ios - Determine if a certain address with a certain port is reachable

Question

I want to know if my server is online via my ios application. Here's what I'm doing:

Boolean result;
CFHostRef hostRef = CFHostCreateWithName(kCFAllocatorDefault, (__bridge CFDataRef)(serverIPAddress)); //serverIPAdress = "10.10.10.100:5010"

    if(hostRef) {
        result = CFHostStartInfoResolution(hostRef, kCFHostAddresses, NULL); // pass an error instead of NULL here to find out why it failed
    }

    if (!result) { //This means that the host was unreachable        
        return ;
    }

My server is online and I can access it later on in the code(meaning that my connection to the server works perfectly fine). However, I want to be able to detect if my server, on a certain port, is reachable.

Also, if I remove the ":5010" from the ip address, it detects that my server is online (it doesn't go in the "!result" condition) and detects that my server is offline if I put "10.10.10.253" which corresponds to no ip address on my network.

How can I manage to determine if my server is online or not ?

I've looked at this question : Reachability with Address - Server AND Port - iOS 5 but it doesn't work since it always return that it is reachable no matter what ip address I enter

Thanks in advance

Answer 1

One approach could be to open a socket connection to a specific port to see if you get any response back. If not, then the destination is unreachable. For example

#include <arpa/inet.h> //for PF_INET, SOCK_STREAM, IPPROTO_TCP etc

CFRunLoopSourceRef gSocketSource;
void ConnectCallBack(CFSocketRef socket, CFSocketCallBackType type, CFDataRef address, const void *data, void *info)
{
    UInt8 buffer[1024];
    bzero(buffer, sizeof(buffer));
    CFSocketNativeHandle sock = CFSocketGetNative(socket); // The native socket, used recv()

    //check here for correct connect output from server
    recv(sock, buffer, sizeof(buffer), 0);
    printf("Output: %s\n", buffer);

    if (gSocketSource)
    {
        CFRunLoopRef currentRunLoop = CFRunLoopGetCurrent();
        if (CFRunLoopContainsSource(currentRunLoop, gSocketSource, kCFRunLoopDefaultMode))
        {
            CFRunLoopRemoveSource(currentRunLoop, gSocketSource, kCFRunLoopDefaultMode);
        }
        CFRelease(gSocketSource);
    }

    if (socket) //close socket
    {
        if (CFSocketIsValid(socket))
        {
            CFSocketInvalidate(socket);
        }
        CFRelease(socket);
    }
}

void ConnectSocket()
{
    //socket
    CFSocketContext context = {0, NULL, NULL, NULL, NULL};
    CFSocketRef theSocket = CFSocketCreate(kCFAllocatorDefault, PF_INET, SOCK_STREAM, IPPROTO_TCP, kCFSocketConnectCallBack , (CFSocketCallBack)ConnectCallBack, &context);

    //address
    struct sockaddr_in socketAddress;
    memset(&socketAddress, 0, sizeof(socketAddress));
    socketAddress.sin_len = sizeof(socketAddress);
    socketAddress.sin_family = AF_INET;
    socketAddress.sin_port = htons(5010);
    socketAddress.sin_addr.s_addr = inet_addr("10.10.10.253");

    gSocketSource = CFSocketCreateRunLoopSource(kCFAllocatorDefault, theSocket, 0);
    CFRunLoopAddSource(CFRunLoopGetCurrent(), gSocketSource, kCFRunLoopDefaultMode);

    CFDataRef socketData = CFDataCreate(kCFAllocatorDefault, (const UInt8 *)&socketAddress, sizeof(socketAddress));
    CFSocketError status = CFSocketConnectToAddress(theSocket, socketData, 30); //30 second timeout
    //check status here
    CFRelease(socketData);
}

Basically, if the server is unreachable at that port, you will most likely get a kCFSocketTimeout for CFSocketError status. If you are looking to parse a specific response back from the server to determine if the server is ready or not, the ConnectCallBack function will be called upon successful socket connection.

This is just a simple example, make sure not to block the UI by calling socket connections on the main thread such as recv()