Automating telnet session using Bash scripts

Question

I am working on automating some telnet related tasks, using Bash scripts.

Once automated, there will be no interaction of the user with telnet (that is, the script will be totally automated).

The scripts looks something like this:

# execute some commands on the local system
# access a remote system with an IP address: 10.1.1.1 (for example)

telnet 10.1.1.1

# execute some commands on the remote system
# log all the activity (in a file) on the local system
# exit telnet
# continue with executing the rest of the script

There are two problems I am facing here:

1. How to execute the commands on the remote system from the script (without human interaction)?

   From my experience with some test code, I was able to deduce that when telnet 10.1.1.1 is executed, telnet goes into an interactive session and the subsequent lines of code in the script are executed on the local system. How can I run the lines of code on the remote system rather than on the local one?

2. I am unable to get a log file for the activity in the telnet session on the local system. The stdout redirect I used makes a copy on the remote system (I do not want to perform a copy operation to copy the log to the local system). How can I achieve this functionality?

Answer 1

Telnet is often used when you learn the HTTP protocol. I used to use that script as a part of my web scraper:

echo "open www.example.com 80"
sleep 2
echo "GET /index.html HTTP/1.1"
echo "Host: www.example.com"
echo
echo
sleep 2

Let's say the name of the script is get-page.sh, then this will give you an HTML document:

get-page.sh | telnet

I hope this will be helpful to someone ;)

Answer 2

The answer by @thiton was helpful but I wanted to avoid the sleep command. Also telnet didn't exit the interactive mode, so my script got stuck.

I solved that by sending telnet command with curl (which seems to wait for the response) and by explicitly telling telnet to quit like this:

curl telnet://10.1.1.1:23 <<EOF
remotecommand 1
remotecommand 2
quit
EOF

Answer 3

While I'd suggest using expect, too, for non-interactive use the normal shell commands might suffice. telnet accepts its command on stdin, so you just need to pipe or write the commands into it through heredoc:

telnet 10.1.1.1 <<EOF
remotecommand 1
remotecommand 2
EOF

(Edit: Judging from the comments, the remote command needs some time to process the inputs or the early SIGHUP is not taken gracefully by telnet. In these cases, you might try a short sleep on the input:)

{ echo "remotecommand 1"; echo "remotecommand 2"; sleep 1; } | telnet 10.1.1.1

In any case, if it's getting interactive or anything, use expect.

Answer 4

Here is a solution that will work with a list of extenders. This only requires bash - some of the answers above require expect and you may not be able to count on expect being installed.

#!/bin/bash

declare -a  Extenders=("192.168.1.48" "192.168.1.50" "192.168.1.51") 
# "192.168.1.52" "192.168.1.56" "192.168.1.58" "192.168.1.59" "192.168.1.143")
sleep 5
# Iterate the string array using for loop
for val in ${Extenders[@]}; do
   { sleep 0.2; echo "root"; sleep 0.2; echo "ls"; sleep 0.2; }  | telnet $val
done

Answer 5

Script for obtain version of CISCO-servers:

#!/bin/sh

servers='
192.168.34.1
192.168.34.3
192.168.34.2
192.168.34.3
'
user='cisco_login'
pass='cisco_password'

show_version() {
host=$1
expect << EOF
set timeout 20
set host $host
set user $user
set pass $pass
spawn telnet $host
expect "Username:"
send "$user\r"
expect "Password:"
send "$pass\r"
expect -re ".*#"
send "show version\r"
expect -re ".*-More-.*"
send " "
expect -re ".*#"
send "exit\r"
EOF
}

for ip in $servers; do
  echo '---------------------------------------------'
  echo "$ip"
  show_version $ip | grep -A3 'SW Version'
done

Answer 6

Write an expect script.

