TELNET and SSH by MUSTAFA SAKHAI

TELNETSSH

MUSTAFA SAKHAI7 77 .m ks @gmail com

Poznan University of Technology

1 .TELNET (Terminal Network)• TELNET is an abbreviation for TErminaL NETwork.• Is an application layer protocol used on the internet or local

area networks, enables the establishment of a connection to a remote system in such a way that the local terminal appears to be a terminal at the remote system.• It is the standard TCP/IP protocol for virtual terminal service

Topics to be Discussed

• Understand how TELNET works• Understand the role of NVT in a TELNET communication• Understand TELNET option and sub-option negotiation• Know how control characters are used• Know the TELNET modes of operation• Why TELNET unsecure!

TELNET is a general-purpose client-server application program

Network Diagram

Figure 1.1 Local Login

Figure 1.2 Remote Login

Network Virtual Terminal (NVT)• Is a universal interface, The TELNET client translates characters (data

or commands) that come from the local terminal into NVT form and delivers them to the network. The TELNET server translates data and commands from NVT form into the form acceptable by the remote computer. Figure 1.3 concept of NVT

NVT uses two sets of characters, one for data and one for control. Both are 8-bits.

Figure 1.5 format of data and control characters

Table 1.1 some of NVT control characters

Mode of OperationsI. Default ModeII. Character ModeIII. Line Mode

Default Mode

• In this example, we use the default mode to show the concept

and its deficiencies even though it is almost obsolete today.

• The client and the server negotiate the terminal type and

terminal speed and then the server checks the login and

password of the user (see figure 1.6).

Figure 1.6

Character Mode•We show how the client switches to the character mode. This requires that the client request the server to enable the SUPPRESS GO AHEAD and ECHO options (seeFigure 1.7).

Figure 1.7

Line Mode

• It’s proposed to compensate for the deficiencies of default mode and character mode• In this line (echoing) in done by the client, then the

client sends the whole line to the server

Why TELNET Unsecure?!TELNET suffers from security problems. TELNET requires a login

name and password (when exchanging text). A microcomputer connected to a broadcast LAN can easily spy using snooper software to capture a login name and the corresponding password (even if it is encrypted).!!

It’s Big problemLet’s see how to hacking ;)

Use ping to check the connection

Figure 1.8

Using Wireshark to capture data from LAN port

Figure 1.9

Figure 1.10

Surprise!!!

2. SSH• Is a protocol for secure remote login and other secure

network services over an unsecure network.• Secure channel between two computers• Provides data confidentiality and integrity

•Many uses other than remote shell

Topics to be Discussed

• History• SSH components• Why using SSH?• SSH security features• SSH Authentication Methods• Port forwarding

History• SSH-1 designed in 1995 by Tatu Ylönen• In response to a password-sniffing attack• Replacement for rlogin, telnet, and rsh• Released as freeware in July 1995

• SSH-2 designed in 1996• Incompatible with SSH-1• Security and feature improvements

• Open source implementations (OSSH and OpenSSH) created in 1999• OSSH is now obsolete• OpenSSH is the most popular SSH implementation as of 2005

SSH Components•Transport Layer Protocol•Provides server authentication, confidentiality, and

integrity•User Authentication Protocol•Authenticates the client-side user to the server

•Connection Protocol•Multiplexes the tunnel into logical channels

SSH Components

Figure 2.1

SSH TLP-Overview

SSH UAP-Overview

Why using SSH?•SSH can handle secure remote logins (SSH)•SSH can handle secure file copy (SCP)•SSH can even drive secure FTP (sFTP)

SSH security features• strong algorithms• uses well established strong algorithms for encryption, integrity, key

exchange, and public key management

• large key size• requires encryption to be used with at least 128 bit keys• supports larger keys too

• algorithm negotiation• encryption, integrity, key exchange, and public key algorithms are negotiated• it is easy to switch to some other algorithm without modifying the base

protocol

SSH Authentication Methods

I. PasswordII. Public/Private Key pairIII. Host-Based Authentication

Prompts for password

Public/Private key pair

Host-Based Authentication

•Doesn’t require (Password or key)•Provide trust based on hostname and user ID•User ID on both system has to be the same

Port Forwarding

Sharing your tunnel