Useful Objective-C methods, classes, objects, and categories. Basically a snippit collection.

BDHost.m

// From: http://www.bdunagan.com/2009/11/28/iphone-tip-no-nshost/

// MIT license
// Remember to add CFNetwork.framework to your project using Add=>Existing Frameworks.
 
#import "BDHost.h"
#import <CFNetwork/CFNetwork.h>
#import <netinet/in.h>
#import <netdb.h>
#import <ifaddrs.h>
#import <arpa/inet.h>
#import <net/ethernet.h>
#import <net/if_dl.h>
 
@implementation BDHost
 
+ (NSString *)addressForHostname:(NSString *)hostname {
    NSArray *addresses = [BDHost addressesForHostname:hostname];
    if ([addresses count] > 0)
        return [addresses objectAtIndex:0];
    else
        return nil;
}
 
+ (NSArray *)addressesForHostname:(NSString *)hostname {
    // Get the addresses for the given hostname.
    CFHostRef hostRef = CFHostCreateWithName(kCFAllocatorDefault, (CFStringRef)hostname);
    BOOL isSuccess = CFHostStartInfoResolution(hostRef, kCFHostAddresses, nil);
    if (!isSuccess) return nil;
    CFArrayRef addressesRef = CFHostGetAddressing(hostRef, nil);
    if (addressesRef == nil) return nil;
 
    // Convert these addresses into strings.
    char ipAddress[INET6_ADDRSTRLEN];
    NSMutableArray *addresses = [NSMutableArray array];
    CFIndex numAddresses = CFArrayGetCount(addressesRef);
    for (CFIndex currentIndex = 0; currentIndex < numAddresses; currentIndex++) {
        struct sockaddr *address = (struct sockaddr *)CFDataGetBytePtr(CFArrayGetValueAtIndex(addressesRef, currentIndex));
        if (address == nil) return nil;
        getnameinfo(address, address->sa_len, ipAddress, INET6_ADDRSTRLEN, nil, 0, NI_NUMERICHOST);
        if (ipAddress == nil) return nil;
        [addresses addObject:[NSString stringWithCString:ipAddress encoding:NSASCIIStringEncoding]];
    }
 
    return addresses;
}
 
+ (NSString *)hostnameForAddress:(NSString *)address {
    NSArray *hostnames = [BDHost hostnamesForAddress:address];
    if ([hostnames count] > 0)
        return [hostnames objectAtIndex:0];
    else
        return nil;
}
 
+ (NSArray *)hostnamesForAddress:(NSString *)address {
    // Get the host reference for the given address.
    struct addrinfo      hints;
    struct addrinfo      *result = NULL;
    memset(&hints, 0, sizeof(hints));
    hints.ai_flags    = AI_NUMERICHOST;
    hints.ai_family   = PF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = 0;
    int errorStatus = getaddrinfo([address cStringUsingEncoding:NSASCIIStringEncoding], NULL, &hints, &result);
    if (errorStatus != 0) return nil;
    CFDataRef addressRef = CFDataCreate(NULL, (UInt8 *)result->ai_addr, result->ai_addrlen);
    if (addressRef == nil) return nil;
    freeaddrinfo(result);
    CFHostRef hostRef = CFHostCreateWithAddress(kCFAllocatorDefault, addressRef);
    if (hostRef == nil) return nil;
    CFRelease(addressRef);
    BOOL isSuccess = CFHostStartInfoResolution(hostRef, kCFHostNames, NULL);
    if (!isSuccess) return nil;
 
    // Get the hostnames for the host reference.
    CFArrayRef hostnamesRef = CFHostGetNames(hostRef, NULL);
    NSMutableArray *hostnames = [NSMutableArray array];
    for (int currentIndex = 0; currentIndex < [(NSArray *)hostnamesRef count]; currentIndex++) {
        [hostnames addObject:[(NSArray *)hostnamesRef objectAtIndex:currentIndex]];
    }
 
    return hostnames;
}
 
+ (NSArray *)ipAddresses {
    NSMutableArray *addresses = [NSMutableArray array];
    struct ifaddrs *interfaces = NULL;
    struct ifaddrs *currentAddress = NULL;
 
    int success = getifaddrs(&interfaces);
    if (success == 0) {
        currentAddress = interfaces;
        while(currentAddress != NULL) {
            if(currentAddress->ifa_addr->sa_family == AF_INET) {
                NSString *address = [NSString stringWithUTF8String:inet_ntoa(((struct sockaddr_in *)currentAddress->ifa_addr)->sin_addr)];
                if (![address isEqual:@"127.0.0.1"]) {
                    NSLog(@"%@ ip: %@", [NSString stringWithUTF8String:currentAddress->ifa_name], address);
                    [addresses addObject:address];
                }
            }
            currentAddress = currentAddress->ifa_next;
        }
    }
    freeifaddrs(interfaces);
    return addresses;
}
 
+ (NSArray *)ethernetAddresses {
    NSMutableArray *addresses = [NSMutableArray array];
    struct ifaddrs *interfaces = NULL;
    struct ifaddrs *currentAddress = NULL;
    int success = getifaddrs(&interfaces);
    if (success == 0) {
        currentAddress = interfaces;
        while(currentAddress != NULL) {
            if(currentAddress->ifa_addr->sa_family == AF_LINK) {
                NSString *address = [NSString stringWithUTF8String:ether_ntoa((const struct ether_addr *)LLADDR((struct sockaddr_dl *)currentAddress->ifa_addr))];
 
                // ether_ntoa doesn't format the ethernet address with padding.
                char paddedAddress[80];
                int a,b,c,d,e,f;
                sscanf([address UTF8String], "%x:%x:%x:%x:%x:%x", &a, &b, &c, &d, &e, &f);
                sprintf(paddedAddress, "%02X:%02X:%02X:%02X:%02X:%02X",a,b,c,d,e,f);
                address = [NSString stringWithUTF8String:paddedAddress];
 
                if (![address isEqual:@"00:00:00:00:00:00"] && ![address isEqual:@"00:00:00:00:00:FF"]) {
                    NSLog(@"%@ mac: %@", [NSString stringWithUTF8String:currentAddress->ifa_name], address);
                    [addresses addObject:address];
                }
            }
            currentAddress = currentAddress->ifa_next;
        }
    }
    freeifaddrs(interfaces);
    return addresses;
}
 
@end

current-ip.m

// This is to get around NSHost being crippled on iOS.

