varnish health probe errors

[Giannis Zach]

Hello all,

a successful health probe will log this flags `4--X-RH` and from the
documentation the flags are:
4 -- IPv4 connection established
X -- Request transmit succeeded
R -- Read response succeeded
H -- Happy with the result

The issue I'm facing is that sometimes the health probe returns the flags
`4--X---` marking the backend as sick.

Can someone please explain to me when a varnish health probe will not log
an `R` (Read response succeeded)? Is this related to the health probe's
timeout value? Does this mean that the TCP connection failed before the
backend sent the response? The backend doesn't even log the request in its
access log, so from my understanding, the backend never received that
request, however, varnish says the request was successfully transmitted
(the `X` flag).

Thank you for your time.

-- 
Giannis
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