Random “http first read error: EOF” errors

[Hazar Güney]

I see sporadic fetch errors (http first read error: EOF) from backend but
backend is normally healthy. It seems that timeout is not the root issue in
this case. What could be the reason?

*   << BeReq    >> 98808229
-   Begin          bereq 98808228 fetch
-   Timestamp      Start: 1490683823.763272 0.000000 0.000000
-   BereqMethod    GET
-   BereqURL       XXXX
-   BereqProtocol  HTTP/1.1
-   BereqHeader    Pragma: no-cache
-   BereqHeader    Accept: */*
-   BereqHeader    From: bingbot(at)microsoft.com
-   BereqHeader    Host: XXXX
-   BereqHeader    User-Agent: Mozilla/5.0 (compatible; bingbot/2.0;
+http://www.bing.com/bingbot.htm)
-   BereqHeader    Accept-Encoding: gzip
-   BereqHeader    X-Varnish: 98808229
-   VCL_call       BACKEND_FETCH
-   VCL_return     fetch
-   BackendOpen    38 reload_2017-03-20T11:32:44.st2 10.35.78.11 80
172.17.0.2 48388
-   BackendStart   10.35.78.11 80
-   Timestamp      Bereq: 1490683823.763758 0.000487 0.000487
-   *FetchError*     *http first read error: EOF*
-   BackendClose   38 reload_2017-03-20T11:32:44.st2
-   Timestamp      Beresp: 1490683823.764271 0.000999 0.000513
-   Timestamp      Error: 1490683823.764277 0.001005 0.000005
-   BerespProtocol HTTP/1.1
-   BerespStatus   503
-   BerespReason   Service Unavailable
-   BerespReason   Backend fetch failed
-   BerespHeader   Date: Tue, 28 Mar 2017 06:50:23 GMT
-   BerespHeader   Server: Varnish
-   VCL_call       BACKEND_ERROR
-   BereqHeader    X-Varnish-Backend-5xx: 1
-   VCL_return     retry
-   Timestamp      Retry: 1490683823.764294 0.001022 0.000017
-   Link           bereq 97940444 retry
-   End
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