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I did try to research the user agent, but for obvious reasons came up empty handed.
Pretty much the same response as your inquiry in the Apache forum, unless you provide the a "raw visitor log" line, nobody will be able to assist you.
then changed to "YahooCacheSystem" and now has been migrating to "missing"
Is there a purpose for them not identifying themselves...
And why *missing*? Is there a purpose to this particular bot/user agent?