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ISA server auto discovery - which is first? DHCP or DNS

Posted on 2003-12-06
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Last Modified: 2008-11-18
I'm working on a multi-site, multi ISA server configuration and I'm puzzled about automatic configuration of proxy settings. Microsquish's documentation does not address my issue.

If you select the "Automatcially detect configuration" checkbox, does the DHCP option 252 setting or the DNS wpad record take priority?

I'd like to have one site to use it's local ISA server by setting the DHCP option in it's scope and have the other site use it's local ISA server by setting it's DHCP option.  I'd also like to have the DNS wpad record set to the main site's ISA servers to serve as a fall back.

Will the clients at the non-main site use the DHCP option first (and its the local ISA server) or use the DNS wpad record (and use the main site's ISA server)?
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Question by:Nevaar
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KingHollis earned 250 total points
ID: 9891660
You keep saying "site". Are these genuinely sites or domains or what? The only reason I ask is it makes a difference in answering your question.
1. DHCP will take precedence if configured.
2. With DHCP, you are able to assign ISA servers per scope [effectively per subnet if you wish].
3. With DNS wpad, you are limited to one ISA per domain: wpad.<domain>. So, since your domain could span sites, you would be unfortunately stuck with every computer autoconfigured to use the same ISA server.

So to answer your question, your clients at the non-main site will use the DHCP option first. Your mainsite DNS wpad will only be of use to non-DHCP clients. If your clients for some reason don't get a DHCP address [DHCP server failure], they won't be local to the network [APIPA] and your "fall back" will be of no consequence unless you manually configure all your clients.

Hope this helps.
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by:Nevaar
ID: 9891747
KingHollis,

They are genuinely sites (different subnets at different locations seperated by routers with their own DHCP servers).  Same domain.

Your answer is EXACTLY what I was looking for. Thanks.

BTW, where did you find this tidbit of information?
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by:KingHollis
ID: 9892171
Excellent Nevaar!

A great source for any ISA ish is www.isaserver.org. Also, Thomas W. Shinder writes some good books for Syngress.

Thanks and good luck!
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