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: Conrad Shultz <email@hidden>
- Date: Wed, 01 Jun 2011 17:52:47 -0700
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On 6/1/11 5:34 PM, G S wrote:
> 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?
Note: I am _not_ an SCNetworkReachability guru, so take this all with a
grain of salt.
How do you know that the routes aren't tested? Quoting from your first
message:
"A remote host is considered reachable when a data packet, sent by an
application into the network stack, can leave the local device."
If the kernel cannot route the packet, this would be interpreted by any
sane implementation as "cannot leave the local device."
A kernel routing table lookup would cover VPN availability issues, LANs
with no gateway (or otherwise defined default route), etc.
> I figured the specific-host test pinged the host.
I can't imagine it would. Ping would lead to many, many false negatives
regarding host availability. A great many sites block ICMP echo
requests (I just tried "ping apple.com" for example - and get no response).
Since the OS has no way of knowing what particular port/service is of
interest, your only good option is to try to connect to the service of
interest and handle failure appropriately.
(Of course, even if you seemingly can't connect by this approach, it is
fundamentally unknowable whether the receiving host actually got your
test message: http://en.wikipedia.org/wiki/Two_Generals'_Problem)
- --
Conrad Shultz
Synthetiq Solutions
www.synthetiqsolutions.com
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