iOS: reachabilityWithHostname YES although it should be NO

问题

I tested different frameworks, e.g.

• https://github.com/tonymillion/Reachability
• https://github.com/VerticodeLabs/VCLReachability
• https://github.com/kstenerud/KSReachability

and I would like to know if a host is reachable. On my iPhone, I set my iMac as proxy (Charles) and block or don't block the connections, but the reachability is always YES. Only if I set a non-existing host, it returns NO. But if the host exists but I block the connection to it, I always get isReachable. Isn't there a way to check if the host is really reachable?

If I try with KSReachability, I'm doing the following:

self.reachability = [KSReachability reachabilityToHost:@"www.stackoverflow.com"];
self.reachability.notificationName = kDefaultNetworkReachabilityChangedNotification;
self.reachability.onReachabilityChanged = ^(KSReachability *reachability) {
    NSLog(@"isReachable: %i", reachability.reachable);
};

I always get isReachable: 1 there with the following configuration:

• connected to Wifi
• configured my iMac as HTTP-Proxy
• blocking www.stackoverflow.com in my Charles Proxy

When I try to reach www.stackoverflow.com in Safari, the page can't be opened (as expected). I would expect the reachability to be false (isReachable: 0) in this case.

EDIT So the most important question for me is - how to achieve the behavior I'm expecting? I.e. that the app continuously checks if the given host is really reachable?

回答1:

The code statement:

self.reachability = [KSReachability reachabilityToHost:@"www.stackoverflow.com"];

actually calls below method:

SCNetworkReachabilityRef reachability = SCNetworkReachabilityCreateWithName(NULL,
                                                          [hostName UTF8String]);

SCNetworkReachability reference says:

The SCNetworkReachability programming interface allows an application to determine the status of a system's current network configuration and the reachability of a target host. A remote host is considered reachable when a data packet, sent by an application into the network stack, can leave the local device. Reachability does not guarantee that the data packet will actually be received by the host.

The explanation clears that iOS system doesn't send any request to outside world to check the reachability. It just tells that data packet can leave the device or not. If system were to send the request it automatically means that it is connected to network.

You can verify this by mentioning a valid host like "www.stackoverflow.com" and check in charles (first unblock it) that no request is sent.You can also check with other valid host names like "www.abcdefgh.com" (verify this by running it in Safari and see in charles) it also gives you the reachability but charles shows no request.Also if you put http:// before any valid host, something like "http://www.stackoverflow.com" it will also fails reachability. So it is clear that it is not an outgoing http request. If system has to send a request outside then what's the point of providing the class? A developer could have created a network connection and try to connect to a host and see if it passes or fails.

However it is interesting that if an invalid host like "www.hjjkhkhk.com" is provided iOS system gives reachability as false. Now the question is how iOS system finds a valid or invalid host without sending any query to outside world? May be it is periodically caching a list of DNS ranges??? Highly improbable to me.

回答2:

In your AppDelegate add this method:

#import "Reachability.h"

-(NSString *)checkNetworkConnectivity
{
    NSString *networkValue;

    Reachability *rc = [Reachability reachabilityWithHostName:@"www.stackoverflow.com"];
    NetworkStatus internetStatus = [rc currentReachabilityStatus];

    if(internetStatus==0)
    {
        networkValue = @"NoAccess";
    }
    else if(internetStatus==1)
    {
        networkValue = @"ReachableViaWiFi";

    } else if(internetStatus==2)
    {
        networkValue = @"ReachableViaWWAN";
    }
    else
    {
        networkValue = @"Reachable";
    }

    return networkValue;
}

Checking if the host is reachable

NSString *netStr = [appDelegate checkNetworkConnectivity];

if([netStr isEqualToString:@"NoAccess"])
{
     [appDelegate callNoNetworkAlert];
}

回答3:

Firstly, unless we see some code, we cannot make sure what you're doing, you're doing the right way. However I will assume you are doing it correctly.

Secondly, you should test what FreeNickname has suggested in his comment. Maybe it's not actually unreachable, and the reachability is acting correctly, when you expect a different response.

Last, but very important, from the Reachability docs :

Note: Reachability cannot tell your application if you can connect to a particular host, only that an interface is available that might allow a connection, and whether that interface is the WWAN.

What it means is that, even though your server might not be returning any responses, Reachability does NOT check that. It only checks if your server is available , such that a packet can be sent. What it does with that packet is of no concern to Reachability. If Reachability is able to transmit the entire packet, it assumes your host is reachable. It will return unreachable iff your server is down, disconnected, or does not exist.

回答4:

I would like to add that To listen the Network changes at Runtime, you need to listen to the Notifications that tell you that a Network state has been changed.

This is how you can do this :

1. Implement a listener in AppDelegate file.

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(reachabilityChanged:) name:kReachabilityChangedNotification object:nil];

• Now reachabilityChanged will be called when there is a change in Network status (perhaps connectivity of internet connection or it's disconnection). You can do appropriate handling in reachabilityChanged event

like:

- (void) reachabilityChanged:(NSNotification *)note
{
     Reachability* reachability = [note object];

     if (reachability == self.hostReachability)
     {
        isHostReachable = YES;  //A flag to keep track of connectivity
     }

     if (reachability == self.internetReachability)
     {
         isInternetAvailable = YES;  
     }

     if (reachability == self.wifiReachability)
     {
         isWifiAvailable = YES;  
     }

     //If all are true that means we have host and Internet available
     if (isHostReachable && isInternetAvailable && isWifiAvailable) 
     {
          isInternetAvailable = true;
     }
}

Hope this will help you.

回答5:

AFNetworking

A delightful iOS and OS X networking framework http://afnetworking.com

---

Network Reachability Manager

AFNetworkReachabilityManager monitors the reachability of domains, and addresses for both WWAN and WiFi network interfaces.

Shared Network Reachability

[[AFNetworkReachabilityManager sharedManager] setReachabilityStatusChangeBlock:^(AFNetworkReachabilityStatus status)
 {
     NSLog(@"Reachability: %@", AFStringFromNetworkReachabilityStatus(status));
 }];

HTTP Manager Reachability

NSURL *baseURL = [NSURL URLWithString:@"http://example.com/"];
AFHTTPRequestOperationManager *manager = [[AFHTTPRequestOperationManager alloc] initWithBaseURL:baseURL];

NSOperationQueue *operationQueue = manager.operationQueue;
[manager.reachabilityManager setReachabilityStatusChangeBlock:^(AFNetworkReachabilityStatus status)
 {
     switch (status)
     {
         case AFNetworkReachabilityStatusReachableViaWWAN:
         case AFNetworkReachabilityStatusReachableViaWiFi:
             [operationQueue setSuspended:NO];
             break;
         case AFNetworkReachabilityStatusNotReachable:
         default:
             [operationQueue setSuspended:YES];
             break;
     }
 }];

来源：https://stackoverflow.com/questions/24260833/ios-reachabilitywithhostname-yes-although-it-should-be-no