SimplePing.m

/*
    File:       SimplePing.m

    Contains:   Implements ping.

    Written by: DTS

    Copyright:  Copyright (c) 2010-2012 Apple Inc. All Rights Reserved.

    Disclaimer: IMPORTANT: This Apple software is supplied to you by Apple Inc.
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                The Apple Software is provided by Apple on an "AS IS" basis. 
                APPLE MAKES NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
                WITHOUT LIMITATION THE IMPLIED WARRANTIES OF NON-INFRINGEMENT,
                MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, REGARDING
                THE APPLE SOFTWARE OR ITS USE AND OPERATION ALONE OR IN
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*/

#import "SimplePing.h"

#include <sys/socket.h>
#include <netinet/in.h>
#include <errno.h>

#pragma mark * ICMP On-The-Wire Format

static uint16_t in_cksum(const void *buffer, size_t bufferLen)
    // This is the standard BSD checksum code, modified to use modern types.
{
	size_t              bytesLeft;
    int32_t             sum;
	const uint16_t *    cursor;
	union {
		uint16_t        us;
		uint8_t         uc[2];
	} last;
	uint16_t            answer;

	bytesLeft = bufferLen;
	sum = 0;
	cursor = buffer;

	/*
	 * Our algorithm is simple, using a 32 bit accumulator (sum), we add
	 * sequential 16 bit words to it, and at the end, fold back all the
	 * carry bits from the top 16 bits into the lower 16 bits.
	 */
	while (bytesLeft > 1) {
		sum += *cursor;
        cursor += 1;
		bytesLeft -= 2;
	}

	/* mop up an odd byte, if necessary */
	if (bytesLeft == 1) {
		last.uc[0] = * (const uint8_t *) cursor;
		last.uc[1] = 0;
		sum += last.us;
	}

	/* add back carry outs from top 16 bits to low 16 bits */
	sum = (sum >> 16) + (sum & 0xffff);	/* add hi 16 to low 16 */
	sum += (sum >> 16);			/* add carry */
	answer = (uint16_t) ~sum;   /* truncate to 16 bits */

	return answer;
}

#pragma mark * SimplePing

@interface SimplePing ()

@property (nonatomic, copy,   readwrite) NSData *           hostAddress;
@property (nonatomic, assign, readwrite) uint16_t           nextSequenceNumber;

- (void)stopHostResolution;
- (void)stopDataTransfer;

@end

@implementation SimplePing
{
    CFHostRef               _host;
    CFSocketRef             _socket;
}

@synthesize hostName           = _hostName;
@synthesize hostAddress        = _hostAddress;

@synthesize delegate           = _delegate;
@synthesize identifier         = _identifier;
@synthesize nextSequenceNumber = _nextSequenceNumber;

- (id)initWithHostName:(NSString *)hostName address:(NSData *)hostAddress
    // The initialiser common to both of our construction class methods.
{
    assert( (hostName != nil) == (hostAddress == nil) );
    self = [super init];
    if (self != nil) {
        self->_hostName    = [hostName copy];
        self->_hostAddress = [hostAddress copy];
        self->_identifier  = (uint16_t) arc4random();
    }
    return self;
}

- (void)dealloc
{
    // -stop takes care of _host and _socket.
    [self stop];
    assert(self->_host == NULL);
    assert(self->_socket == NULL);
}

+ (SimplePing *)simplePingWithHostName:(NSString *)hostName
    // See comment in header.
{
    return [[SimplePing alloc] initWithHostName:hostName address:nil];
}

+ (SimplePing *)simplePingWithHostAddress:(NSData *)hostAddress
    // See comment in header.
{
    return [[SimplePing alloc] initWithHostName:NULL address:hostAddress];
}

- (void)noop
{
}

- (void)didFailWithError:(NSError *)error
    // Shut down the pinger object and tell the delegate about the error.
{
    assert(error != nil);
    
    // We retain ourselves temporarily because it's common for the delegate method 
    // to release its last reference to use, which causes -dealloc to be called here. 
    // If we then reference self on the return path, things go badly.  I don't think 
    // that happens currently, but I've got into the habit of doing this as a 
    // defensive measure.
    
