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I have DNS server running which caches for records from a main DNS server.

What would happen when the main DNS server is down? Will the cache DNS server still serve the records it has or will all the records in the cache server expire after TTL of respective domains?

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  • Which DNS server? Have you looked in its documentation for this question? Feb 1, 2014 at 16:35
  • @AndrewSchulman: I'm guessing bind since that's a tag on the question.
    – Bill Weiss
    Feb 1, 2014 at 16:45
  • BIND9 My bad. forgot to mention. Feb 1, 2014 at 16:46

2 Answers 2

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The cache will stop serving those records after the TTL expires. This is why multiple name servers for your zones are important :)

There's also an expire time in the SOA, but that's only for secondary/slave nameservers.

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When the authoritative server (an authoritative DNS server is what you call the "main" DNS server in your question; it establishes what IP, say, some-name.example has) goes down, the caching server (a DNS server that gets its information from authoritative servers) will only have in its cache names that have recently been requested.

So, supposing that your authoritative server has a TTL of 1 day for its records, when the auth server goes down, any records retrieved in the last day will be around for under one day and then stop working. Any records not fetched in the last day will not work.

The purpose of DNS caching is simply to speed up resolution of names. DNS' way of making records reliable is by using something called a "DNS zone transfer", where the "main" server becomes what is called a "master" server in DNS. The one sets up a second "slave" DNS server that periodically gets its DNS record from the "master". More information: http://www.microhowto.info/howto/configure_bind_as_a_slave_dns_server.html

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