res_search only incremented got_servfail for h_errno == TRY_AGAIN *AND*
hp->rcode == SERVFAIL. However, there are cases such as timeouts where
rcode is not always set to SERVFAIL. This leads to inconsistent nameserver
operation during multi-domain and truncated dot searches, especially during
booting when portions of the network are being brought up simultanious with
dns lookups.
This patch attempts to correct the problem by unconditionally terminating
the search if TRY_AGAIN is returned (after res_query has gone through all
retries and name servers) instead of trying other domain elements in the
domain seach path.
This patch should fix reported problems (which I can reproduce) with some
NFS mounts failing during boot. This occured because mount_nfs thought the
host name lookup returned a definitive failure using a non-dotted host name
when, in fact, it timed out on the first part (host.search.domain.name) and
got a definitive host-not-found response on the second part (host.).
Generally speaking, search path name server timeouts can exceed 60 seconds
per element and most machines which consistently timeout on earlier portions
of a search path are effectively non-operational due to the imposed delays.
It is more important for DNS lookups to return the proper error code then
to be able to recover a valid lookup in later portions of the search path
in these situations.
Obtained from: DragonFly
MFC after: 3 weeks
1cc11684ac