.stir.ac.uk/jsbin/wwpi
  ng)

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Tools Available on the InternetTracecon

• 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 InternetFinger

• Finger - can help you to find an email address or
  correct an undeliverable or looping message
• Finger hits a users 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.uk8080/www_sa/persona
  l/finger.cgi)
• Finger Gateway at NASA (http//eos.nasa.gov/cgi
  -bin/finger)

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Tools Available on the InternetWhois

• 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.wust
  l.edu70/7/email/uwho_frm)
• Whois at Germany (http//www.th-darmstadt.de/c
  gi-bin/whois.old)

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Tools Available on the InternetNetfind

• 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
  .net4320/7netfind20dblookup)
• Netfind at Nova (gopher//cs4sun.cs.ttu.edu4320/
  7netfind20dblookup)

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Tools Available on the InternetPh

• 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.edu8080/cg
  i-bin/phfd)

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Tools Available on the InternetLookup

• 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/mem
  ber.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.ht
  ml)
• RFC 854 Telnet Protocol (http//www.cis.ohio-st
  ate.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.h
  tml)