仅仅向google的蜘蛛发送HTTP 503：
Options +FollowSymLinks
RewriteEngine On
RewriteBase /

RewriteCond %{HTTP_USER_AGENT} ^.*(Googlebot|Googlebot|Mediapartners|Adsbot|Feedfetcher)-?(Google|Image)? [NC]
# or RewriteCond %{HTTP_USER_AGENT} ^.*google.* [NC]
RewriteRule .* /cgi-bin/error/503.php
向除了指定ip外的任何来访者发送 503：
Options +FollowSymLinks
RewriteEngine On
RewriteBase /

RewriteCond %{REMOTE_HOST} !^1\.1\.1\.1
RewriteCond %{REQUEST_URI} !^/cgi-bin/error/503\.php [NC]
RewriteRule .* /cgi-bin/error/503.php

向蜘蛛发送 503，其他来访者返回一个 404 页面：
Options +FollowSymLinks
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_USER_AGENT} ^.*(Googlebot|Googlebot|Mediapartners|Adsbot|Feedfetcher)-?(Google|Image)? [NC]
RewriteCond %{REQUEST_URI} !^/cgi-bin/error/503\.php [NC]
RewriteRule .* /cgi-bin/error/503.php

RewriteCond %{REMOTE_HOST} !^1\.1\.1\.1
RewriteCond %{REQUEST_URI} !^/cgi-bin/error/404\.php [NC]
RewriteRule .* /under-development-explain.html [R=302,L]

PHP代码中返回 503：:
ob_start();
header(‘HTTP/1.1 503 Service Temporarily Unavailable’);
header(‘Status: 503 Service Temporarily Unavailable’);
header(‘Retry-After: 3600′);
header(‘X-Powered-By:’);
?>

Service Temporarily Unavailable

The server is temporarily unable to service your
request due to maintenance downtime or capacity
problems. Please try again later.