Dns_clientSourceTODO ideally there'd be something like mirage-flow-lwt that didn't depend on lwt and a ton of other things, and still provided map and connect and so on. leaving this stuff here for now until a better solution presents itself.
stdlib_random len is a buffer of size len, filled with random data. This function is used by default (in the Unix and Lwt implementations) for filling the ID field of the DNS packet. Internally, the Random module from the OCaml standard library is used, which is not cryptographically secure. If desired Nocrypto.Rng.generate can be passed to S.create.
default_resolver is the IPv4 address in dotted-decimal form of the default resolver. Currently it is the IP address of the UncensoredDNS.org anycast service.