[squid-users] forward_max_tries 1 has no effect

Tue Nov 28 21:27:44 UTC 2017

Thanks Alex, this is very helpful.

Another interesting fact is that I can't reproduce this issue if squid has
no other traffic except my testing requests. But it's easy to reproduce
when server has other traffic.

The problem is that with other traffic I can't provide the whole log file
with debug ALL,7 enabled because it has other requests.

So I tried to select only parts related to my test request (this is ALL,7):

https://www.dropbox.com/s/udzeipeerf5o38t/squid_retry_logs.tgz?dl=1

On Tue, Nov 28, 2017 at 7:32 AM, Alex Rousskov <
rousskov at measurement-factory.com> wrote:

> On 11/27/2017 05:19 PM, Ivan Larionov wrote:
>
> > I see retries only when squid config has 2 parents. If I comment out
> > everything related to "newproxy" I can't reproduce this behavior anymore.
>
> The posted logs are not detailed enough to confirm or deny that IMO, but
> I suspect that you are dealing with at least one bug.
>
>
> > https://wiki.squid-cache.org/SquidFaq/InnerWorkings#When_
> does_Squid_re-forward_a_client_request.3F
> >
> >> Squid does not try to re-forward a request if at least one of the
> following conditions is true:
> >>
> >> The number of forwarding attempts exceeded forward_max_tries. For
> >> example, if you set forward_max_tries to 1 (one), then no requests
> >> will be re-forwarded.
>
>
> AFAICT, there is an off-by-one bug in Squid that violates the above:
>
> >     if (n_tries > Config.forward_max_tries)
> >         return false;
>
> The n_tries counter is incremented before Squid makes a request
> forwarding attempt. With n_tries and Config.forward_max_tries both set
> to 1, the quoted FwdState::checkRetry() code will not prevent
> re-forwarding. There is a similar problem in FwdState::reforward(). This
> reasoning needs confirmation/testing.
>
> Please note that simply changing the ">" operator to ">=" may break
> other things in a difficult-to-detect-by-simple-tests ways. The correct
> fix may be more complex than it looks and may involve making policy
> decisions regarding forward_max_tries meaning. The best fix would remove
> checkRetry() and reforward() duplication. This code is difficult to work
> with; many related code names are misleading.
>
>
> >> Squid has no alternative destinations to try. Please note that
> >> alternative destinations may include multiple next hop IP addresses
> >> and multiple peers.
>
> The fact that Squid sends two requests to the same peer with only one
> peer address selected suggests that Squid is retrying a failed
> persistent connection rather than re-forwarding after receiving a bad
> response. Again, the logs are not detailed enough to distinguish the two
> cases. I can only see that a single peer/destination address was
> selected (not two), which is correct/expected behavior. I cannot see
> what happened next with sufficient detail.
>
> Going forward, you have several options, including:
>
> A. Post a link to compressed ALL,7+ logs to confirm bug(s).
> B. Fix the broken condition(s) in FwdState. See above.
>
> HTH,
>
> Alex.
>
>
> > 2017/11/27 15:53:40.542| 5,2| TcpAcceptor.cc(220) doAccept: New
> connection on FD 15
> > 2017/11/27 15:53:40.542| 5,2| TcpAcceptor.cc(295) acceptNext: connection
> > on local=0.0.0.0:3128 <http://0.0.0.0:3128> remote=[::] FD 15 flags=9
> > 2017/11/27 15:53:40.543| 11,2| client_side.cc(2372) parseHttpRequest:
> > HTTP Client local=127.0.0.1:3128 <http://127.0.0.1:3128>
> > remote=127.0.0.1:53798 <http://127.0.0.1:53798> FD 45 flags=1
> > 2017/11/27 15:53:40.543| 11,2| client_side.cc(2373) parseHttpRequest:
> > HTTP Client REQUEST:
> > ---------
> > GET http://HOST:12345/ HTTP/1.1
> > Host: HOST:12345
> > User-Agent: curl/7.51.0
> > Accept: */*
> > Proxy-Connection: Keep-Alive
> >
> >
> > ----------
> > 2017/11/27 15:53:40.543| 85,2| client_side_request.cc(745)
> > clientAccessCheckDone: The request GET http://HOST:12345/ is ALLOWED;
> > last ACL checked: localhost
> > 2017/11/27 15:53:40.543| 85,2| client_side_request.cc(721)
> > clientAccessCheck2: No adapted_http_access configuration. default: ALLOW
> > 2017/11/27 15:53:40.543| 85,2| client_side_request.cc(745)
> > clientAccessCheckDone: The request GET http://HOST:12345/ is ALLOWED;