    [self performSelector:@selector(noop) withObject:nil afterDelay:0.0];
    
    [self stop];
    if ( (self.delegate != nil) && [self.delegate respondsToSelector:@selector(simplePing:didFailWithError:)] ) {
        [self.delegate simplePing:self didFailWithError:error];
    }
}

- (void)didFailWithHostStreamError:(CFStreamError)streamError
    // Convert the CFStreamError to an NSError and then call through to 
    // -didFailWithError: to shut down the pinger object and tell the 
    // delegate about the error.
{
    NSDictionary *  userInfo;
    NSError *       error;

    if (streamError.domain == kCFStreamErrorDomainNetDB) {
        userInfo = [NSDictionary dictionaryWithObjectsAndKeys:
            [NSNumber numberWithInteger:streamError.error], kCFGetAddrInfoFailureKey,
            nil
        ];
    } else {
        userInfo = nil;
    }
    error = [NSError errorWithDomain:(NSString *)kCFErrorDomainCFNetwork code:kCFHostErrorUnknown userInfo:userInfo];
    assert(error != nil);

    [self didFailWithError:error];
}

- (void)sendPingWithData:(NSData *)data
    // See comment in header.
{
    int             err;
    NSData *        payload;
    NSMutableData * packet;
    ICMPHeader *    icmpPtr;
    ssize_t         bytesSent;
    
    // Construct the ping packet.
    
    payload = data;
    if (payload == nil) {
        payload = [[NSString stringWithFormat:@"%28zd bottles of beer on the wall", (ssize_t) 99 - (size_t) (self.nextSequenceNumber % 100) ] dataUsingEncoding:NSASCIIStringEncoding];
        assert(payload != nil);
        
        assert([payload length] == 56);
    }
    
    packet = [NSMutableData dataWithLength:sizeof(*icmpPtr) + [payload length]];
    assert(packet != nil);

    icmpPtr = [packet mutableBytes];
    icmpPtr->type = kICMPTypeEchoRequest;
    icmpPtr->code = 0;
    icmpPtr->checksum = 0;
    icmpPtr->identifier     = OSSwapHostToBigInt16(self.identifier);
    icmpPtr->sequenceNumber = OSSwapHostToBigInt16(self.nextSequenceNumber);
    memcpy(&icmpPtr[1], [payload bytes], [payload length]);
    
    // The IP checksum returns a 16-bit number that's already in correct byte order 
    // (due to wacky 1's complement maths), so we just put it into the packet as a 
    // 16-bit unit.
    
    icmpPtr->checksum = in_cksum([packet bytes], [packet length]);
    
    // Send the packet.
    
    if (self->_socket == NULL) {
        bytesSent = -1;
        err = EBADF;
    } else {
        bytesSent = sendto(
            CFSocketGetNative(self->_socket),
            [packet bytes],
            [packet length], 
            0, 
            (struct sockaddr *) [self.hostAddress bytes], 
            (socklen_t) [self.hostAddress length]
        );
        err = 0;
        if (bytesSent < 0) {
            err = errno;
        }
    }

    // Handle the results of the send.
    
    if ( (bytesSent > 0) && (((NSUInteger) bytesSent) == [packet length]) ) {

        // Complete success.  Tell the client.

        if ( (self.delegate != nil) && [self.delegate respondsToSelector:@selector(simplePing:didSendPacket:)] ) {
            [self.delegate simplePing:self didSendPacket:packet];
        }
    } else {
        NSError *   error;
        
        // Some sort of failure.  Tell the client.
        
