DNS Server using heavily cpu

[cabe49]

I have a 1921 router, which are being used as DNS server.

Over time the DNS cache reaches typically around 4-5000 entries, and the process begins to use cpu heavily around 95 %.
As an intermediate solution I have made a EEM script which monitors SYS-3-CPUHOG in the log, and do a

clear host *.

But why is this happen suddenly - the router have been run ning for several years without problems.

Thanks for your time.

[pieterh]

"But why is this happen suddenly - the router have been running for several years without problems."

this could be out of your control!
if a number of the external DNS servers  changed their TTL of frequently queried records to a low interval (5min instead or 8hours)
your local query is not resolved from cache, but records needs to be refreshed from outside DNS server almost every new query

which results in additional load on you router

[Joseph W. Doherty]

@pieterh a very interesting possibility.  A possible example of an external change, which I also mentioned (i.e. external changes) might be the cause of the suddenly new issue.

Something similar, if clients aren't caching their DNS lookups for as long, either.

Personally, from what OP has described, I suspect it has more to do with volume of entries in router's DNS cache, then additionally querying.  (I.e. more like a memory leak situation.)  Reason I think this, it seems to take some time for problem to develop.  If it were volume of queries, I would expect it to occur more frequently and/or vary, during the day, as usage activity varies.  (Of course, I might be misreading/misunderstanding what OP has described.)

[cabe49]

Hi all,

I ended up with using public DNS servers instead, and disabled DNS on my router.

Thanks all for your contributions.