how can I detect an active Internet connection?

[Steven]

Objective:

Currently, I design a project to try to discover the active connection to Internet and filter some packets.Because I use windows service to run my application, if the connection or the number of adapters have any changes, I want to use winpkfilter event notification or IP Helper functions to find a new Internete connection automatically for the user.
But I can not find a good way to figure it out .

My question is :
How can I find an active connection to Internet?f
or examles, sometimes, wireless adapter can access to Internet, sometimes not. Maybe, sometimes, the user also use wire cable to connect to Internet.

Test:
Because my laptop has two adapters,one is wire adapter,another is wireless adapter.

[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.

[Steven]

Now I am doing a test:
On my laptop, I have a stable Internet connection through wire calbe;
for the test purpose, I add one wireless adapter on my laptop, which can find out one AccessPoint, but, which needs ID and password to provide all services.
Anyway, the current routing table is like:

===========================================================================
Interface List
0x1 ……………………… MS TCP Loopback interface
0x2 …00 06 1b cf c3 41 …… Intel(R) PRO/100 VE Network Connection – packet Scheduler Miniport
0x30004 …00 03 2f 17 5e 8f …… GL242201 – packet Scheduler Miniport
===========================================================================
===========================================================================
Active Routes:
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 172.19.0.2 172.19.3.182 30
0.0.0.0 0.0.0.0 207.162.10.253 207.162.10.161 20
127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1
172.19.0.0 255.255.128.0 172.19.3.182 172.19.3.182 30
172.19.3.182 255.255.255.255 127.0.0.1 127.0.0.1 30
172.19.255.255 255.255.255.255 172.19.3.182 172.19.3.182 30
207.162.10.0 255.255.255.0 207.162.10.161 207.162.10.161 20
207.162.10.161 255.255.255.255 127.0.0.1 127.0.0.1 20
207.162.10.255 255.255.255.255 207.162.10.161 207.162.10.161 20
224.0.0.0 240.0.0.0 172.19.3.182 172.19.3.182 30
224.0.0.0 240.0.0.0 207.162.10.161 207.162.10.161 20
255.255.255.255 255.255.255.255 172.19.3.182 172.19.3.182 1
255.255.255.255 255.255.255.255 207.162.10.161 207.162.10.161 1
Default Gateway: 207.162.10.253
===========================================================================
Persistent Routes:
None

However, 172.19.3.182 is ip address on my wireless adapter, which can not really access to Internet; 207.162.10.161 on the wire adapter is working.

But, in my application, I want to know how to detect which one can really access to Internet?????

[Vadim Smirnov]

Two lines are most important here:

Network Destination Netmask Gateway Interface Metric

0.0.0.0 0.0.0.0 172.19.0.2 172.19.3.182 30

0.0.0.0 0.0.0.0 207.162.10.253 207.162.10.161 20 

One route has metric 20, and another one 30. First of all Windows tries to connect thru adapter with better (less) metric, if it fails then it tries another one. So your default internet connection is thru 207.162.10.161 and it is also confirmed by your default gateway 207.162.10.253.

[Author]

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