Varnish 503 error

[Ragnar Kurm]

> 12 FetchError   c http first read error: -1 0 (No error recorded)

Jean,

Got into same situation myself and replying mostly for documentation 
purposes,
since your original post was quite some time ago.

Found that Apache was closing keep-alive connection improperly in some cases
(at least this is how I understand keep-alive).

Varnish sent request during keep-alive session,
Apache closed connection in return.

Apache configuration "KeepAlive Off" solved it for me.
For complete details I posted my findings here:
https://www.varnish-cache.org/forum/topic/1468

Ragnar