> > last ACL checked: localhost
> > 2017/11/27 15:53:40.543| 17,2| FwdState.cc(133) FwdState: Forwarding
> > client request local=127.0.0.1:3128 <http://127.0.0.1:3128>
> > remote=127.0.0.1:53798 <http://127.0.0.1:53798> FD 45 flags=1,
> > url=http://HOST:12345/
> > 2017/11/27 15:53:40.543| 44,2| peer_select.cc(258) peerSelectDnsPaths:
> > Find IP destination for: http://HOST:12345/' via 127.0.0.1
> > 2017/11/27 15:53:40.543| 44,2| peer_select.cc(280) peerSelectDnsPaths:
> > Found sources for 'http://HOST:12345/'
> > 2017/11/27 15:53:40.543| 44,2| peer_select.cc(281) peerSelectDnsPaths:
> >  always_direct = DENIED
> > 2017/11/27 15:53:40.543| 44,2| peer_select.cc(282) peerSelectDnsPaths:
> >   never_direct = ALLOWED
> > 2017/11/27 15:53:40.543| 44,2| peer_select.cc(292) peerSelectDnsPaths:
> >     cache_peer = local=127.0.0.3 remote=127.0.0.1:18070
> > <http://127.0.0.1:18070> flags=1
> > 2017/11/27 15:53:40.543| 44,2| peer_select.cc(295) peerSelectDnsPaths:
> >       timedout = 0
> > 2017/11/27 15:53:40.543| 11,2| http.cc(2229) sendRequest: HTTP Server
> > local=127.0.0.3:57091 <http://127.0.0.3:57091> remote=127.0.0.1:18070
> > <http://127.0.0.1:18070> FD 40 flags=1
> > 2017/11/27 15:53:40.543| 11,2| http.cc(2230) sendRequest: HTTP Server
> > REQUEST:
> > ---------
> > GET http://HOST:12345/ HTTP/1.1
> > User-Agent: curl/7.51.0
> > Accept: */*
> > Host: HOST:12345
> > Cache-Control: max-age=259200
> > Connection: keep-alive
> >
> >
> > ----------
> >
> > [SKIPPED 40 seconds until parent closes TCP connection with FIN,ACK]
> >
> > 2017/11/27 15:54:20.627| 11,2| http.cc(1299) continueAfterParsingHeader:
> > WARNING: HTTP: Invalid Response: No object data received for
> > http://HOST:12345/ AKA HOST/
> > 2017/11/27 15:54:20.627| 17,2| FwdState.cc(655)
> > handleUnregisteredServerEnd: self=0x3e31838*2 err=0x409b338
> > http://HOST:12345/
> > 2017/11/27 15:54:20.627| 11,2| http.cc(2229) sendRequest: HTTP Server
> > local=127.0.0.3:41355 <http://127.0.0.3:41355> remote=127.0.0.1:18070
> > <http://127.0.0.1:18070> FD 40 flags=1
> > 2017/11/27 15:54:20.627| 11,2| http.cc(2230) sendRequest: HTTP Server
> > REQUEST:
> > ---------
> > GET http://HOST:12345/ HTTP/1.1
> > User-Agent: curl/7.51.0
> > Accept: */*
> > Host: HOST:12345
> > Cache-Control: max-age=259200
> > Connection: keep-alive
> >
> >
> > ----------
> >
> > [SKIPPED 40 seconds again until parent closes TCP connection with
> FIN,ACK]
> >
> > 2017/11/27 15:55:00.728| ctx: enter level  0: 'http://HOST:12345/'
> > 2017/11/27 15:55:00.728| 11,2| http.cc(719) processReplyHeader: HTTP
> > Server local=127.0.0.3:41355 <http://127.0.0.3:41355>
> > remote=127.0.0.1:18070 <http://127.0.0.1:18070> FD 40 flags=1
> > 2017/11/27 15:55:00.728| 11,2| http.cc(720) processReplyHeader: HTTP
> > Server REPLY:
> > ---------
> > HTTP/1.0 502 Bad Gateway
> > Cache-Control: no-cache
> > Connection: close
> > Content-Type: text/html
> >
> > <html><body><h1>502 Bad Gateway</h1>
> > The server returned an invalid or incomplete response.
> > </body></html>
> >
> > ----------
> > 2017/11/27 15:55:00.728| ctx: exit level  0
> > 2017/11/27 15:55:00.728| 20,2| store.cc(996) checkCachable:
> > StoreEntry::checkCachable: NO: not cachable
> > 2017/11/27 15:55:00.728| 20,2| store.cc(996) checkCachable:
> > StoreEntry::checkCachable: NO: not cachable
> > 2017/11/27 15:55:00.728| 88,2| client_side_reply.cc(2073)
> > processReplyAccessResult: The reply for GET http://HOST:12345/ is
> > ALLOWED, because it matched (access_log stdio:/var/log/squid/access.log
> > line)
> > 2017/11/27 15:55:00.728| 11,2| client_side.cc(1409) sendStartOfMessage:
> > HTTP Client local=127.0.0.1:3128 <http://127.0.0.1:3128>
> > remote=127.0.0.1:53798 <http://127.0.0.1:53798> FD 45 flags=1
> > 2017/11/27 15:55:00.728| 11,2| client_side.cc(1410) sendStartOfMessage:
> > HTTP Client REPLY:
> > ---------
> > HTTP/1.1 502 Bad Gateway
> > Date: Mon, 27 Nov 2017 23:54:20 GMT
> > Cache-Control: no-cache
> > Content-Type: text/html
> > X-Cache: MISS from ip-172-23-18-130
> > X-Cache-Lookup: MISS from ip-172-23-18-130:3128
> > Transfer-Encoding: chunked
> > Connection: keep-alive
> >
> >
> > ----------
>

-- 
With best regards, Ivan Larionov.
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