+ (NSString *)getIPAddress { 
	NSString *address = @"error"; 
	struct ifaddrs *interfaces = NULL; struct ifaddrs *temp_addr = NULL;
	int success = 0; // retrieve the current interfaces - returns 0 on success  
	success = getifaddrs(&interfaces); 
	if (success == 0)  { 
		// Loop through linked list of interfaces  
		temp_addr = interfaces; 
		while(temp_addr != NULL)  { 
			if(temp_addr->ifa_addr->sa_family == AF_INET)  { 
				// Check if interface is en0 which is the wifi connection on the iPhone  
				if([[NSString stringWithUTF8String:temp_addr->ifa_name] isEqualToString:@"en0"])  { 
					// Get NSString from C String  
					address = [NSString stringWithUTF8String:inet_ntoa(((struct sockaddr_in *)temp_addr->ifa_addr)->sin_addr)]; 
				} 
			} 
			temp_addr = temp_addr->ifa_next; 
		} 
	} 
	// Free memory  
	freeifaddrs(interfaces); 
	return address; 
}

dns-resolver.m

// From: http://wiki.gonzofamily.com/a/DNS_resolution_with_IPv6_in_Objective_C

/**
 Give the IPs corresponding to a Hostname
 
 Sometime only 1 IPv4 is shown even if there's more.
 Sometime only 1 IPv6 is shown even if there's more.
 Certainly due to iOS Memory optimisation when locally cached
 
 @author Christian Gonzalvez, http://wiki.gonzofamily.com
 @param hostName A hostname
 @return an Array of NSString of all the corresponding IP addresses. The first
 is the Canonical name, the following are IPs (all NSString)
 */
+ (NSArray *)addressesForHostname:(NSString *)hostname
{
	const char* hostnameC = [hostname UTF8String];
 
	struct addrinfo hints, *res;
	struct sockaddr_in *s4;
	struct sockaddr_in6 *s6;
	int retval;
	char buf[64];
	NSMutableArray *result; //the array which will be return
	NSMutableArray *result4; //the array of IPv4, to order them at the end
	NSString *previousIP = nil;
 
	memset (&hints, 0, sizeof (struct addrinfo));
	hints.ai_family = PF_UNSPEC;//AF_INET6;
	hints.ai_flags = AI_CANONNAME;
		//AI_ADDRCONFIG, AI_ALL, AI_CANONNAME,	AI_NUMERICHOST
		//AI_NUMERICSERV, AI_PASSIVE, OR AI_V4MAPPED
 
	retval = getaddrinfo(hostnameC, NULL, &hints, &res);
	if (retval == 0)
	  {
 
		if (res->ai_canonname)
		  {
			result = [NSMutableArray arrayWithObject:[NSString stringWithUTF8String:res->ai_canonname]];
		  }
		else
		  {
				//it means the DNS didn't know this host
			return nil;
		  }
		result4= [NSMutableArray array];
		while (res) {
			switch (res->ai_family){
				case AF_INET6:				
					s6 = (struct sockaddr_in6 *)res->ai_addr;
					if(inet_ntop(res->ai_family, (void *)&(s6->sin6_addr), buf, sizeof(buf))
					   == NULL)
					  {
						NSLog(@"inet_ntop failed for v6!\n");
					  }
					else
					  {
							//surprisingly every address is in double, let's add this test
						if (![previousIP isEqualToString:[NSString stringWithUTF8String:buf]]) {
							[result addObject:[NSString stringWithUTF8String:buf]];
						}
					  }
					break;
 
				case AF_INET:				
					s4 = (struct sockaddr_in *)res->ai_addr;
					if(inet_ntop(res->ai_family, (void *)&(s4->sin_addr), buf, sizeof(buf))
					   == NULL)
					  {
						NSLog(@"inet_ntop failed for v4!\n");
					  }
					else
					  {
							//surprisingly every address is in double, let's add this test
						if (![previousIP isEqualToString:[NSString stringWithUTF8String:buf]]) {
							[result4 addObject:[NSString stringWithUTF8String:buf]];
						}
					  }
					break;
				default:
					NSLog(@"Neither IPv4 nor IPv6!");
 
			}
				//surprisingly every address is in double, let's add this test
			previousIP = [NSString stringWithUTF8String:buf];
 
			res = res->ai_next;
		}
	  }else{
		  NSLog(@"no IP found");
		  return nil;
	  }
 
	return [result arrayByAddingObjectsFromArray:result4];
}

see-also.textile

• http://developer.apple.com/library/ios/#qa/qa1652/_index.html
• http://blog.zachwaugh.com/post/309927273/programmatically-retrieving-ip-address-of-iphone
• http://blog.zachwaugh.com/post/309924609/how-to-use-json-in-cocoaobjective-c
• http://stackoverflow.com/questions/258284/srv-record-lookup-with-iphone-sdk