Help needed: Java Socket and close detection

[Chris Hegarty]

Some further reading…
  https://docs.oracle.com/javase/8/docs/technotes/guides/net/articles/connection_release.html

-Chris.

> On 13 Jul 2016, at 21:37, Dmitry Samersoff <dmitry.samersoff at oracle.com> wrote:
> 
> Christoph,
> 
> My $0.2
> 
> Typically you see RST packet when the data come to a *closed* socket.
> You shouldn't get RST if client/server communication shutdown properly.
> 
> Also balancer may take a care about connection shutdown (it needs to
> update internal tables) so it's possible that you never get RST after
> balancer.
> 
> Moreover, there are RST based attack and scanning methods so firewall
> might be set to reject RST packets and doesn't pass it to inside network.
> 
> I.e. IMHO, customer server should be updated to live without RST.
> 
> -Dmitry
> 
> On 2016-07-13 18:29, Langer, Christoph wrote:
>> Hi folks,
>> 
>> 
>> 
>> I have a question to the experts - regarding an issue that was reported
>> to me by a customer.
>> 
>> 
>> 
>> In the customer scenario they are running a Servlet engine and the
>> Servlet is constantly sending data to a browser client. When the browser
>> client is closed, the server does not get a notification of the other
>> end having been terminated and is constantly sending out data and
>> blocking an application thread. I’m under the assumption that the server
>> should get an RST packet from the network upon writing/flushing data to
>> the OutputStream as soon as the client is gone and hence an Exception
>> should pop up but this isn’t happening.
>> 
>> 
>> 
>> There is a load balancer and maybe other network infrastructure involved
>> in between the Servlet JVM and the browser client. We did some TCPDUMP
>> tracing at the load balancer and we could not see an RST packet coming
>> in from the client side. But when I’m running the scenario without all
>> this network infrastructure involved, e.g. between servers and clients
>> in the same network, I would always observe an RST packet once I close
>> the browser. A FIN packet is received, too, but this does not lead to an
>> Exception and to all my knowledge this can’t be detected, not from the
>> java Socket API and even less from the Servlet API which is just dealing
>> with Streams.
>> 
>> 
>> 
>> So my question to the experts is most of all: Would you agree that an
>> RST packet should be generated in the network and received by the
>> server? Or is it a normal behavior that servers must deal with not
>> receiving RSTs and hence needing to wait for a timeout until e.g. the
>> load balancer generates an RST? Also, is there any way to detect a FIN
>> in the JVM and react on it?
>> 
>> 
>> 
>> Thanks in advance and best regards
>> 
>> Christoph
>> 
>> 
>> 
> 
> 
> -- 
> Dmitry Samersoff
> Oracle Java development team, Saint Petersburg, Russia
> * I would love to change the world, but they won't give me the sources.