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Old 01-15-2018, 04:54 AM   #1
hoes
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Understanding dns and dhcp interaction on disconnect


Dear all,

I am working with a set of computers that share the same hostname.
Regulary, I am testing multiple of these computers on the same network on after another.

I sometimes notice that I cannot connect to the newly connected computer, because the local DNS record seems to point to the computer that was previously connected.

This let me to wonder what kind of caching exists for dns records.
Do name servers have a cache or are they always up-to-date?
I know my linux host has a cache. Is its time configurable or related to the TTL of a dns record?

Also could it happen that a DHCP lease would expire, while a dns cache still points to this IP?
I also had some difficulty finding the default DHCP lease time.
Could somebody also point me to a good source for that?

I hope you can give me some insight in these interactions.
It will surely help me a lot to know what is actually going on instead of just waiting for some cache somewhere.

Best wishes,
Hugo
 
Old 01-16-2018, 08:20 AM   #2
michaelk
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I don't claim to be an expert but most of the answers depend on what LOCAL DNS and DHCP server you are running and how they are configured.

The DNS can cache information as well as the client. While the DNS cache uses TTL the arp cache has its own timeout value. I run dnsmasq which sets local TTL at 64 and I believe the default arp cache is 60 seconds.

When the computer shuts down the DHCP server should release the address but will not reuse it until the lease expires or if necessary due to running out of unused addresses. Default lease times for SOHO routers are typically in days.
 
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