Sibling cache returns 504 when accessing cached objects

[Tomas Mozes <hydrapolic@xxxxxxxxx>]

Hello,
I had a squid (3.5.15) instance (named t1) in reverse-proxy mode:

http_port 80 accel
cache_peer up.example.com parent 80 0 default no-query no-digest no-netdb-exchange originserver name=origin

It works great, no problems so far.

Now I wanted to add an instance (named t2), having both in sibling relation (cache hierarchy):

on t1:
http_port 80 accel
cache_peer up.example.com parent 80 0 default no-query no-digest no-netdb-exchange originserver
cache_peer t2 sibling 80 4827 htcp no-digest

on t2:
http_port 80 accel
cache_peer up.example.com parent 80 0 default no-query no-digest no-netdb-exchange originserver
cache_peer t1 sibling 80 4827 htcp no-digest

Since t1 had many files cached, t2 started pulling files from t1, but sometimes, the following error occurs.

Client request from t2:
GET /test.zip HTTP/1.1
User-Agent: Wget/1.17.1 (linux-gnu)
Accept: */*
Accept-Encoding: identity
Host: cache.example.com
Connection: Keep-Alive

t2 asks t1 via htcp:
GET http://cache.example.com/test.zip
User-Agent: Wget/1.17.1 (linux-gnu)
Accept: */*
Accept-Encoding: identity
Host: cache.example.com
Via: 1.1 t2 (squid/3.5.15)
Surrogate-Capability: t2="Surrogate/1.0"
X-Forwarded-For: 1.2.3.4
Cache-Control: max-age=259200

t1 replies to htcp with:
Age: 782932
Expires: Sat, 01 Oct 2016 09:07:09 GMT
Last-Modified: Sun, 15 Nov 2015 00:09:20 GMT
Cache-to-Origin: cache.example.com 1 0.001000 1

then t2 requests the file from t1 via http:
GET http://cache.example.com/test.zip HTTP/1.1
User-Agent: Wget/1.17.1 (linux-gnu)
Accept: */*
Accept-Encoding: identity
Host: cache.example.com
Via: 1.1 t2 (squid/3.5.15)
Surrogate-Capability: t2="Surrogate/1.0"
X-Forwarded-For: 1.2.3.4
Cache-Control: max-age=259200, only-if-cached
Connection: keep-alive

and t1 replies with 504:
HTTP/1.1 504 Gateway Timeout
Server: squid/3.5.15
Mime-Version: 1.0
Date: Wed, 13 Apr 2016 10:36:01 GMT
Content-Type: text/html;charset=utf-8
Content-Length: 4182
X-Squid-Error: ERR_ONLY_IF_CACHED_MISS 0
Vary: Accept-Language
Content-Language: en
Age: 1460543762
Warning: 113 t1 (squid/3.5.15) This cache hit is still fresh and more than 1 day old
Warning: 110 squid/3.5.15 "Response is stale"
Warning: 111 squid/3.5.15 "Revalidation failed"
X-Cache: HIT from t1
X-Cache-Lookup: HIT from t1:80
Via: 1.1 t1 (squid/3.5.15)
Connection: keep-alive

I checked upstream (up.example.com) and there was no connection attempt. Tried with ICP/HTCP, same result. I'm not sure, is this bug http://bugs.squid-cache.org/show_bug.cgi?id=4223?

Thanks,
Tomas Mozes

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