Hi Jim,

From your description, I'd like to confirm if my understanding is correct,
you are trying to determine if a client is able to connect to certain
server, not just if a client has a network connection( which might not be
able to connect to the destination server) , is it correct?

IMO, there is no better way to resolve this issue, the tcp "connection" is
simulated by "time-out and retry" mechanism on the two end points, the
underlying network routers do not have the concept of connection state, in
other words, we could not know a tcp connection failure until we get
timeout when we try to send something to the destination.

Your current solution is ok. If you can reach the destination server using
ping command , you may try P/Invoking the IsDestinationReachable API, it
provides a similiar function by sending ping packets to the destination and
return a boolean value instead of exception, maybe it could reduce the
performance penalty somehow.  You may define the P/Invoke definition of
this API in C# like below:
//bool bReachable = IsDestinationReachable("www.microsoft.com",IntPtr.Zero);
[DllImport("sensapi.dll")]
private extern static bool IsDestinationReachable(string dest,IntPtr ptr);

Ofcourse, many servers turned off the Ping response for security concerns,
if this is the case, maybe we at least could polling the status in a
seperate thread.
Also, If both the server and clients are in the same LAN, maybe you can try
making  a "heartbeat" program on the server side, this program could create
a multicast group and send echo message to the group address periodically,
then the client could get the connection status by receiving echo messages.

If you still have problem on this issue, please feel free to replyl this
thread.

Thanks!

Best regards,

Ying-Shen Yu [MSFT]
Microsoft Community Support
Get Secure! - www.microsoft.com/security

This posting is provided "AS IS" with no warranties and confers no rights.
This mail should not be replied directly, please remove the word "online"
before sending mail.

Hi Jim,

This is a FAQ in windows programing. Please refer to this article that I collected before. It is talking about connecting to Internet. But you can
also get some idea from it.

--------------
Actually, there is no single function for determining if a machine is connected to the Internet, and it is impossible to reliably determine what
is happening without side effects - such as automatic network connections taking place. What you can do is reliably detect when there
definitely isn't an Internet Link: in the absence of any dial up or LAN connection the system is definitely off line.

Some techniques include:

1. IsNetworkAlive()
If you are targeting system with IE5 or later, this is the best API call yet it even listens for traffic on a LAN. There is a secondary function
IsDestinationReachable() which tries to resolve the hostname and ping it. This does not work through firewalls, and overestimates speed as
the max the LAN card can support, rather than the actual point to point bandwidth.

2. RasEnumConnections()
A reliable technique for modems and direct dial up networking, but not for situations where Internet access is via a LAN. You should
dynamically load "RasEnumConnectionA" from "RASAPI32.DLL", as LAN installations of Windows may not include the library.

3. InternetGetConnectedState()
This Wininet /IE4 function call can distinguish between modem and LAN, but can't handle complex LAN+autodial router situations. It is "offline
state aware". Important: handling of the offline flage changed for IE5 -it returns TRUE for connected'  even when off line, but signals the flags in
the LPDWORD parameter.

4. InternetCheckConnection()
A Winnet/IE4 function call. This is meant to determine if a URL is reachable- in practice it is pretty unreliable and best voided.

5. NT SP4, NT5: The IP helper API can tell you which network interface to use to connect to a supplied IP address, and what the bandwidth
and current status of that link is

6. Using the Offline flag which is part of IE4 to allow users to manually control the online/offline state of applications. This flag is stored in
the registry and can be manipulated via some funcions calls

These calls mostly determine the presence or absence of network connections -not Internet access, so can't handle a home network sharing
a dial up connection, or two laptops connected directly to each other.

The global offline state flag of IE4 (and hence win98, NT5) and the call to test it - InternetGetConnectedState()- look the best long term options,
but will take time to become universal. The IP Helper APIs even let you find out how much traffic is going over a link, but only detect the
'loopback' interface on Windows 98, so is not a lot of use. Wouldn't a 'GetSpeedToHost() function call be great?

Finally, whatever technique you use, when it's time to talk to a remote site, always add timeouts or a cancel button. Even a quick functions like
gethostbyname() can lock up an app if something in the network chain is broken.
--------------

Best regards,
Yanhong Huang
Microsoft Community Support

Get Secure! ?C www.microsoft.com/security
This posting is provided "AS IS" with no warranties, and confers no rights.