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While attempting to diagnose another issue (which I asked about over here), I discovered that the apache 1.3.37 installation on a machine I inherited doesn't log 404 errors to error_log. I've scoured the config files for CustomLog and ErrorLog directives, but they're all just the standard 'set a path and (for CustomLog) combined format' type.

It does log other errors normally, such as 'Permission denied', but 404s are silently ignored.

What might be causing this and how can I get the 404s to appear (at least temporarily)?

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Have you checked your access.log for the 404? – Nifle Jan 2 '12 at 15:12
@Nifle: Yes, the request shows in access_log with a 404 status. – Dave Sherohman Jan 2 '12 at 15:30

maybe the CustomLog directive is configured (also check .htaccess files) which excludes 404s.

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I turned out that the context of the original issue was relevant here - the problem URL contained the sequence %2F (a URI-encoded forward slash). In an attempt to protect security-oblivious CGI programmers from their own ignorance, apache immediately rejects (as 404 Not found) any requested URL which contains that sequence at a very early stage in the request-handling process, which apparently bypasses any writes to error_log.

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