testing for Internet connection

Dar Scott dsc at swcp.com
Sun Dec 19 12:27:23 EST 2004


On Dec 19, 2004, at 5:04 AM, Alex Tweedly wrote:

> This problem happens to me all the time, because I use my laptop 
> connected to an ethernet which connects to the internet through 
> another computer using a dial-up line, and the dial-up is often 
> disconnected. Laptop thinks it has a connection (the ethernet is up) 
> and thinks it has DNS servers it can reach (because the main computer 
> acts as a DNS proxy) - but when it queries a host name, it is blocked 
> until the DNS response times out.

This illustrates that there are several meanings to "is connected to 
the Internet".

They might be:

A.  This computer thinks there is a route to a public IP address.
B.  #A and the route works.
C.  This computer can modify routing on demand to get to a public IP 
address.

I have not used a dialup connection in a while so, I don't remember all 
the problems involved.  One problem is that the act of checking can 
cause dialing to start.

Some ideas:

1
Open TCP to some http port on a reliable server.  This does a B check.

2
Open UPD to any port on a public address.  This does an A check and is 
fast.  Maybe this is less likely to cause a dial than #1.  This is 
non-intrusive.

3
Parse the results of a command-line utility.  Dial-up may take special 
consideration.  This does an A test.

4
External.  A test.

Dar

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     DSC (Dar Scott Consulting & Dar's Lab)
     http://www.swcp.com/dsc/
     Programming Services and Software
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