Do not keep more than 3 DNS servers and init default DNS struct

[hellt]

fix #1670

[codecov]

Codecov Report

Merging #1671 (70b803d) into main (cd3c3cf) will decrease coverage by 0.01%.
Report is 5 commits behind head on main.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1671      +/-   ##
==========================================
- Coverage   51.05%   51.04%   -0.01%     
==========================================
  Files         138      138              
  Lines       13339    13350      +11     
==========================================
+ Hits         6810     6815       +5     
- Misses       5776     5780       +4     
- Partials      753      755       +2     

Files	Coverage Δ
types/topology.go	68.01% <100.00%> (+0.49%)	⬆️
utils/resolve.go	94.44% <100.00%> (+0.89%)	⬆️

... and 2 files with indirect coverage changes