NetworkManager: I want to believe, but...

[Hans Ulrich Niedermann]

Matthew Miller wrote:

> But "is the network up" a generally useful question?

I can picture several situations where "this system is online" or "this 
system is offline" can cause more harm than it solves problems.

"Is *the* network up?" "No."

   Now software will refuse to connect to 127.0.0.1, ::1 or any
   other locally configured network address, for that matter - even
   though these addresses are local and can be reached locally without
   any network being connected.

   cf. https://bugzilla.redhat.com/show_bug.cgi?id=443385
       Firefox refuses to connect to http://127.0.0.1/ when "offline"

"Is address 1.2.3.4 or dead:beef::1 reachable?" "Yes/No."

   This would be a better question locally running software could
   ask.

   The answer could be determined by just looking at the local routing
   tables (aka "ip route" and "ip -6 route" output).

> I find that "can I
> reach the network resource I need" is the more important one, and the "is
> the network up" issue basically a detail.I mean, who cares if the network
> is up if the gateway is down? This is why external monitoring (big brother
> or the like) is more practical.

"Is resource //FOO on server 12.34.56.78 port 12345 available?" "Yes/No"

   That is the real question you want to know and can be easily
   determined by the software which wants to access //FOO on
   12.34.56.78:12345 in the first place.

Having a common system service maintaining resource availability state 
by actually communicating with other nodes in the network only to 
determine resource reachability is probably not very desirable. It 
requires communication with all potentially interesting network nodes, 
knowledge of many different protocols, etc. That is a lot of effort just 
for determining availability which the services themselves will be in a 
much better position to determine.

Exploiting the local routing table for a more fine-grained 
online/offline status should be quite easy in comparison.

-- 
Hans Ulrich Niedermann

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