SSH auto login on Linux and Unix

Security it’s important, a host secured with a strong password is required, but sometimes we need to use other kind of authentification methods, in this article we will walk-through public and private key pair authentification and auto login with Expect script.

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

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Method 1 – Public and private key pair authentification

1. Generate RSA public and private key pair on the client host, ~/.ssh/id_rsa private and ~/.ssh/id_rsa.pub public key will be generated.

2. Append the public key to the ~/.ssh/authorized_keys file stored on the remote host through SSH

Ready! Now you can login into remote_user@remote_host without password.

(Optional) If multiple private keys are used for different servers, we need to generate multiple key pairs and create ~/.ssh/config file.

Method 2 – Auto login with Expect script

1. Make sure that you have installed Expect and the PATH from the first line of the script it’s correct for example on FreeBSD 11.1 the PATH is /usr/local/bin/expect.

2. Create ssh.sh script with your favorite text editor and modify the user, password and port.

#!/usr/bin/expect -f
if {[llength $argv] < 1} {
    puts "Usage: ./ssh.sh host";
    exit 1;
}
set timeout 10
set host [lindex $argv 0]
set user "my_user"
set password "my_password"
spawn ssh $user@$host -p 22
expect {
        "yes/no*" {
                send "yes\n"
                exp_continue
       }
        "?assword*" {
            send "$password\n"
            interact
            exit 0;
       }
}
exit 1

3. Change permission and execute.

Warning: Keep your private keys and scripts in secure places! Anyone who can read your files will be able to login into your hosts!