Time to broadcast, iPhone style
Ran into a need to dynamically determine the current UDP broadcast address for the WiFi interface on the ole’ iPhone. Since NSHost appears to be a private API even w/the 3.0 software, it seems one must go lower. I wrapped it up in a neat little bundle that seems fairly usable if not verbose and full of magic (but understandable) numbers.
A few things of note. en0 is the WiFi interface. There are others. Instrument the following code w/some debug to get them all out. The ip/netmask methods return nil when the WiFi interface is not active. I would also be shocked if there were no corner cases I am ignoring…
#include <arpa/inet.h> #include <net/if.h> #include <sys/ioctl.h> #include <sys/socket.h> #include <errno.h> #include <ifaddrs.h> #include <stdio.h> static NSString *kWifiInterface = @"en0"; @implementation NetUtil + (NSString *)broadcastAddressForAddress:(NSString *)ipAddress withMask:(NSString *)netmask { NSAssert(nil != ipAddress, @"IP address cannot be nil"); NSAssert(nil != netmask, @"Netmask cannot be nil"); NSArray *ipChunks = [ipAddress componentsSeparatedByString:@"."]; NSAssert([ipChunks count] == 4, @"IP does not have 4 octets!"); NSArray *nmChunks = [netmask componentsSeparatedByString:@"."]; NSAssert([nmChunks count] == 4, @"Netmask does not have 4 octets!"); NSUInteger ipRaw = 0; NSUInteger nmRaw = 0; NSUInteger shift = 24; for (NSUInteger i = 0; i < 4; ++i, shift -= 8) { ipRaw |= [[ipChunks objectAtIndex:i] intValue] << shift; nmRaw |= [[nmChunks objectAtIndex:i] intValue] << shift; } NSUInteger bcRaw = ~nmRaw | ipRaw; return [NSString stringWithFormat:@"%d.%d.%d.%d", (bcRaw & 0xFF000000) >> 24, (bcRaw & 0x00FF0000) >> 16, (bcRaw & 0x0000FF00) >> 8, bcRaw & 0x000000FF]; } + (NSString *)ipAddressForInterface:(NSString *)ifName { NSAssert(nil != ifName, @"Interface name cannot be nil"); struct ifaddrs *addrs = NULL; if (getifaddrs(&addrs)) { NSLog(@"Failed to enumerate interfaces: %@", [NSString stringWithCString:strerror(errno)]); return nil; } /* walk the linked-list of interfaces until we find the desired one */ NSString *addr = nil; struct ifaddrs *curAddr = addrs; while (curAddr != NULL) { if (AF_INET == curAddr->ifa_addr->sa_family) { NSString *curName = [NSString stringWithCString:curAddr->ifa_name]; if ([ifName isEqualToString:curName]) { char* cstring = inet_ntoa(((struct sockaddr_in *)curAddr->ifa_addr)->sin_addr); addr = [NSString stringWithCString:cstring]; break; } } curAddr = curAddr->ifa_next; } /* clean up, return what we found */ freeifaddrs(addrs); return addr; } + (NSString *)ipAddressForWifi { return [NetUtil ipAddressForInterface:kWifiInterface]; } + (NSString *)netmaskForInterface:(NSString *)ifName { NSAssert(nil != ifName, @"Interface name cannot be nil"); struct ifreq ifr; strncpy(ifr.ifr_name, [ifName UTF8String], IFNAMSIZ-1); int fd = socket(AF_INET, SOCK_DGRAM, 0); if (-1 == fd) { NSLog(@"Failed to open socket to get netmask"); return nil; } if (-1 == ioctl(fd, SIOCGIFNETMASK, &ifr)) { NSLog(@"Failed to read netmask: %@", [NSString stringWithCString:strerror(errno)]); close(fd); return nil; } close(fd); char *cstring = inet_ntoa(((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr); return [NSString stringWithCString:cstring]; } + (NSString *)netmaskForWifi { return [NetUtil netmaskForInterface:kWifiInterface]; } @end
