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Chapter 19

TELNETand

Rlogin

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CONTENTSCONTENTS• CONCEPT• NETWORK VIRTUAL TERMINAL (NVT)• NVT CHARACTER SET• EMBEDDING• OPTIONS• OPTION NEGOTIATION• SUBOPTION NEGOTIATION• CONTROLLING THE SERVER• OUT-OF-BAND SIGNALING

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CONTENTS CONTENTS (Continued)(Continued)

• ESCAPE CHARACTER• MODE OF OPERATION• EXAMPLES• USER INTERFACE• RLOGIN (REMOTE LOGIN)• SECURITY ISSUE

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TELNET and Rlogin are TELNET and Rlogin are general-purpose general-purpose

client-server application programs.client-server application programs.

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CONCEPT

19.119.1

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Figure 19-1

Local login

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Figure 19-2

Remote login

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NETWORKVIRTUAL

TERMINAL

19.219.2

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Figure 19-3

NVT

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NVTCHARACTER

SET

19.319.3

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Figure 19-4

Format of data characters

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Figure 19-5

Format of control characters

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EMBEDDING

19.419.4

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Figure 19-6

Embedding

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OPTIONS

19.519.5

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OPTIONNEGOTIATION

19.619.6

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Figure 19-7

Offer to enable

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Figure 19-8

Request to enable

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Figure 19-9

Offer to disable

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Figure 19-10

Request to disable

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Figure 19-11

Echo option example

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SUBOPTIONNEGOTIATION

19.719.7

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Figure 19-12

Example of suboption negotiation

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CONTROLLINGTHE

SERVER

19.819.8

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Figure 19-13

Example of interrupting an application program

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OUT-OF-BANDSIGNALING

19.919.9

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Figure 19-14

Out-of-band signaling

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ESCAPECHARACTER

19.1019.10

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Figure 19-15Two different interruptions

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MODEOF

OPERATION

19.1119.11

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EXAMPLES

19.1219.12

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Example 1Example 1

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 19.16.

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Figure 19-16 Example1

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Example 2Example 2

In this example, 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. See Figure 19.17.

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Figure 19-17

Example 2

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USERINTERFACE

19.1319.13

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RLOGIN(REMOTE LOGIN)

19.1419.14

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The Rlogin process uses the TCP port 513.

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Figure 19-18

Connection establishment

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Figure 19-19

Sending ss command from the client to the server

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SECURITYISSUE

19.1519.15