Exercise 1: No Trace in Any Log At All — Possible Solution
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THE MOST LIKELY EXPLANATION
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Per this chapter, "if a user reports being unable to log in and
nothing in any log on this server... shows any trace of a request
from them at all, the request most likely never arrived." This points
to Layer 1 - a network/perimeter issue (DNS problem, network routing
issue, or client-side connectivity problem) that prevented the request
from ever reaching this server in the first place.

WHY THE APPLICATION LOGS COULDN'T HAVE SHOWN ANYTHING DIFFERENT
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Per this chapter, "this server's own logs, by definition, cannot
record a request that never reached it." A log - at any layer, access,
error, or application - can only record events the server actually
received and processed. If the request genuinely never arrived, there
is nothing for any log on that server to have captured, no matter how
thoroughly each individual log file is searched.

CONNECTION TO EARLIER MATERIAL
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Per this chapter, this is explicitly tied back to "Chapter 1's own
'silent failure' applies directly" - the complete absence of any log
trace is not proof that nothing happened, it's consistent with a
failure that occurred somewhere the logs on this particular server
were never in a position to observe.

WHY THIS WORKS AS AN ANSWER
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It correctly identifies Layer 1 (network/perimeter, request never
arrived) as the most likely explanation, and correctly explains that
this server's logs are structurally incapable of recording a request
that never reached it, rather than treating the empty logs as some
kind of application-level mystery.