Determining if a client PC has an Internet connection

[torment]

Have you tried just parsing the output from the command "ipconfig"?

It's pretty obvious from the output that might give you if a connection
is availible. I hope it helps.

-Matt

Dave Brueck wrote:
> Cliff Wells wrote:
> > I'm writing an application that needs to know if an Internet
connection
> > is available.
> [snip]
> > Is there any way to reliably determine the state of the client's
> > internet connectivity?
>
> Hi Cliff,
>
> On a Win32 system you can reliably determine the state of the
Internet
> connection but it requires a fair amount of work. Here's the recipe I
use:
>
> 1) If the user has a modem and is connected via a modem, you know for
sure that
> the user is online (use ctypes to call rasapi32.RasEnumEntriesA to
see if the
> user has a modem, and rasapi32.RasEnumConnectionsA to see if the user
is
> connected via a modem)
>
> 2) If that test fails, next check for open connections to remote
hosts. If there
> are any open connections to public IP addresses, the user is
"online". (use
> iphlpapi.GetTcpTable to get a list of connections, and keep only
those where
> dwState is MIB_TCP_STATE_ESTAB. Remove any addresses that are
127.0.0.1, 10.*,
> 192.168.*, or 172.16-31.*). If after all this the list is non-empty,
you're
> probably online.
>
> 3) Still no luck? Call IcmpSendEcho to a well-known server - this
sends a "ping"
> packet, which won't bring up the dial-up networking box to annoy your
users. I
> usually start with a list of all the DNS root servers as well as
pingable
> company IP addresses, and randomly choose one of them. Then, anytime
I make a
> connection to the Internet, I save the results of the hostname lookup
and use
> them in my list as well (so that very few pings actually go to the
DNS root
> servers). Also, it's good to make sure that your code has
restrictions in place
> to prevent it from pinging anything too often. Due to instant
messengers & email
> clients, step #2 usually detects when the user is online, so the need
for an
> actual ping is greatly reduced.
>
> Anyway, after doing the above you know with a high degree of
certainty whether
> or not the user is online.
>
> The above approach is obviously a lot of work, but once you take the
time to do
> it you can just stick it in a library somewhere and not have to think
about it
> again. On import of the library, I start up a background thread that
updates a
> status variable every second or so, so that any time my app can query
to see the
> state of the Internet connection. Although it's a lot of work, I've
found that
> (1) it's pretty accurate and (2) it's non-intrusive (it doesn't e.g.
pop up the
> dial-up networking dialog box if the user is offline).
> 
> Hope at least some of this helps,
> Dave