Use Nginx basic auth to protect HTTP services like Solr ( http://distinctplace.com/howto/2014/07/16/use-nginx-basic-auth-to-protect-http-services-like-solr/

gistfile1.nginxconf

server
{
  # To avoid changing your app config
  # point Nginx to the Solr port
  listen 8983;

  # Set read/write variables
  set $solr_write "0";
  set $solr_read "1";
  set $solr_admin "0;
  
  # Define users permissions
  # $remote_user translates to Basic Auth user
  if ($remote_user = "solr_r") {
    set $solr_write "0";
    set $solr_read "1";
  }

  if ($remote_user = "solr_w") {
    set $solr_write "1";
    set $solr_read "0";
  }

  if ($remote_user = "solr_rw") {
    set $solr_write "1";
    set $solr_read "1";
    set $solr_admin "1";
  }

  # Force Basic Auth for all requests to Solr
  # IMPORTANT:
  # Make sure to add new users to htpasswd!
  auth_basic "Private Beta";
  auth_basic_user_file /etc/nginx/conf/htpasswd;

  # Write Solr enpoint
  location ~* /solr/\w+/update {
    if ($solr_write != "1") {
      return 403;
    }
    proxy_pass http://localhost:8900; # Proxy to solr
  }

  # Read Solr endpoint
  location ~* /solr/\w+/select {
    if ($solr_read != "1") {
      return 403;
    }
    proxy_pass http://localhost:8900;
  }
  
  # Admin UI
  location ~* /solr {
    if ($solr_admin != "1") {
      return 403;
    }   
    proxy_pass http://localhost:8900;
  }


  # Disable proxy buffering because it was causing problems
  proxy_buffering off;

  # All other urls expect admin permission
  location / {
    proxy_pass http://localhost:8900;
  }
}

Just an FYI, there's a typo in the config at line 10... missing quote... =)