"CACHE_URL",
"DOCUMENT_ROOT",
"HTTP_URL",
"HTTP_HOST",
"PATH_INFO",
"PATH_TRANSLATED",
"QUERY_STRING",
"REQUEST_FILENAME",
"REQUEST_URI",
"SCRIPT_FILENAME",
"SCRIPT_NAME",
"SCRIPT_TRANSLATED",
"UNENCODED_URL",
"URL",
"URL_PATH_INFO",
"APP_POOL_ID",
"APPL_MD_PATH",
"APPL_PHYSICAL_PATH",
"GATEWAY_INTERFACE",
"SERVER_SOFTWARE",
"SSI_EXEC_DISABLED"
link: http://www.iis.net/learn/extensions/url-rewrite-module/url-rewrite-module-configuration-reference