high availability configuration error?

Mark Andrews Mark_Andrews at isc.org
Tue Feb 27 21:49:37 UTC 2007

> Hi all!
> what do you think about the following configuration for high
> availability of web servers?
> In the domain zone the filed www has "nslp" as name server and nslp
> has 2 A field
> www        NS           nslp
> nslp       A            ip1
>            A            ip2
> so if a client tries to resolve www and the ip1 is down, the the
> client should use the next ip2.
> Unluckly this configuration does not work because the client uses only
> ip1 and never ip2.
> Do you have suggestions?
> Thanks
> Marco

	Fix the client.  It's not like multihomed machines have been
	around a long time :-)   RFC 1123 gave guidance on this issue
	nearly 18 years ago.


Network Working Group                    Internet Engineering Task Force
Request for Comments: 1123                             R. Braden, Editor
                                                            October 1989

       Requirements for Internet Hosts -- Application and Support

   2.3  Applications on Multihomed hosts
      When the remote host is multihomed, the name-to-address
      translation will return a list of alternative IP addresses.  As
      specified in Section, this list should be in order of
      decreasing preference.  Application protocol implementations
      SHOULD be prepared to try multiple addresses from the list until
      success is obtained.  More specific requirements for SMTP are
      given in Section 5.3.4.
      When the local host is multihomed, a UDP-based request/response   
      application SHOULD send the response with an IP source address    
      that is the same as the specific destination address of the UDP
      request datagram.  The "specific destination address" is defined
      in the "IP Addressing" section of the companion RFC [INTRO:1].
      Similarly, a server application that opens multiple TCP
      connections to the same client SHOULD use the same local IP
      address for all.

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews at isc.org

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