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domain name system - Intranet BIND server fallback to internet DNS servers?

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On our local small business network,
we have a need to run a intranet-only BIND (named) DNS server for only local,
intranet-only addresses. For example, we do a lot of web development on the network, so
having a DNS server to manage local addresses (example: testsite3.local) is
beneficial.



One aspect of this we are unsure of
is, currently all the business desktops have their DNS servers set to
75.75.75.75 and 75.75.76.76, which are
Comcast internet DNS servers (Comcast is our business ISP). So if we change the
computer's DNS to point instead to our local DNS server, how do you setup bind to
"forward" all failed requests out to the Comcast DNS
servers?




For example, if someone on
the network trys to visit www.google.com, their computer will
first check with our local DNS server, which obviously doesn't have internet DNS records
in it like google.com. So then, either the computer needs to
know to check with the secondary or tertiary DNS servers OR can the local DNS server
somehow request the info from 75.75.75.75 or
75.75.76.76? Is this something called zone
transfers?



I know that on each individual
computer (OSX in this case) that I can manually set the primary DNS to be the local
server, and then secondary and tertiary servers to be the Comcast DNS servers. But I
wanted to see if there was a better, more manageable way on the server side to do
this.



Answer




DNS clients do not work like this, they do
not "fall back" to the other records. A properly-functioning DNS client should choose a
server at random from its configured list to spread the query load evenly, and have
alternate servers in case one should go
offline.



If you need to serve both internal and
external DNS records then you should have your client machines configured to only use
the internal DNS servers, and configure your internal server to
attempt to resolve any non-locally defined DNS records from the external
server.



The beginning of this would be the href="http://www.zytrax.com/books/dns/ch7/queries.html#allow-recursion" rel="nofollow
noreferrer">allow-recursion directive in BIND. Just be sure that you do
not allow recursive lookups from machines not on your network as
that is how href="https://www.riskanalytics.com/2013/05/23/dns-amplification-attacks/" rel="nofollow
noreferrer">DNS Amplification Attacks happen.



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