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

[Tom]

Hi

I'm trying to understand the column "drops" in the "showServers()"-function:

 > showServers()
#   Name                 Address                       State     Qps 
Qlim Ord Wt    Queries   Drops Drate   Lat Outstanding Pools
0   backend01            192.168.1.1:53                  up    76.7 
  0   1  1    5121235   14926   0.0  30.7           6 primary
1   backend02            192.168.1.2:53                  up    49.8 
  0   1  1    5163504   15051   0.0  34.5           6 primary

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)?

Any hints for this?
Thanks a lot.

Kind regards,
Tom