DNS server process, high CPU usage

Question

Hello,
I am having a problem with high CPU usage on DNS process. We have 1921 setup in network of about 100 workstations that use it as DNS server. It also performs split view functionality for couple of domains in order to avoid hairpin NAT. I know that a router is not fit for heavy server duty but such setup was unfortunately necessary.
In peak work hours we oftenly have CPU hitting 100%  with DNS server process being the root cause,and on several occasions we had DNS process stuck in such state for prolonged periods of time during which other router functions were affected (t's a default gateway, IPsec concentrator etc.). Sometimes shutting the process and restarting it would solve the problem, but once it required a full reload in order to restore functionality (such was the description that engineer on duty reported to me).
Forwarder queue statistics:
Current size = 0
Maximum size = 86
Drops        = 0
IOS upgrade did not help, we are currently running 15.2(2)T.
The question is ... is such behaviour (CPU load) to be expected on LAN with 100 workstations due to slow CPU on router or we have bugged IOS DNS server (requiring a TAC case)?
The previous solution seem to do this functionality quite nicely (even though it was also a router), so I am not inclined to think that we are dealing with someone DoS-ing the DNS (WAN access to DNS is of course forbidden)
P.S.
Since we moved servers off the routers DNS, we do not receive complaints, but we had a couple of unresolved messages a day while mail server was using router for DNS. I am suspecting that an old bug where IOS DNS servers occasionaly sends clients back empty DNS replies (properly formatted message but without A record) could be still around?

Answer 1

hi,
i have the same problem on UC540 and 2911 on IOS 15.0.1. CPU high, router crashed. Reboot helps but until CPU usage spikes again. the only fix is to remove "ip dns server" from config and use a different DNS server
                    111                       1111    1111111111
          999999999900077777777799999777778888000099990000000000
    322222111119999900077776666622222111119999000088880000000000
100            ********                       ******************
 90       *************         *****     **********************
 80       ***************************     **********************
 70       ******************************************************
 60       ******************************************************
 50       ******************************************************
 40       ******************************************************
 30       ******************************************************
 20       ******************************************************
 10       ******************************************************
   0....5....1....1....2....2....3....3....4....4....5....5....6
             0    5    0    5    0    5    0    5    0    5    0
               CPU% per second (last 60 seconds)
after removed IP dns server
                                       111                      
                             99999999990007777777779999977777888
              33333333332222211111999990007777666662222211111999
100                               ********                      
 90                          *************         *****     ***
 80                          ***************************     ***
 70                          ***********************************
 60                          ***********************************
 50                          ***********************************
 40                          ***********************************
 30                          ***********************************
 20                          ***********************************
 10                          ***********************************
   0....5....1....1....2....2....3....3....4....4....5....5....6
             0    5    0    5    0    5    0    5    0    5    0
               CPU% per second (last 60 seconds)