Sometime My WLC 5520 cannot Ping DNS authoritative

[Mr dum]

Hello Friend's
I really need help for this problem

Sometimes the client of WIFI it connect to Internet and sometimes it cannot connect to Internet
and when the client cannot connect to Internet from WLC 5520 CLI I cannot ping the DNS Server but sometimes I can ping the DNS Server 

I'am using WLC 5520 and running Software version 8.5.151.0

Mybe someone can explain me 
why my WLC sometimes got the problem like that
and this I will show you my general setting my WLC 

I'am Using Management Port for the Access Point
And Now My WLC connect to Cisco 3850 LAG enable with 10G 

[marce1000]

 - As per https://www.cisco.com/c/en/us/support/docs/wireless/wireless-lan-controller-software/200046-tac-recommended-aireos.html  
                        Upgrade to https://software.cisco.com/download/home/286284738/type/280926587/release/8.10.190.0
  And try again , 

 M.

[Mr dum]

he problem from software WLC ?

[marce1000]

  - Possibly yes  ; you are running an old version for the particular WLC model. Also as the aireos based controller platforms are getting old nowadays it is always advised to use the latest/last version that they can run , 

 M.

[Mr dum]

for this situation 
could I updated the software version from 8.5.151 to 8.10.190 for free ??

[marce1000]

       >...could I updated the software version from 8.5.151 to 8.10.190 for free ??
 - No, you need a service contract on your WLC in order to be able to download software for it , 

 M.

[Rich R]

Software upgrades normally require a Cisco service contract.

If you do not have a contract then refer to https://www.cisco.com/c/en/us/support/docs/csa/cisco-sa-wlc-auth-bypass-JRNhV4fF.html and read the section "Customers Without Service Contracts".
Then email TAC quoting the URL of that advisory, quoting that section of text, quoting the URL of the software you require eg: https://software.cisco.com/download/home/286284738/type/280926587/release/8.10.190.0 and provide the serial number of your WLC.
Then TAC should publish the software for you.

[Haydn Andrews · Accepted solution]

Also a good check to test from the VLAN the WLC is dropping the client into to see if DNS is responding. Could be the DNS server is having issues

[Mr dum]

thankyou for your respons @Haydn Andrews 
Iam already test vlan from my laptop directly to the switch and connection no issue
maybe I can cek for bug the Software version 

[Rich R]

There are hundreds of bug fixes since 8.5.151.0!
You can read the release notes for each successive release to see the major fixes.  Note that the release notes do not list every fix - only those which are severity 1, 2 and selected severity 3 bugs.
https://www.cisco.com/c/en/us/support/wireless/wireless-lan-controller-software/products-release-notes-list.html
Look for Fixed Issues or Resolved Caveats.
For more detail on specific fixes in each release you can search on the Bug Search Tool.