The effects of short DHCP leases with long DNS scavenging times

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Do you have the issue where you create a share on a client and cannot  connect to it from another computer?  I have seen many customers with  this issue and many times the solution is a simple one.

 

The issue can be caused by the interaction of DNS entries and how quickly DHCP addresses are renewed.   How do you tell if this is the case?  One simple way is to open up DNS  look for duplicate IP entries.  Look for a single IP address with 2 or  more computers registered to it.

 

How does this happen? Lets use the following example to illustrate one way the issue can occur.

 

  • DHCP has its lease duration set to 2 days.  (Why 2 days?  The site  was running out of usable IP addresses and this was the easiest way to  fix it for the administrator.)
  • DNS Scavenging is set to the default of 7 days for both the No-refresh interval and Refresh interval.
  • Client A connects to the network gets IP address 10.1.1.100.  The person using that computer goes on a 1 week vacation.
  • Client A registers with DNS.
  • Three Days later Client B turns on the PC and gets IP address 10.1.1.100 from the DHCP server.
  • Client B cannot remove Client A’s entry so it registers with DNS and creates a duplicate.

 

Now any other machine on the network could have issues connecting to Client B.  The exact symptoms very widely and are largely dependent on the software loaded in the environment.

 

 

My Rules of Thumb are as follows:

  • DHCP make this a long as you can, but be aware of how many IP  addresses you are consuming, etc.  If the lease is too long you may run  out of available IP’s.
  • Set No-refresh scavenging at 1/2 of the DHCP lease (no less than 1 day.
  • Set Refresh scavenging at 1/2 of the DHCP lease (no less than 1 day).
  • On my network I have DHCP lease set at 7 days.
  • No-refresh scavenging set at 3 days.
  • Refresh scavenging set at 3 days.

 

To fix my example above?  I made the DHCP duration be 5 days, the DNS  scavenging 2 days for both No-refresh and Refresh.  I also made the  wireless network use a different IP range then the wired network.

 

Josh Jones wrote a great blog about Scavenging check it out.   Follow his advice on scavenging.  There is a DNScmd switch, if run  without knowing the consequences, may cause you to either restore from a  good backup or spend hours adding back in static DNS entries.

 

Lance Caven

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