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1. Overview

In this short tutorial, we’ll discuss different ways to automate telnet sessions using the expect command.

2. Example expect Scripts

The expect program provides a means of automating interactive programs. It’s able to call one or more programs, emulating a user operating them through a standard terminal.

expect also provides a special language, allowing us to script the dialogue with the controlled programs. In general, programs will be centered around the idea of sending inputs and waiting for specific outputs to/from the controlled programs.

Next, we’ll discuss examples of how to use expect to automate telnet sessions.

2.1. Logging Into a Server

telnet has been largely superseded by the more secure ssh as the preferred protocol for remote host connections.

However, telnet presents some advantages. For instance, it’s a lightweight protocol suitable for low-powered devices. Moreover, within secured networks, the gains provided by more secure protocols may be marginal.

In the script below, expect calls telnet to log into a host automatically:

#!/usr/bin/expect

set timeout 5

set host [lindex $argv 0]
set port [lindex $argv 1]
set login [lindex $argv 2]
set password [lindex $argv 3]

spawn telnet $host $port

expect "login: "
send "$login\r"

expect "password: "
send "$password\r"

interact

Now, let’s analyze the above script:

  • set timeout 5: the called program (telnet, in our case) may hang, or the string we’re waiting for may not show up. By setting the timeout variable, we make sure that expect will wait only for the specified amount of time, in seconds.
  • set host, port, and other variables: here we’re using statements in the form of [lindex $argv <index>] to set variables with values provided by the caller as command-line arguments
  • spawn telnet $host $port: calls telnet passing the values of host and port variables
  • expect statements: instructs expect to wait for a particular string pattern to show up on the controlled program’s output
  • send statements: emulates the user typing a value
  • interact: pass the control to the user who called the script. Now, we can interact with telnet using the keyboard.

Here’s how we can invoke the script:

$ ./automate-telnet.sh localhost 23 myusername mypassword

2.2. Requesting an HTTP Page

When connecting to ports other than 23, telnet assumes it isn’t connecting with a telnet server. Then, it won’t attempt to perform the regular protocol negotiations. This allows us to use telnet as a general-purpose TCP/IP client to interact with servers using arbitrary protocols.

In the example below, expect invokes a telnet process to establish an HTTP connection to the www.example.com host:

#!/usr/bin/expect

set timeout 5

spawn telnet www.example.com 80

expect "Connected"

send "GET /index.html HTTP/1.1\r"
send "Host: www.example.com\r\r"

expect "</html>"

Firstly, the expect script will wait for the “Connected” string (a sign that telnet could establish a connection). Then, it transmits a simple GET request and waits for the string “</html>”, signaling the end of the HTML output.

3. Conclusion

In this short article, we’ve discussed possibilities for automating telnet sessions using expect scripts.

Authors Bottom

If you have a few years of experience in the Linux ecosystem, and you’re interested in sharing that experience with the community, have a look at our Contribution Guidelines.

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