SSH password automation in Linux with sshpass

[Editor's Note, Nov 29, 2021: All examples of providing a password on the command line include the risk of the password being captured in the user's shell history (if supported) or visible to all system users in the process listing. Security experts recommend deleting all files and clearing shell logs.]

Connecting and transferring files to remote systems is something system administrators do all the time. One essential tool used by many system administrators on Linux platforms is SSH. SSH supports two forms of authentication:

1. Password authentication
2. Public-key Authentication

Public-key authentication is considered the most secure form of these two methods, though password authentication is the most popular and easiest. However, with password authentication, the user is always asked to enter the password. This repetition is tedious. Furthermore, SSH also requires manual intervention when used in a shell script. If automation is needed when using SSH password authentication, then a simple tool called sshpass is indispensable.

What is sshpass?

The sshpass utility is designed to run SSH using the keyboard-interactive password authentication mode, but in a non-interactive way.

SSH uses direct TTY access to ensure that the password is indeed issued by an interactive keyboard user. sshpass runs SSH in a dedicated TTY, fooling SSH into thinking it is getting the password from an interactive user.

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Install sshpass

You can install sshpass with this simple command:

# yum install sshpass

Use sshpass

Specify the command you want to run after the sshpass options. Typically, the command is ssh with arguments, but it can also be any other command. The SSH password prompt is, however, currently hardcoded into sshpass.

The synopsis for the sshpass command is described below:

sshpass [-ffilename|-dnum|-ppassword|-e] [options] command arguments

Where:

-ppassword
    The password is given on the command line. 
-ffilename
    The password is the first line of the file filename. 
-dnumber
    number is a file descriptor inherited by sshpass from the runner. The password is read from the open file descriptor. 
-e
    The password is taken from the environment variable "SSHPASS".

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Examples

To better understand the value and use of sshpass, let's look at some examples with several different utilities, including SSH, Rsync, Scp, and GPG.

Example 1: SSH

Use sshpass to log into a remote server by using SSH. Let's assume the password is!4u2tryhack. Below are several ways to use the sshpass options.

A. Use the -p (this is considered the least secure choice and shouldn't be used):

$ sshpass -p !4u2tryhack ssh username@host.example.com

The -p option looks like this when used in a shell script:

$ sshpass -p !4u2tryhack ssh -o StrictHostKeyChecking=no username@host.example.com

B. Use the -f option (the password should be the first line of the filename):

$ echo '!4u2tryhack' >pass_file
$ chmod 0400 pass_file
$ sshpass -f pass_file ssh username@host.example.com

The $ chmod 0400 pass_file is critical for ensuring the security of the password file. The default umask on RHEL is 033, which would permit world readability to the file.

Here is the -f option when used in shell script:

$ sshpass -f pass_file ssh -o StrictHostKeyChecking=no username@host.example.com

C. Use the -e option (the password should be the first line of the filename):

$ SSHPASS='!4u2tryhack' sshpass -e ssh username@host.example.com

The -e option when used in shell script looks like this:

$ SSHPASS='!4u2tryhack' sshpass -e ssh -o StrictHostKeyChecking=no username@host.example.com

Example 2: Rsync

Use sshpass with rsync:

$ SSHPASS='!4u2tryhack' rsync --rsh="sshpass -e ssh -l username" /custom/ host.example.com:/opt/custom/ 

The above uses the -e option, which passes the password to the environment variable SSHPASS

We can use the -f switch like this:

$ rsync --rsh="sshpass -f pass_file ssh -l username" /custom/ host.example.com:/opt/custom/

Example 3: Scp

Use sshpass with scp:

$ scp -r /var/www/html/example.com --rsh="sshpass -f pass_file ssh -l user" host.example.com:/var/www/html

Example 4: GPG

You can also use sshpass with a GPG-encrypted file. When the -f switch is used, the reference file is in plaintext. Let's see how we can encrypt a file with GPG and use it.

First, create a file as follows:

$ echo '!4u2tryhack' > .sshpasswd

Next, encrypt the file using the gpg command:

$ gpg -c .sshpasswd

Remove the file which contains the plaintext:

$ rm .sshpasswd

Finally, use it as follows:

$ gpg -d -q .sshpasswd.gpg | sshpass ssh user@srv1.example.com

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Wrap up

sshpass is a simple tool that can be of great help to sysadmins. This doesn't, by any means, override the most secure form of SSH authentication, which is public-key authentication. However, sshpass can also be added to the sysadmin toolbox.

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