Why does nginx sometimes send Connection: close to Connection: keep-alive requests?

This may be a bit difficult to explain but I will try my best:

We have the following setup:

[ BOX 1 : NGINX Frontend ] —reverse-proxy—> [ BOX 2: NGINX Backend
—>
PHP-FPM ]

Upstream keepalives are enabled on BOX 1 as follows:

upstream backend{
server 1.2.3.4;
keepalive 512;
}

Keepalives are enabled on BOX 2 as follows:

    keepalive_timeout       86400s;
    keepalive_requests      10485760;

Yes, really high values… BOX 2 never sees any external traffic. Its
all
coming just from the front end (BOX1).

We have noticed, sometimes BOX 2 will return a Connection: close header,
and
leave the connection in TIME_WAIT state EVEN THO the request came with a
Connection: keep-alive header. This is correct behavior if BOX 2 wanted
to
close the connection… But why would it want to?

We have sniffed this info via netstat AND ngrep.

We are 100% sure BOX 2 sometimes sends back a Connection: close header,
and
the connection is left in a TIME_WAIT state. When we run the ngrep
utility
and watch netstat -na | grep TIME_WAIT | grep BOX1IP | etc etc etc… As
soon as a connection:close is sent, the count of TIME_WAIT sockets
increases.

So to summarize:

In what situations would nginx dispatch a Connection: close to a client
who
makes a request with Connection: keep-alive.

Worth nothing:

Generally, the upstream keep alives work… There is a high number of
reqs /
sec happening. These connection: close events happen rarely, but
frequently
enough to warrant this lengthy post :wink:

Thanks!

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,234746,234746#msg-234746

Hello!

On Sun, Jan 06, 2013 at 08:03:05PM -0500, anonymous-one wrote:

    server          1.2.3.4;

soon as a connection:close is sent, the count of TIME_WAIT sockets
sec happening. These connection: close events happen rarely, but frequently
enough to warrant this lengthy post :wink:

Even if you configure nginx to allow infinite keepalive timeout
and lots of requests per connection - it might still need to close
a connection e.g. while returning certain errors like 400 Bad
Request (which might assume connection might be out of sync).
There are also some browser workaround which might disable
keepalive in certain situations, see
http://nginx.org/r/keepalive_disable.

Note that in general keepalive should not be relied on as
something required, it’s just an optimization. If you depend on
connections being kept alive forever and never closed - you’ve
probably did something wrong.


Maxim D.
http://nginx.com/support.html

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