Frequently Asked Questions
Q. How can I detect if my internet connection is active at this moment?
A. Unfortunately, there is no
simple answer to this question. Allen Weng of Microsoft provided a
insight into this question in a newsgroup posting:
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.
[ Allen Weng's
newsgroup posting of May 4, 2001 ]
Most of the information of provided in this topic is an editorialized version
of Allen's comments. Some NDIS-specific supplementary information from PCAUSA is provided near
the end.
There is also mention of an
Intel® paper on Network Status Detection at the end of this page.
System Event Notification Service (SENS)
- IsNetworkAlive
- IsDestinationReachable
You can use 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.
You can also use 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.
Remote Access Service (RAS)
- RasEnumConnections
RasEnumConnections is 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.
Win32 Internet (WinInet)
- InternetGetConnectedState
- InternetCheckConnection
InternetGetConnectedState is a 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.
InternetCheckConnection is a Winnet/IE4 function call. This is meant to
determine if a URL is reachable- in practice it is pretty unreliable and best
voided.
IP Helper API
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. 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.
The IP Helper API documented in the Platform SDK, is available on
Windows NT SP4 and higher as well as some Windows 9X/ME platforms.
Registry Information
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 function calls
Allen's Summary
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.
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.
Network Driver Interface Specification (NDIS)
NDIS 5.0 and higher requires NDIS miniport drivers to support information
called "media connect status". This is defined to be the "connection status of
the NIC on the network".
It is important to understand
that the NDIS media connect status does not actually provide the answer to
the original question!
For an Ethernet NIC a media connect status result of
NdisMediaStateConnected simply means that the Ethernet cable is plugged
in.
If the NIC is connected to a dial-on-demand router (for example), the fact
that the cable is plugged in offers no insight into whether the dialup
connection is active. If the NIC is attached to a DSL router or cable modem,
media connect status does not offer any insight into whether the connection to
the ISP is actually active.
A NDIS device driver components, such as a NDIS protocol driver, can use the
NdisRequest function to query information on the NDIS
OID_GEN_MEDIA_CONNECT_STATUS "object identifier". However, NDIS media connect status is not simple to fetch from a user mode
application.
User mode applications can fetch media connect status
indirectly using the Windows Management Instrumentation (WMI) API as well
as other techniques. See the FAQ topic "How do I query NDIS miniport OIDs using WMI?".
This is an interesting paper published on the Intel®
Developer Services website.
[
Network Status Detection by Majd Naciri ]
