Apache Solr does not have any security feature by its own, either at the document level or the communication level.
The simplest solution is the use of a firewall but this is not possible if you are using Solr as a Tomcat application. In this case you could want to require authentication only when accessing the entire Solr application or only some part of it, such as admin or update.

Let's see how to implement a basic authentication mechanism. We have to modify only two files: conf/tomcat-users.xml, inside the main folder of your Tomcat server, and WEB-INF/web.xml in your solr war file.

In tomcat-users.xml add:

<role rolename="yourRole"/>
<user username="yourUser" password="yourPassword" roles="yourRole"/>


In web.xml you have to specify the security restriction for your application:

  <security-constraint>
    <web-resource-collection>
      <web-resource-name> 
        Solr authenticated application
      </web-resource-name>

      <url-pattern>/*</url-pattern>
      <http-method>GET</http-method>
      <http-method>POST</http-method>
    </web-resource-collection>
    <auth-constraint>
      <role-name>yourRole</role-name>
    </auth-constraint>
  </security-constraint>
   <login-config>
    <auth-method>BASIC</auth-method>
    <realm-name>Basic Authentication</realm-name>
  </login-config>
  <security-role>
    <description>My role</description>
    <role-name>yourRole</role-name>
  </security-role>

In this example we protect the entire Solr application, but you can change the url-pattern node to protect only a part of Solr.

You could need to modify your client or your crawler to send the authentication header.
For example if you use PHP you have to send authentication parameters:

file_get_contents('http://yourUser:yourPassword@www.yourdomain.com:8080/solr/select/?q='.$query);