[krbdev.mit.edu #8687] Repeated lookups of local computer name on Windows

[Greg Hudson via RT]

Every time we want to look up a SRV or URI record, we call 
krb5int_dns_init(), which calls res_init() (absent res_ninit() 
support or being on a Mac).  On Windows this is implemented via the 
wshelper library.  The wshelper res_init() initializes a global 
structure _res, but does not check if _res is already initialized (as 
it could via _res.options & RES_INIT).

As part of its work, it tries to determine a local domain name and 
search path.  Unless the LOCALDOMAIN environment variable is set, it 
does this by calling GetComputerName() and looking up the result in 
DNS with DnsQuery_A() with the DNS_QUERY_BYPASS_CACHE and 
DNS_QUERY_NO_LOCAL_NAME flags.  On a VM I set up using Windows 10, 
this lookup takes approximately one second to fail (I'm not sure 
why), causing kinit to take several seconds in a typical exchange.

We do not currently make use of the local domain or search path in 
the resolver configuration, since we always look up absolute domain 
names (dnssrv.c:prepare_lookup_buf()).  We have some interest in a 
local domain name to expand short names when 
dns_canonicalize_hostname is set to false, but it would need to be 
determined from on-machine configuration, not DNS.