Here is an example:

#!/usr/bin/expect

#If it all goes pear shaped the script will timeout after 20 seconds.
set timeout 20
#First argument is assigned to the variable name
set name [lindex $argv 0]
#Second argument is assigned to the variable user
set user [lindex $argv 1]
#Third argument is assigned to the variable password
set password [lindex $argv 2]
#This spawns the telnet program and connects it to the variable name
spawn telnet $name 
#The script expects login
expect "login:" 
#The script sends the user variable
send "$user "
#The script expects Password
expect "Password:"
#The script sends the password variable
send "$password "
#This hands control of the keyboard over to you (Nice expect feature!)
interact

To run:

./myscript.expect name user password

Answer 7

Following is working for me... put all of your IPs you want to telnet in IP_sheet.txt

while true
read a
do
{
    sleep 3
    echo df -kh
    sleep 3
    echo exit
} | telnet $a
done<IP_sheet.txt

Answer 8

Here is how to use telnet in bash shell/expect

#!/usr/bin/expect
# just do a chmod 755 one the script
# ./YOUR_SCRIPT_NAME.sh $YOUHOST $PORT
# if you get "Escape character is '^]'" as the output it means got connected otherwise it has failed

set ip [lindex $argv 0]
set port [lindex $argv 1]

set timeout 5
spawn telnet $ip $port
expect "'^]'."

Answer 9

#!/bin/bash
ping_count="4"
avg_max_limit="1500"
router="sagemcom-fast-2804-v2"
adress="192.168.1.1"
user="admin"
pass="admin"

VAR=$(
expect -c " 
        set timeout 3
        spawn telnet "$adress"
        expect \"Login:\" 
        send \"$user\n\"
        expect \"Password:\"
        send \"$pass\n\"
        expect \"commands.\"
        send \"ping ya.ru -c $ping_count\n\"
        set timeout 9
        expect \"transmitted\"
        send \"exit\"
        ")

count_ping=$(echo "$VAR" | grep packets | cut -c 1)
avg_ms=$(echo "$VAR" | grep round-trip | cut -d '/' -f 4 | cut -d '.' -f 1)

echo "1_____ping___$count_ping|||____$avg_ms"
echo "$VAR"

Answer 10

This worked for me..

I was trying to automate multiple telnet logins which require a username and password. The telnet session needs to run in the background indefinitely since I am saving logs from different servers to my machine.

telnet.sh automates telnet login using the 'expect' command. More info can be found here: http://osix.net/modules/article/?id=30

telnet.sh

#!/usr/bin/expect
set timeout 20
set hostName [lindex $argv 0]
set userName [lindex $argv 1]
set password [lindex $argv 2]

spawn telnet $hostName

expect "User Access Verification"
expect "Username:"
send "$userName\r"
expect "Password:"
send "$password\r";
interact

sample_script.sh is used to create a background process for each of the telnet sessions by running telnet.sh. More information can be found in the comments section of the code.

sample_script.sh

#!/bin/bash
#start screen in detached mode with session-name 'default_session' 
screen -dmS default_session -t screen_name 
#save the generated logs in a log file 'abc.log' 
screen -S default_session -p screen_name -X stuff "script -f /tmp/abc.log $(printf \\r)"
#start the telnet session and generate logs
screen -S default_session -p screen_name -X stuff "expect telnet.sh hostname username password $(printf \\r)"

1. Make sure there is no screen running in the backgroud by using the command 'screen -ls'.
2. Read http://www.gnu.org/software/screen/manual/screen.html#Stuff to read more about screen and its options.
3. '-p' option in sample_script.sh preselects and reattaches to a specific window to send a command via the ‘-X’ option otherwise you get a 'No screen session found' error.

Answer 11

You can use expect scripts instaed of bash. Below example show how to telnex into an embedded board having no password

#!/usr/bin/expect

set ip "<ip>"

spawn "/bin/bash"
send "telnet $ip\r"
expect "'^]'."
send "\r"
expect "#"
sleep 2

send "ls\r"
expect "#"

sleep 2
send -- "^]\r"
expect "telnet>"
send  "quit\r"
expect eof

Answer 12

Play with tcpdump or wireshark and see what commands are sent to the server itself

Try this

printf (printf "$username\r\n$password\r\nwhoami\r\nexit\r\n") | ncat $target 23

Some servers require a delay with the password as it does not hold lines on the stack

printf (printf "$username\r\n";sleep 1;printf "$password\r\nwhoami\r\nexit\r\n") | ncat $target 23**

Answer 13

Use ssh for that purpose. Generate keys without using a password and place it to .authorized_keys at the remote machine. Create the script to be run remotely, copy it to the other machine and then just run it remotely using ssh.

I used this approach many times with a big success. Also note that it is much more secure than telnet.