        if (err == 0) {
            err = ENOBUFS;          // This is not a hugely descriptor error, alas.
        }
        error = [NSError errorWithDomain:NSPOSIXErrorDomain code:err userInfo:nil];
        if ( (self.delegate != nil) && [self.delegate respondsToSelector:@selector(simplePing:didFailToSendPacket:error:)] ) {
            [self.delegate simplePing:self didFailToSendPacket:packet error:error];
        }
    }
    
    self.nextSequenceNumber += 1;
}

+ (NSUInteger)icmpHeaderOffsetInPacket:(NSData *)packet
    // Returns the offset of the ICMPHeader within an IP packet.
{
    NSUInteger              result;
    const struct IPHeader * ipPtr;
    size_t                  ipHeaderLength;
    
    result = NSNotFound;
    if ([packet length] >= (sizeof(IPHeader) + sizeof(ICMPHeader))) {
        ipPtr = (const IPHeader *) [packet bytes];
        assert((ipPtr->versionAndHeaderLength & 0xF0) == 0x40);     // IPv4
        assert(ipPtr->protocol == 1);                               // ICMP
        ipHeaderLength = (ipPtr->versionAndHeaderLength & 0x0F) * sizeof(uint32_t);
        if ([packet length] >= (ipHeaderLength + sizeof(ICMPHeader))) {
            result = ipHeaderLength;
        }
    }
    return result;
}

+ (const struct ICMPHeader *)icmpInPacket:(NSData *)packet
    // See comment in header.
{
    const struct ICMPHeader *   result;
    NSUInteger                  icmpHeaderOffset;
    
    result = nil;
    icmpHeaderOffset = [self icmpHeaderOffsetInPacket:packet];
    if (icmpHeaderOffset != NSNotFound) {
        result = (const struct ICMPHeader *) (((const uint8_t *)[packet bytes]) + icmpHeaderOffset);
    }
    return result;
}

- (BOOL)isValidPingResponsePacket:(NSMutableData *)packet
    // Returns true if the packet looks like a valid ping response packet destined 
    // for us.
{
    BOOL                result;
    NSUInteger          icmpHeaderOffset;
    ICMPHeader *        icmpPtr;
    uint16_t            receivedChecksum;
    uint16_t            calculatedChecksum;
    
    result = NO;
    
    icmpHeaderOffset = [[self class] icmpHeaderOffsetInPacket:packet];
    if (icmpHeaderOffset != NSNotFound) {
        icmpPtr = (struct ICMPHeader *) (((uint8_t *)[packet mutableBytes]) + icmpHeaderOffset);

        receivedChecksum   = icmpPtr->checksum;
        icmpPtr->checksum  = 0;
        calculatedChecksum = in_cksum(icmpPtr, [packet length] - icmpHeaderOffset);
        icmpPtr->checksum  = receivedChecksum;
        
        if (receivedChecksum == calculatedChecksum) {
            if ( (icmpPtr->type == kICMPTypeEchoReply) && (icmpPtr->code == 0) ) {
                if ( OSSwapBigToHostInt16(icmpPtr->identifier) == self.identifier ) {
                    if ( OSSwapBigToHostInt16(icmpPtr->sequenceNumber) < self.nextSequenceNumber ) {
                        result = YES;
                    }
                }
            }
        }
    }

    return result;
}

- (void)readData
    // Called by the socket handling code (SocketReadCallback) to process an ICMP 
    // messages waiting on the socket.
{
    int                     err;
    struct sockaddr_storage addr;
    socklen_t               addrLen;
    ssize_t                 bytesRead;
    void *                  buffer;
    enum { kBufferSize = 65535 };

    // 65535 is the maximum IP packet size, which seems like a reasonable bound 
    // here (plus it's what <x-man-page://8/ping> uses).
    
    buffer = malloc(kBufferSize);
    assert(buffer != NULL);
    
    // Actually read the data.
    
    addrLen = sizeof(addr);
    bytesRead = recvfrom(CFSocketGetNative(self->_socket), buffer, kBufferSize, 0, (struct sockaddr *) &addr, &addrLen);
    err = 0;
    if (bytesRead < 0) {
        err = errno;
    }
    
