503 Error, first byte timeout. Which Backend?

[Jason Price]

So, I have backends which are occasionally slow.  So varnish responds with
a 503 error.

How can I determine WHICH backend was used, which failed to respond in a
timely manner?

In varnishlog, I see errors like the following:

  180 FetchError   c http first read error: -1 11 (Resource temporarily
unavailable)

Reading the code, the -1 SEEMS to indicate that the backend closed the
connection on this error.  vcl_error doesn't give me access to beresp
variables (like beresp.backend.name or IP, which would answer the
question).  This indicates that obj.* does NOT include a backend
indication...<https://www.varnish-cache.org/docs/3.0/reference/vcl.html#variables>

Any way I can figure out which backend is failing me?

--Jason
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