high availability configuration error?
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?
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
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 18.104.22.168, 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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