    // Process the data we read.
    
    if (bytesRead > 0) {
        NSMutableData *     packet;

        packet = [NSMutableData dataWithBytes:buffer length:(NSUInteger) bytesRead];
        assert(packet != nil);

        // We got some data, pass it up to our client.

        if ( [self isValidPingResponsePacket:packet] ) {
            if ( (self.delegate != nil) && [self.delegate respondsToSelector:@selector(simplePing:didReceivePingResponsePacket:)] ) {
                [self.delegate simplePing:self didReceivePingResponsePacket:packet];
            }
        } else {
            if ( (self.delegate != nil) && [self.delegate respondsToSelector:@selector(simplePing:didReceiveUnexpectedPacket:)] ) {
                [self.delegate simplePing:self didReceiveUnexpectedPacket:packet];
            }
        }
    } else {
    
        // We failed to read the data, so shut everything down.
        
        if (err == 0) {
            err = EPIPE;
        }
        [self didFailWithError:[NSError errorWithDomain:NSPOSIXErrorDomain code:err userInfo:nil]];
    }
    
    free(buffer);
    
    // Note that we don't loop back trying to read more data.  Rather, we just 
    // let CFSocket call us again.
}

static void SocketReadCallback(CFSocketRef s, CFSocketCallBackType type, CFDataRef address, const void *data, void *info)
    // This C routine is called by CFSocket when there's data waiting on our 
    // ICMP socket.  It just redirects the call to Objective-C code.
{
    SimplePing *    obj;
    
    obj = (__bridge SimplePing *) info;
    assert([obj isKindOfClass:[SimplePing class]]);
    
    #pragma unused(s)
    assert(s == obj->_socket);
    #pragma unused(type)
    assert(type == kCFSocketReadCallBack);
    #pragma unused(address)
    assert(address == nil);
    #pragma unused(data)
    assert(data == nil);
    
    [obj readData];
}

- (void)startWithHostAddress
    // We have a host address, so let's actually start pinging it.
{
    int                     err;
    int                     fd;
    const struct sockaddr * addrPtr;

    assert(self.hostAddress != nil);

    // Open the socket.
    
    addrPtr = (const struct sockaddr *) [self.hostAddress bytes];

    fd = -1;
    err = 0;
    switch (addrPtr->sa_family) {
        case AF_INET: {
            fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_ICMP);
            if (fd < 0) {
                err = errno;
            }
        } break;
        case AF_INET6:
            assert(NO);
            // fall through
        default: {
            err = EPROTONOSUPPORT;
        } break;
    }
    
    if (err != 0) {
        [self didFailWithError:[NSError errorWithDomain:NSPOSIXErrorDomain code:err userInfo:nil]];
    } else {
        CFSocketContext     context = {0, (__bridge void *)(self), NULL, NULL, NULL};
        CFRunLoopSourceRef  rls;
        
        // Wrap it in a CFSocket and schedule it on the runloop.
        
        self->_socket = CFSocketCreateWithNative(NULL, fd, kCFSocketReadCallBack, SocketReadCallback, &context);
        assert(self->_socket != NULL);
        
        // The socket will now take care of cleaning up our file descriptor.
        
        assert( CFSocketGetSocketFlags(self->_socket) & kCFSocketCloseOnInvalidate );
        fd = -1;
        
        rls = CFSocketCreateRunLoopSource(NULL, self->_socket, 0);
        assert(rls != NULL);
        
        CFRunLoopAddSource(CFRunLoopGetCurrent(), rls, kCFRunLoopDefaultMode);
        
        CFRelease(rls);

        if ( (self.delegate != nil) && [self.delegate respondsToSelector:@selector(simplePing:didStartWithAddress:)] ) {
            [self.delegate simplePing:self didStartWithAddress:self.hostAddress];
        }
    }
    assert(fd == -1);
}

