@bortzmeyer I had done a simple `dig nixos.org` (I don't know what +cd does) but I got the same result with `+cd` :

; <<>> DiG 9.11.2 <<>> nixos.org
;; global options: +cmd
;; connection timed out; no servers could be reached

A result I interpret as the DNS process on their server crashing and thus maybe closing the socket unexpectedly ? Because I can garantee the server can be reached : I can resolve any other domain name without any trouble at all.