When the proxy is configured to cache requests, it can cache FTP file requests as well as HTTP file requests. However, because FTP files do not contain the same type of header information as HTTP files, expiration dates for cached FTP files are calculated differently than for other cached files.
When a request is made to the FTP server to retrieve a file, the proxy first sends to the FTP server a LIST request for the file to obtain FTP directory information about the file. If the FTP server responds to the LIST request with a positive completion reply and the directory information for the file, the proxy creates an HTTP Last-Modified header with the date parsed from the FTP directory information. The caching function of the proxy then uses this Last-Modified header, together with the value set in the CacheLastModifiedFactor directive in the configuration file, to determine the length of time that the FTP file remains in the cache before expiring.
For more information on how the Last-Modified header and the CacheLastModifiedFactor directive are used to determine the length of time a file remains in the cache, see Maintaining cache content.
FTP files that are retrieved for a specific user ID rather than by anonymous login are considered to be private files and are not cached.