Data Communications PowerPoint

1. Rlogin/Telnet/SSH
Team 5
Lacher, Robert
Lam, David
Lentz, Tonnie
Mazlik, Kyle

2. Rlogin (Remote Login)
● Rlogin was introduced in 1983 in the BSD
● Is a UNIX command that allows users to login to other Unix machines
○ Rlogin only works on UNIX machines
● Rlogin uses TCP and port 513
● Rlogin communicates with Rlogind on the remote host
● Can login without using password
● Can cause security issues
● Rlogin Syntax: rlogin [-l username] host
Example: rlogin -tommy domain.com

3. hosts.equiv and .rhosts
● hosts.equiv file lies in the ROOT directory
● .rhosts lies in the user’s home directory
● These files create a hole in system’s security
● Syntax is the same for both files (Hostname Username)
○ Examples
■ + + (Anyone can login)
■ + Tom (Tom can login from any remote host)
■ Workstation + (Any user from workstation can login)
■ Workstation Tom (Tom from workstation can login)

4. RSH and RCP
● RSH = remote-shell
○ Allows users to execute shell commands on the host without logging in.
● RCP = remote-copy
○ Allow users to copy from the server to the user's home directory without
logging in

5. Telnet
● Telnet is a simple, text based program in which network protocols are utilized to
allow a device to remotely access and control another device via the internet.
● Generally, permission to access another device must be granted by the device's
administrator or by using log-in credentials.
● Once connected, Telnet allows the user to enter commands to access programs
and other services on the remote computer.
● Accessing e-mail accounts, databases, or simple files are among the many
examples of how Telnet can be used.
● It's like a remote control for the internet!

6. Telnet Origins
● The term Telnet is short for Telecommunications Network.
● It was one of the earliest network protocols developed, and is one of the only
original protocols still in use today.
● Telnet was developed with the Request for Comments (RFC) in 1969, first
defined in RFC 97. It continued to be developed throughout the 1970s as many
modifications needed to be made before it worked properly.
● The final version, Telnet Protocol Specification, was not finished until several
years later. It was published as RFC 854 in May 1983.

7. Examples of Telnet Use:
● Accessing a server running BBS software and using various features
● Accessing a server running an MUD and playing games
● Connecting to a server to run a command line application to check mail
● Connecting to a Linux server and issuing various Linux commands
● Connecting to a router to issue configuration commands (commands for Cisco
IOS, Junos, etc)
● Connecting to an HTTP server port to issue test HTTP commands

8. How Telnet is Used:
● Telnet utilizes a protocol that ensures compatibility between terminal types and
computers while allowing special features to be used by computers and terminals
that agree to support them. The protocol is built upon a foundation of three main
concepts:
■ The Network Virtual Terminal (NVT)
■ Options and Option Negotiation
■ Symmetric Operation

9. Secure Shell (SSH) Origins
● Developed in 1995 by Tatu Ylönen, a researcher at the Helsinki University of
Technology in Finland
● SSH is a newer protocol than Rlogin and Telnet
● Designed as a replacement for other insecure remote shells
○ A shell “is a program that takes your commands from the keyboard and gives them to
the operating system to perform”
● Notably, other shells at the time would send information (including passwords) as
plaintext (Digital NI)
○ Precursors to SSH include Rlogin and Telnet, which were insecure
● SSH is considered a high-security protocol. It uses cryptography to protect the
connection against eavesdropping, hijacking, and other attacks.
● SSH is a common utility for Unix and Linux systems

10. What Does SSH Do?
● SSH is a protocol providing secure remote login and network services
○ Provides access to remote computers for file transfer, execution of processes, e-mail
● Authentication protocol
○ Allows the client to verify server authenticity and to verify itself
● Public key authentication
○ Required by both client and server
○ Client is expected to have two keys generated using encryption algorithms (RSA or DSA)
○ Client sends public key to server, and if server finds public key in its list of allowed keys then the client encrypts and
sends packets using its private key along with the public key
● Password authentication
○ User provides a username and password. Simplest form
● Host-based authentication
○ Client access is restricted to certain hosts
○ Similar to public key authentication, but server maintains list of hosts and their public keys
○ Using a public key on another host would not authenticate the client
● Authentication protocol runs on top of transport layer protocol to provide an authenticated tunnel for the connection
protocol

11. SSH Layers and TCP/IP
● Transport Layer Protocol: Provides server
authentication, data confidentiality, and data
integrity with forward secrecy (that is, if a key
is compromised during one session, the
knowledge does not affect the security of
earlier sessions); the transport layer may
optionally provide compression
● User Authentication Protocol: Authenticates
the user to the server
● Connection Protocol: Multiplexes multiple
logical communications channels over a
single underlying SSH connection
Source: Cisco
http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_12-
4/124_ssh.html

12. References
● Rlogin
○ http://searchnetworking.techtarget.com/definition/rlogin
○ http://www.computerhope.com/unix/urlogin.htm
○ http://cc-ipcp.icp.ac.ru/Section1.1.html#1.1
○ http://wiki.wireshark.org/RLogin
○ http://searchsoa.techtarget.com/definition/daemon
○ https://www.ietf.org/rfc/rfc1282.txt
○ http://foldoc.org/rlogin
○ https://www.mkssoftware.com/docs/man4/rhosts.4.asp
○ https://servercheck.in/blog/brief-history-ssh-and-remote-access

13. References
● Telnet
○ Notess, G. R. (1994). Telnet explored. Online, 18(1), 94.
○ http://www.wisegeek.com/what-is-telnet.htm
○ http://www.tcpipguide.com/free/t_TelnetOverviewHistoryandStandards.htm
○ http://telnet.org/
○ http://windows.microsoft.com/en-us/windows/telnet-faq#1TC=windows-7
● SSH
○ http://docstore.mik.ua/orelly/networking_2ndEd/ssh/ch01_05.htm
○ http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_12-
4/124_ssh.html
○ http://www.serverwatch.com/news/print.php/3551081
○ http://tools.ietf.org/html/rfc4252
○ http://digital.ni.com/public.nsf/allkb/C49602A79827DDBE86256CE9005757D5
○ http://cc-ipcp.icp.ac.ru/Section1.2.html
9/26/14