Network change detection on macOS

callback.go

package main

/*
#cgo CFLAGS: -mmacosx-version-min=10.6 -D__MAC_OS_X_VERSION_MAX_ALLOWED=1080
#cgo LDFLAGS: -framework CoreFoundation -framework SystemConfiguration
#include <SystemConfiguration/SystemConfiguration.h>
*/
import "C"
import "unsafe"

//export nativeNetworkChangeCallback
func nativeNetworkChangeCallback(_ unsafe.Pointer, changedKeys C.CFArrayRef) {
	callback(changedKeys)
}

main.go

package main

/*
#cgo CFLAGS: -mmacosx-version-min=10.6 -D__MAC_OS_X_VERSION_MAX_ALLOWED=1080
#cgo LDFLAGS: -framework CoreFoundation -framework SystemConfiguration
#include <SystemConfiguration/SystemConfiguration.h>

extern void nativeNetworkChangeCallback(void * context, CFArrayRef changedKeys);

// cfstring_utf8_length returns the number of characters successfully converted to UTF-8 and
// the bytes required to store them.
static inline CFIndex cfstring_utf8_length(CFStringRef str, CFIndex *need) {
  CFIndex n, usedBufLen;
  CFRange rng = CFRangeMake(0, CFStringGetLength(str));
  return CFStringGetBytes(str, rng, kCFStringEncodingUTF8, 0, 0, NULL, 0, need);
}

// MacOS/X Code taken from http://developer.apple.com/technotes/tn/tn1145.html
static OSStatus MoreSCErrorBoolean(Boolean success)
{
    OSStatus err = noErr;
    if (!success)
    {
        int scErr = SCError();
        if (scErr == kSCStatusOK) scErr = kSCStatusFailed;
        err = scErr;
    }
    return err;
}

static OSStatus MoreSCError(const void *value) {return MoreSCErrorBoolean(value != NULL);}
static OSStatus CFQError(CFTypeRef cf) {return (cf == NULL) ? -1 : noErr;}
static void CFQRelease(CFTypeRef cf) {if (cf != NULL) CFRelease(cf);}

// Create a SCF dynamic store reference and a corresponding CFRunLoop source.  If you add the
// run loop source to your run loop then the supplied callback function will be called when local IP
// address list changes.
static OSStatus CreateIPAddressListChangeCallbackSCF(SCDynamicStoreCallBack callback, void *contextPtr, SCDynamicStoreRef *storeRef, CFRunLoopSourceRef *sourceRef)
{
    OSStatus                err;
    SCDynamicStoreContext   context = {0, NULL, NULL, NULL, NULL};
    SCDynamicStoreRef       ref = NULL;
    CFStringRef             patterns[2] = {NULL, NULL};
    CFArrayRef              patternList = NULL;
    CFRunLoopSourceRef      rls = NULL;

    assert(callback   != NULL);
    assert( storeRef  != NULL);
    assert(*storeRef  == NULL);
    assert( sourceRef != NULL);
    assert(*sourceRef == NULL);

    // Create a connection to the dynamic store, then create
    // a search pattern that finds all entities.
    context.info = contextPtr;
    ref = SCDynamicStoreCreate(NULL, CFSTR("AddIPAddressListChangeCallbackSCF"), callback, &context);
    err = MoreSCError(ref);
    if (err == noErr)
    {
        // This pattern is "State:/Network/Service/[^/]+/IPv4".
        patterns[0] = SCDynamicStoreKeyCreateNetworkInterfaceEntity(NULL, kSCDynamicStoreDomainState, kSCCompAnyRegex, kSCEntNetIPv4);
        err = MoreSCError(patterns[0]);
        if (err == noErr)
        {
            // This pattern is "State:/Network/Service/[^/]+/IPv6".
            patterns[1] = SCDynamicStoreKeyCreateNetworkInterfaceEntity(NULL, kSCDynamicStoreDomainState, kSCCompAnyRegex, kSCEntNetIPv6);
            err = MoreSCError(patterns[1]);
        }
    }

