Due to a bug in unbound, we were passing a string containing a null
character to ub_ctx_resolvconf and ub_ctx_hosts rather than a NULL
pointer. On *nix this wasn't causing headache, but on Windows this was
causing unbound to not correctly load DNS settings from the OS.
Note on the bug: in a Windows-specific code branch in the function
ub_ctx_hosts(), if the hosts file specified was a NULL pointer, a call
to getenv() was stored in a local char* and later freed. This is
incorrect, as we do not own that data, and caused the program to crash.
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