The effects of short DHCP leases with long DNS scavenging times

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.

1. 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.)
2. DNS Scavenging is set to the default of 7 days for both the No-refresh interval and Refresh interval. 
3. Client A connects to the network gets IP address 10.1.1.100. 
4. Client A registers with DNS. The person using Computer A goes on a 1 week vacation leaving Computer A turned off.
5. Three Days later Client B's computer is turned on after being off for 4 days.  It gets IP address 10.1.1.100 from the DHCP server.
6. Client B cannot remove Client A's entry so it registers with DNS and creates a duplicate DNS entry.

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 for these three settings are:

1. DHCP lease - 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.
2. Set No-refresh scavenging at 1/2 of the DHCP lease (no less than 1 day).
3. 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 than 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.

Happy DHCPing and DNSing!

Lance Caven

IT Software Support - Why does it suck so?

Don't you hate it when:  You have a problem with your computer software and when you contact the company that supports the software they insinuate or indicate that you are to blame for the problem?

I have had a recent issue where the company I was working with didn't believe that I had a real problem.  The support department had been working on my problem for 3 weeks and I couldn't get the issue escalated to the developers.  The people in the support dept had never seen the issue before, but the developers didn't believe that the issue I was having was possible and would not assist.

The attitude that the issue is not possible, is one of my pet peeves and an attitude that no software company should ever have.  There is always an explanation for a customers software issue.  Covering your eyes and ears to your customers issues only make a new customer for your competitor.    


Unfortunately for many good software products, technical support is terrible and sales dip or dry up. 

Good customer support keeps customers allows for additional sales to them and creates new customers.

Here is my short list of what characteristics a company needs to perform above average support. 

Exhibit a "get things done attitude" - be the customers advocate internally.  Good support organizations have the ability to escalate issues quickly and get others involved as needed without multiple layers of approvals

Patience with new customer questions.  New customers don't know that the question has been asked hundreds of times. 

Quickly getting to a root cause of a problem.  This makes customers happy (not waiting for the issue to be diagnosed)

Automate support where it makes sense. I don't mean the 25 level automated support tree so that you know a customer has x version of the software running on y hardware.  Automate with the following goals. 

• Getting data to the right person faster
•  Making it easier for the customer to help themselves
• Being able to data mine to identify subtle problems in the software

Comments?  Make them here.

I published a SQL server deck about data and SQL Server

 A link to my discussion about how SQL Server interacts with data.


How to Optimize SQL Server’s Interaction with Your Data

Let me know if this is helpful to anyone.

Lance

Your computer is gone, now what?

What do you mean your computer is gone?  You have had a major problem and the computer doesn't work or maybe it is as simple as someone stole your laptop during the night.

You have a full backup of your data right? No? that is not surprising according to a recent blog that I read only 10% of people perform a daily backup.  http://blog.backblaze.com/2012/06/12/10-now-back-up-daily-90-to-go/


What now? You have had a major data loss.  You will be distracted for awhile, this issue with cause you to spend time recovering and/or recreating critical data and hinder new work from occurring.

The good news:  You can recover some of your data without recreating it from scratch.  Look at each of your email accounts.  Your sent and received mail will have some of the data you have sent to other people. The data may not be 100 percent current, but will prevent you from recreating the data totally from scratch.

If you had a hardware failure, sometimes (very dependent on the type of failure that has occurred) data can be recovered by taking out the hard-drive from the failed computer, attaching the old hard-drive to  a new computer and recovering data.

Do you use Google Drive, Drop-box or Microsoft SkyDrive?  You can get your data back from those cloud based storage solutions. 

Lost photos?   This is one time where Facebook, friends and family are very helpful in getting photos of your vacation or kids back to your computer.

Missing music?  If you use Google play or the Amazon cloud player your music can be downloaded to your new device.  I don't use iTunes, but have heard that iCloud is worth looking at to prevent data loss


Full disclosure:  I backup my data 2 times per day.  I work for a company that provides solutions for small/mid size businesses to backup all of their data securely, on-site and off-site.  I have helped restore data for disasters big and small.

Most of the recovery issues I work on relate to human error, malicious files of some sort, hardware failure or theft of equipment.  I have helped members of my own family recover data.  My son learned the hard way when the only copy of a file is on a memory stick and the memory stick gets washed, the data is forever gone and the homework has to be re-created from scratch.

What should you do now before losing computer data?  You will eventually lose data

With the advent of more and more cloud based computing, store your data in the cloud.  Office 365, SkyDrive, Drop-box, Google Drive, etc.  If you are a remote user for a corporation, make an effort to copy your files to the designated location advocated by your IT department. 

I like to have data on-site in a NAS device to allow me to manually put files on another computer system.  (This is not true backup but allows a second copy of data in many cases which is good) I use a QNAP NAS with 4 hard drives installed.  This device has tons of functionality more than I use and allows me to have iTunes, files, photos etc all accessible from one location.  This device also allows me to access files from my phone while on the road.  (some minor firewall work did need to be performed for this feature)

I also advocate true backups and keeping them off-site.  The easiest in my opinion are cloud based from companies like backblaze for end users.  You can also sign up for the one that my company hosts at DeltaWare Data Solutions.  We have customers primarily in the Midwest USA, but the great thing about cloud backup is that distance is not an issue. 

Your backup/recovery solution could be as simple as buying 2 external hard drives and using the no cost automated backup in Windows 7/8.  You would backup to one of the hard drives while keeping the other hard drive in a different location.  Swap the 2 hard drive locations about 1 time per week. 

The key is to create a solution that works for you and doesn't take much time or effort.  The day is coming when you will lose data, be prepared for it.