In a recent app I wrote I used the Reachability API but then when attempting to log on to my host I pinged 5 tries at the same time as attempting to log on. My thinking was that I could either wait the very long time for the logon attempt to time out or I could decide after 5 failed ping attempts that the host was down.
-----Original Message----- From: cocoa-dev-bounces+jim.adams=sas....@lists.apple.com [mailto:cocoa-dev-bounces+jim.adams=sas....@lists.apple.com] On Behalf Of Greg Parker Sent: Wednesday, June 01, 2011 9:12 PM To: G S Cc: cocoa-dev Subject: Re: What is the point of a host-reachability test that doesn't test the reachability of the host? 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/jim.adams%40sas.com This email sent to jim.ad...@sas.com _______________________________________________ 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
