Reply To: how can I detect an active Internet connection?

[Vadim Smirnov]

Internet connection can be identified by checking systems IP routing table (sample output from “route print”):

===========================================================================

Interface List

0x1 ........................... MS TCP Loopback interface

0x2 ...00 0e a6 a6 7c b6 ...... Realtek RTL8139 Family PCI Fast Ethernet NIC - Packet Scheduler Miniport

===========================================================================

===========================================================================

Active Routes:

Network Destination        Netmask          Gateway       Interface  Metric

0.0.0.0          0.0.0.0      192.168.1.1   192.168.1.101	  20

127.0.0.0        255.0.0.0        127.0.0.1       127.0.0.1	  1

192.168.1.0    255.255.255.0    192.168.1.101   192.168.1.101	  20

192.168.1.101  255.255.255.255        127.0.0.1       127.0.0.1	  20

192.168.1.255  255.255.255.255    192.168.1.101   192.168.1.101	  20

224.0.0.0        240.0.0.0    192.168.1.101   192.168.1.101	  20

255.255.255.255  255.255.255.255    192.168.1.101   192.168.1.101	  1

Default Gateway:       192.168.1.1

===========================================================================

Persistent Routes:

None

First line with the default route network 0.0.0.0 mask 0.0.0.0 gateway 192.168.1.1 interface 192.168.1.101 points that I’m currently connected to Internet thru the network interface with IP 192.168.1.101. I may have many Internet connection but this is the default one.

You can request routing information programmatically using IP helper API (GetIpForwardTable) and register the callback handler for any routing table changes (NotifyRouteChange) in order to be notified about changing the Internet connection.