logtail's dialer only used dnscache in its fallback path, and built a
new Resolver per dial there, so every new connection to the log server
did a fresh DNS lookup (twice when the first dial failed). On systems
without a caching resolver, that adds up to a lot of queries for
log.tailscale.com.
Share one dnscache.Resolver across all log dialers, and have the
bootstrap path go straight to dnsfallback rather than repeating the
regular lookup. Since the cache TTL is a fixed guess, make
dnscache.Dialer expire a cached entry and look the host up again when
dialing its cached IPs fails, so a moved host isn't stuck on stale IPs.
Updates #15326
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I6a55d72a73ed878faebc15317421a54d9ae293ca
Brad Fitzpatrick
67f8c81e61