아래 두 문서는 꼭 읽어두세요.
http://httpd.apache.org/docs/1.3/mod/mod_setenvif.html#setenvif
http://httpd.apache.org/docs/1.3/mod/core.html#allowoverride

USE: server config, virtual host, directory, .htaccess

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AllowOverride FileInfo
SetEnvIF Referer  nehome.net local_referral
<FilesMatch ".(gif|jpg|jpeg|png)$">  
    Order deny,allow  
    allow from env=local_referral
    deny from all  
    ErrorDocument 403 "Don't link to my images."  
</FilesMatch>
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