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一般情况下，就是利用ping来检测网络连通。不过这样需要指定一个IP，一旦这个地址受到攻击或者网络的短暂迟缓，就会不可靠
能否通过检测在一定时间内收到的数据包大小来判断网络连通是否呢
能否通过检测在一定时间内收到的数据包大小来判断网络连通是否呢
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kingzai 2004-11-03
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基本上用InternetGetConnectedState API判断就行了，当然ping是最直接的
mmm00000 2004-10-29
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To eastnofail(Char *) :
虽然看的头晕，不过还是很感谢你~
过两天回来慢慢研究
To bcb_alone(孤心) :
握手机制只是听说过，能解释一下么
我去搜索，结果又是系统维护~晕
虽然看的头晕，不过还是很感谢你~
过两天回来慢慢研究
To bcb_alone(孤心) :
握手机制只是听说过，能解释一下么
我去搜索，结果又是系统维护~晕
TypeCsdnLearn 2004-10-29
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学习,请高手来点代码分析一下
lidawen 2004-10-29
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學習
bcb_alone 2004-10-29
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也可以使用心跳（也就是握手机制）
定时发送数据来检测。
定时发送数据来检测。
eastnofail 2004-10-29
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From: Allen Weng <yweg_spawa@microsoft.com>
Subject: RE: Detecting if internet connection is active at this moment.
Date: Friday, May 04, 2001 2:26 AM
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.
Allen Weng
Microsoft Developer Support
Subject: RE: Detecting if internet connection is active at this moment.
Date: Friday, May 04, 2001 2:26 AM
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.
Allen Weng
Microsoft Developer Support
eastnofail 2004-10-29
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IsNetworkAlive
The IsNetworkAlive function determines whether the local system is connected to a network and the type of network connection. For example, whether the local system has connectivity to a LAN, WAN, or both networks.
Bool IsNetworkAlive(
LPDWORD lpdwFlags
);
Parameters
lpdwFlags
[in] Provides information on the type of network connection available when the return value is TRUE. The flags can be:
NETWORK_ALIVE_LAN
The computer has one or more LAN cards that are active.
NETWORK_ALIVE_WAN
The computer has one or more active RAS connections.
NETWORK_ALIVE_AOL
This flag is only valid in Windows 95 and Windows 98. Indicates the computer is connected to the America Online network.
Return Values
TRUE
The local system is connected to a network.
FALSE
Call GetLastError to determine the reason for no connectivity.
The IsNetworkAlive function determines whether the local system is connected to a network and the type of network connection. For example, whether the local system has connectivity to a LAN, WAN, or both networks.
Bool IsNetworkAlive(
LPDWORD lpdwFlags
);
Parameters
lpdwFlags
[in] Provides information on the type of network connection available when the return value is TRUE. The flags can be:
NETWORK_ALIVE_LAN
The computer has one or more LAN cards that are active.
NETWORK_ALIVE_WAN
The computer has one or more active RAS connections.
NETWORK_ALIVE_AOL
This flag is only valid in Windows 95 and Windows 98. Indicates the computer is connected to the America Online network.
Return Values
TRUE
The local system is connected to a network.
FALSE
Call GetLastError to determine the reason for no connectivity.
