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PureDevs

Use PureDevs WP locker To safeguard your entire WordPress site, by using a single password.
Only the visitors who know your password would be able to access your WordPress site.
This plugin is just what you need if you’re setting up a developmental version of your WordPress site or if your website is going through a major revamping and your website is simply off-limit from the general public or the known bots.
After activating the plugin, head over to Settings> PureDevs WP Locker> Set Password to set a site-wide password.
Note: When a user enters a password, their IP is saved both on cookie and in database table as future login credentials.
Optional: 
Form heading: Clarify to visitors the reason for locking your website and provide clear guidance on the next steps to take. 
Button Label: customize the text on your password submission button.

PureDevs WP Locker

Use PureDevs WP Locker to hide your dev site from general visitors and known bots.

loftocean

Loft Maintenance is a small plugin to help you lock down your site quickly whenever your site is down for maintenance or currently in development. You can choose to enable the maintenance mode, then set the maintenance page which will show to all visitors, except the site administrators. And you can also choose some other public pages. The site administrators still have the full privileges to view the whole site without any restriction (front end and back end) even in maintenance mode.
PS: The plugin does not create a maintenance page for your site. You need to create a page and add content to it to let your visitors know the site is under construction. By working with our another WP plugin <a href="https://wordpress.org/plugins/loftbuilder" rel="ugc">LoftBuilder</a>, you can easily create a beautiful page in minutes.
<h4>Features & Options</h4>
<ul>
<li>Turn on or off the maintenance mode.</li>
<li>Set any existing page as the maintenance page.</li>
<li>Choose other public pages.</li>
<li>Site administrators have the full privileges to view the whole site.</li>
</ul>

Loft Maintenance

A toolkit to help you lock down your site quickly when your site is not ready to go public.

PureDevs WP Locker vs Loft Maintenance