[dnsdist] Explanation for "drops" in "showServers()"?

Tom lists at verreckte-cheib.ch
Thu Jun 10 12:27:01 UTC 2021

Hi Remi

Thanks a lot for this clarification.

Our UDP-Timeouts defaults to 2. With the "grepq("2000ms")" command, I 
can see a lot of entries with the mentioned "T.O" (timeout). But I see 
also the following entry with a latency of 2891.8ms which should be 
dropped if UDP, right?:

-12.4   xx.xx.xx.xxx:50105                              zz.zz.zz.zzz:53 
4417  abc.example.com. A     2891.8    RD    No Error. 13 answers

In the documentation I don't see a default-value for tcp-timeout. So 
could the line above be a tcp-request without the default 2000ms 
timeout? If not, why do grepq report this line and/or why does dnsdist 
not drop this query?

Many thanks.

Kind regards,

On 10.06.21 10:16, Remi Gacogne via dnsdist wrote:
> Hi Tom,
> On 6/10/21 8:03 AM, Tom via dnsdist wrote:
>> In the case above, I see 14926 drops from backend01. According the 
>> documentation, the backend server discards these requests. Is there a 
>> way in dnsdist, to see which queries where dropped? What can cause a 
>> backend server to drop requests? For example timeouts while doing name 
>> resolution? Does "drops" mean that dnsdist has not received a response 
>> back from the backendserver within a certain time (e.g. > setUDPTimeout)?
> "Drops" values are indeed the number of queries for which dnsdist did 
> not a response before the setUDPTimeout value. You can see if there was 
> any recent "drops" by looking at the in-memory ring buffers storing a 
> summary of the last queries and responses. For that you need to connect 
> to the dnsdist console and issue a 'grepq("1000ms")' command. It will 
> display queries and responses that took more than 1000ms, so the "drops" 
> should be there with "T.O" in the response time column.
> As for why the backend is not responding fast enough, or at all, to 
> these queries, that needs to be investigated on the backend itself.
> Best regards,

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