    // Create a pattern list containing just one pattern,
    // then tell SCF that we want to watch changes in keys
    // that match that pattern list, then create our run loop
    // source.
    if (err == noErr)
    {
        patternList = CFArrayCreate(NULL, (const void **) patterns, 2, &kCFTypeArrayCallBacks);
        err = CFQError(patternList);
    }
    if (err == noErr) err = MoreSCErrorBoolean(SCDynamicStoreSetNotificationKeys(ref, NULL, patternList));
    if (err == noErr)
    {
        rls = SCDynamicStoreCreateRunLoopSource(NULL, ref, 0);
        err = MoreSCError(rls);
    }

    // Clean up.
    CFQRelease(patterns[0]);
    CFQRelease(patterns[1]);
    CFQRelease(patternList);
    if (err != noErr)
    {
        CFQRelease(ref);
        ref = NULL;
    }
    *storeRef = ref;
    *sourceRef = rls;

    assert( (err == noErr) == (*storeRef  != NULL) );
    assert( (err == noErr) == (*sourceRef != NULL) );

    return err;
}

void IPConfigChangedCallback(SCDynamicStoreRef store, CFArrayRef changedKeys, void * context)
{
    nativeNetworkChangeCallback(context, changedKeys);
}
*/
import "C"
import (
	"log"
	"net"
	"os"
	"os/signal"
	"reflect"
	"runtime"
	"strings"
	"unsafe"
)

var (
	running   bool
	storeRef  C.SCDynamicStoreRef
	sourceRef C.CFRunLoopSourceRef
)

func main() {
	log.Println("Starting network change detection")

	returnCode := C.CreateIPAddressListChangeCallbackSCF((*[0]byte)(C.IPConfigChangedCallback), nil, &storeRef, &sourceRef)
	if returnCode == C.noErr {
		go nativeLoop()
	} else {
		log.Fatalf("failed to setup native hook: error code %#v", returnCode)
	}

	interruptHandle := make(chan os.Signal)
	signal.Notify(interruptHandle, os.Interrupt)
	<-interruptHandle

	running = false
	log.Println("Stopping network change detection")
}

func nativeLoop() {
	runtime.LockOSThread()
	defer runtime.UnlockOSThread()

	C.CFRunLoopAddSource(C.CFRunLoopGetCurrent(), sourceRef, C.kCFRunLoopDefaultMode)
	running = true

	for running {
		C.CFRunLoopRun()
	}

	C.CFRunLoopRemoveSource(C.CFRunLoopGetCurrent(), sourceRef, C.kCFRunLoopDefaultMode)
	C.CFRelease((C.CFTypeRef)(storeRef))
	C.CFRelease((C.CFTypeRef)(sourceRef))
}

func callback(changedKeys C.CFArrayRef) {
	if (int)(C.CFArrayGetCount(changedKeys)) == 0 {
		return
	}

	changeDetails := cfstringGo((C.CFStringRef)(
		C.CFArrayGetValueAtIndex(changedKeys, (C.CFIndex)(0)),
	))

	ipInterfaceName := strings.Split(changeDetails, "/")[3]
	ipInterface, _ := net.InterfaceByName(ipInterfaceName)

	log.Printf("Interface changed state: %#v", ipInterface)
}

// cfstring will convert a OSX
// CFStringRef string into a Golang
// compatible string
func cfstringGo(cfs C.CFStringRef) string {
	var usedBufLen C.CFIndex
	n := C.cfstring_utf8_length(cfs, &usedBufLen)
	if n <= 0 {
		return ""
	}
	rng := C.CFRange{location: C.CFIndex(0), length: n}
	buf := make([]byte, int(usedBufLen))

	bufp := unsafe.Pointer(&buf[0])
	C.CFStringGetBytes(cfs, rng, C.kCFStringEncodingUTF8, 0, 0, (*C.UInt8)(bufp), C.CFIndex(len(buf)), &usedBufLen)

	sh := &reflect.StringHeader{
		Data: uintptr(bufp),
		Len:  int(usedBufLen),
	}
	return *(*string)(unsafe.Pointer(sh))
}

Hey, does this code have a license? I'm using it to fix an issue with pf and would like to be able to make the hack open-source.