On Jun 1, 2011, at 5:34 PM, G S wrote: > Thanks for the response, Greg. > >> There are circumstances where a subset of the Internet's hosts may be >> reachable. For example, you may be able to reach link-local names without a >> broader Internet connection. Or you may be able to reach a host behind a VPN >> only when the VPN is active. (I don't know what Reachability actually does >> in these cases, but it's the sort of thing that the API was designed to >> handle.) > > But since this test doesn't actually check the routes to these hosts, > how would it be able to provide meaningful results even in these > cases?
Reachability can check the first half of the first hop of the route. Consider the link-local case. You're connected to an ad-hoc network with no Internet router present. Reachability may be able to deduce that there is definitely no route to apple.com (17.149.160.49), but there may be a route to the link local-address second-floor-printer.local. (169.254.167.45). Reachability's host-specific test could return NO for "apple.com" and MAYBE for "second-floor-printer.local.". > I figured the specific-host test pinged the host. That would not be reliable anyway. Perhaps ICMP Echo packets are blocked by a router between you and the host. Then the "ping test" would say NO but a connection attempt would succeed. Perhaps ICMP Echo works but the service you actually want (like HTTP) is blocked by a firewall. Then the "ping test" would say YES but a connection attempt would fail. Reachability does not answer all questions about Internet connectivity, but the answers it can give are reliable. -- Greg Parker gpar...@apple.com Runtime Wrangler _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
