Updates tailscale/corp#18960
iOS uses Apple's NetworkMonitor to track the default interface and
there's no reason we shouldn't also use this on macOS, for the same
reasons noted in the comments for why this change was made on iOS.
This eliminates the need to load and parse the routing table when
querying the defaultRouter() in almost all cases.
A slight modification here (on both platforms) to fallback to the default
BSD logic in the unhappy-path rather than making assumptions that
may not hold. If netmon is eventually parsing AF_ROUTE and able
to give a consistently correct answer for the default interface index,
we can fall back to that and eliminate the Swift dependency.
Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>
In prep for most of the package funcs in net/interfaces to become
methods in a long-lived netmon.Monitor that can cache things. (Many
of the funcs are very heavy to call regularly, whereas the long-lived
netmon.Monitor can subscribe to things from the OS and remember
answers to questions it's asked regularly later)
Updates tailscale/corp#10910
Updates tailscale/corp#18960
Updates #7967
Updates #3299
Change-Id: Ie4e8dedb70136af2d611b990b865a822cd1797e5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