- (void)hostResolutionDone
    // Called by our CFHost resolution callback (HostResolveCallback) when host 
    // resolution is complete.  We just latch the first IPv4 address and kick 
    // off the pinging process.
{
    Boolean     resolved;
    NSArray *   addresses;
    
    // Find the first IPv4 address.
    
    addresses = (__bridge NSArray *) CFHostGetAddressing(self->_host, &resolved);
    if ( resolved && (addresses != nil) ) {
        resolved = false;
        for (NSData * address in addresses) {
            const struct sockaddr * addrPtr;
            
            addrPtr = (const struct sockaddr *) [address bytes];
            if ( [address length] >= sizeof(struct sockaddr) && addrPtr->sa_family == AF_INET) {
                self.hostAddress = address;
                resolved = true;
                break;
            }
        }
    }

    // We're done resolving, so shut that down.
    
    [self stopHostResolution];
    
    // If all is OK, start pinging, otherwise shut down the pinger completely.
    
    if (resolved) {
        [self startWithHostAddress];
    } else {
        [self didFailWithError:[NSError errorWithDomain:(NSString *)kCFErrorDomainCFNetwork code:kCFHostErrorHostNotFound userInfo:nil]];
    }
}

static void HostResolveCallback(CFHostRef theHost, CFHostInfoType typeInfo, const CFStreamError *error, void *info)
    // This C routine is called by CFHost when the host resolution is complete. 
    // It just redirects the call to the appropriate Objective-C method.
{
    SimplePing *    obj;

    NSLog(@">HostResolveCallback");
    
    obj = (__bridge SimplePing *) info;
    assert([obj isKindOfClass:[SimplePing class]]);
    
    #pragma unused(theHost)
    assert(theHost == obj->_host);
    #pragma unused(typeInfo)
    assert(typeInfo == kCFHostAddresses);
    
    if ( (error != NULL) && (error->domain != 0) ) {
        [obj didFailWithHostStreamError:*error];
    } else {
        [obj hostResolutionDone];
    }
}

- (void)start
    // See comment in header.
{
    // If the user supplied us with an address, just start pinging that.  Otherwise 
    // start a host resolution.
    
    if (self->_hostAddress != nil) {
        [self startWithHostAddress];
    } else {
        Boolean             success;
        CFHostClientContext context = {0, (__bridge void *)(self), NULL, NULL, NULL};
        CFStreamError       streamError;

        assert(self->_host == NULL);

        self->_host = CFHostCreateWithName(NULL, (__bridge CFStringRef) self.hostName);
        assert(self->_host != NULL);
        
        CFHostSetClient(self->_host, HostResolveCallback, &context);
        
        CFHostScheduleWithRunLoop(self->_host, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode);
        
        NSLog(@">CFHostStartInfoResolution");
        success = CFHostStartInfoResolution(self->_host, kCFHostAddresses, &streamError);
        NSLog(@"<CFHostStartInfoResolution");
        if ( ! success ) {
            [self didFailWithHostStreamError:streamError];
        }
    }
}

- (void)stopHostResolution
    // Shut down the CFHost.
{
    if (self->_host != NULL) {
        CFHostSetClient(self->_host, NULL, NULL);
        CFHostUnscheduleFromRunLoop(self->_host, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode);
        CFRelease(self->_host);
        self->_host = NULL;
    }
}

- (void)stopDataTransfer   
    // Shut down anything to do with sending and receiving pings.
{
    if (self->_socket != NULL) {
        CFSocketInvalidate(self->_socket);
        CFRelease(self->_socket);
        self->_socket = NULL;
    }
}

- (void)stop
    // See comment in header.
{
    [self stopHostResolution];
    [self stopDataTransfer];
    
    // If we were started with a host name, junk the host address on stop.  If the 
    // client calls -start again, we'll re-resolve the host name.
    
    if (self.hostName != nil) {
        self.hostAddress = NULL;
    }
}

@end