RE: What is the point of a host-reachability test that doesn't test the reachability of the host?
RE: What is the point of a host-reachability test that doesn't test the reachability of the host?
- Subject: RE: What is the point of a host-reachability test that doesn't test the reachability of the host?
- From: Jim Adams <email@hidden>
- Date: Thu, 02 Jun 2011 13:04:26 +0000
- Thread-topic: What is the point of a host-reachability test that doesn't test the reachability of the host?
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=email@hidden [mailto:cocoa-dev-bounces+jim.adams=email@hidden] 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 email@hidden Runtime Wrangler
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