Telnet auto login on Linux and Unix with Expect

Telnet is obsolete because it’s insecure, an attacker that is eavesdropping your network traffic can easily steal your credentials, because telnet it’s a text-based protocol without encryption, your username and password will be sent in clear text over the network. If it’s available, always use SSH! (/linux/ssh-auto-login-on-linux-and-unix/), but if you must use telnet create and use the script from below.

Tested operating system:  Debian 9.1, CentOS 7.3, FreeBSD 11.1 and macOS 10.12

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1. Check where is Expect installed, on FreeBSD 11.1  the PATH is /usr/local/bin/expect.

2. Create telnet.sh script and modify the username and password

#!/usr/bin/expect -f
if {[llength $argv] < 1} {
    puts "Usage: ./telnet.sh host";
    exit 1;
}
set timeout 10
set host [lindex $argv 0]
set user "my_user"
set password "my_password"
spawn telnet -l $user $host
expect {
    "?ser*" {
        send "$user\n"
        exp_continue
    }
    "?ogin*" {
        send "$user\n"
        exp_continue
    }
    "?assword*" {
        send "$password\n"
        interact
        exit 0;
    }
}
exit 1

3. Change permission and execute.

Warning: Keep your scripts in a safe place! Anyone who has access to your scripts can steal your credentials!