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That tutorial was written for Ubuntu 12.04. What OS are you running? There have been some changes in more recent versions of Apache. As of Ubuntu 14.04 disk_cache.conf is now cache_disk:
These modules have been removed: mod_authn_default, mod_authz_default, mod_mem_cache. If you were using mod_mem_cache in 2.2, look at mod_cache_disk in 2.4.
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