Go C ffi example

pcap.go

/*
 * Copyright (c) 2013 Landon Fuller <landonf@mac68k.info>
 * All rights reserved.
 */

/* Interface to the native pcap(3) library */
package pcap

/*
#cgo LDFLAGS: -lpcap
#include <stdlib.h>
#include <string.h>
#include <pcap/pcap.h>
#include <sys/socket.h>
#include <netinet/in.h>

#include <stdint.h>

// Linux, BSD, and Mac OS X compatible address fetching implementation
// TODO: Untested on Linux
#if defined(__linux__)
#define AF_LINK AF_PACKET
#else
#include <net/if_dl.h>
#endif

static int fetch_mac_addr (const char *ifname, struct sockaddr_storage *ss, uint8_t mac48[5]) {
#if defined(__linux__)
    struct ifreq ifr;

    int fd = socket(AF_INET, SOCK_DGRAM, 0);
    if (fd < 0)
        return -1;

    ifr.ifr_addr.sa_family = AF_INET;
    strcpy(ifr.ifr_name, ifname);
    if (ioctl(fd, SIOCGIFHWADDR, &ifr) != 0)
        return -1;
    close(fd);

    memcpy(mac48, ifr.ifr_hwaddr.sa_data, sizeof(mac48));

    return 0;
#elif defined(AF_LINK)
    struct sockaddr_dl *sdl = (struct sockaddr_dl *) ss;
    memcpy(mac48, LLADDR(sdl), sizeof(mac48));
    return 0;
#else
#error Unsupported OS
#endif
} 

*/
import "C"

import (
	"errors"
	"fmt"
	"runtime"
	"unsafe"
)

const (
	// IPv4
	AF_INET = C.AF_INET

	// IPv6
	AF_INET6 = C.AF_INET6

	// BSD-style AF_LINK. Mapped to AF_PACKET on Linux.
	AF_LINK = C.AF_LINK
)

/* A standard UNIX sockaddr structure. */
type SocketAddress struct {
	// The parent interface name
	ifname string

	// Backing storage
	cval C.struct_sockaddr_storage
}

// Return the socket address family
func (sa *SocketAddress) Family() int {
	return int(sa.cval.ss_family)
}

// Return the socket address in network byte order, or nil
// if the address' family is unknown
func (sa *SocketAddress) AddressBytes() []byte {
	switch sa.Family() {
	case AF_INET:
		sin := (*C.struct_sockaddr_in)(unsafe.Pointer(&sa.cval))
		return C.GoBytes(unsafe.Pointer(&sin.sin_addr), 4)

	case AF_INET6:
		sin6 := (*C.struct_sockaddr_in6)(unsafe.Pointer(&sa.cval))
		return C.GoBytes(unsafe.Pointer(&sin6.sin6_addr), 16)

	case AF_LINK:
		cstr := C.CString(sa.ifname)
		defer C.free(unsafe.Pointer(cstr))

		var mac64 [5]C.uint8_t
		C.fetch_mac_addr(cstr, &sa.cval, &mac64[0])
		return C.GoBytes(unsafe.Pointer(&mac64[0]), 5)
	}

	return nil
}

// Convert a (possibly nil) sockaddr struct pointer to a SocketAddress instance
func newSocketAddress(ifname string, sa *C.struct_sockaddr) *SocketAddress {
	if sa == nil {
		return nil
	}

	address := new(SocketAddress)
	C.memcpy(unsafe.Pointer(&address.cval), unsafe.Pointer(sa), C.size_t(sa.sa_len))
	address.ifname = ifname

	return address
}

/* Address associated with a pcap interface */
type InterfaceAddress struct {
	// The address assigned to this interface
	source *SocketAddress

	// The netmask. May be nil.
	netmask *SocketAddress

	// The broadcast address. May be nil.
	broadcast *SocketAddress

	// The destination address (eg, for point-to-point links). May be nil.
	destination *SocketAddress
}

// Return the address assigned to this interface   
func (addr *InterfaceAddress) Source() *SocketAddress {
	return addr.source
}

/*
 * A pcap interface.
 */
type Interface struct {
	// Interface name.
	name string

	// Human readable description. May be nil.
	description string

	// If true, this is a loopback interface
	loopback bool

	// InterfaceAddress associated with this interface
	addresses []*InterfaceAddress
}

// Return the interface name.
func (iface *Interface) Name() string {
	return iface.name
}

// Return the interface description. May be nil
func (iface *Interface) Description() string {
	return iface.description
}

// Return true if this is a loopback interface
func (iface *Interface) Loopback() bool {
	return iface.loopback
}

// Return the addresses associated with this interface
func (iface *Interface) Addresses() []*InterfaceAddress {
	return iface.addresses
}

type CaptureSource interface {
	/*
	 * Set the snapshot length via pcap_set_snaplen().
	 *
	 * The snaplen must be greater than 0. This call will fail if the
	 * capture source has already been actived.
	 */
	SetSnapshotLength(snaplen int) error

	/*
	 * Set the buffer size via pcap_set_buffer_size().
	 * The bufsize must be greater than or equal to 0. This call will fail if the
	 * capture source has already been actived.
	 */
	SetBufferSize(bufsize int) error

	/**
	 * Activate the capture source via pcap_activate()
	 */
	Activate() error

	/*
	 * Inject a packet via pcap_sendpacket().
	 */
	SendPacket(data []byte) error

	/*
	 * Close a previously opened capture source via pcap_close()
	 */
	Close()
}

/* Packet capture reference. */
type captureSource struct {
	cptr *C.pcap_t
}

/*
 * Create a new pcap capture reference via pcap_create().
 *
 * The ifname must be a valid network device name.
 */
func NewCaptureSource(ifname string) (CaptureSource, error) {
	/* Set up our error buffer */
	errbuf := (*C.char)(C.calloc(C.PCAP_ERRBUF_SIZE, 1))
	defer C.free(unsafe.Pointer(errbuf))

	/* