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From Igor Galić <i.ga...@brainsware.org>
Subject Re: [users@httpd] Problem with DNS lookup caching in reverse proxy
Date Fri, 26 Nov 2010 13:00:12 GMT

----- "Slawomir R. Janotta" <s.janotta@aviope.de> wrote:

> Hi everybody,
> I am facing a problem which I cannot solve in a satisfying way and
> after
> searching the known resources for a while I wonder if anyone has ever
> solved a similiar one.
> 
> Let me first describe the situation I have here.
> 
> I am using a reverse proxy installation to access a backend server
> transparently. However, there are in fact two backend machines instead
> of
> one and they are configured in a cold stand-by way meaning only one
> is
> ever running.
> 
> Additionally there is a special dns-resolver which supplies the ip
> address
> of only one of the backend machines depending on which one is running
> at
> the moment during DNS name resolving. This works perfectly as I can
> check
> each time after the switch-over with a simple nslookup command.
> 
> The definition of the reverse proxy clause is like this:
> 
> NameVirtualHost 192.168.100.10:443
> <VirtualHost 192.168.100.10:443>
>     SSLEngine ON
>     SSLProxyEngine ON
>     ProxyPass / https://backend.mydomain.com:8080/
>     ProxyPassReverse / https://backend.mydomain.com:8080/
> </VirtualHost>
> 
> Where 192.168.100.10 is the address of the reverse proxy machine and
> the
> fully qualified name of backend.mydomain.com is the logical name of
> the
> service which the dns-resolver resolves to either one of the backend
> servers depending on their run status.
> 
> When I access the backend service over the reverse proxy the Apache
> server
> running as the reverse proxy caches the IP address of the then
> running
> backend server. Now if I switch the servers, shutting one down and
> bringing the other up and updating the dns-resolver, I can see that
> the
> dns-resolving mechanism works but the reverse proxy still uses the
> cached
> IP address of the now defunct backend server.

Would it be a viable option for you to not use the DNS method, and
instead do something like:

<Proxy balancer://notsohotcluster>
  BalanceMember https://backend1.mydomain.com:8080/
  BalanceMember https://backend1.mydomain.com:8080/ status=+H
</Proxy>

ProxyPass / balancer://notsohotcluster
ProxyPassReverse / balancer://notsohotcluster

This will (probably) simply mitigate the DNS problem.
 
> Regards
> Slawo

i

-- 
Igor Galić

Tel: +43 (0) 664 886 22 883
Mail: i.galic@brainsware.org
URL: http://brainsware.org/

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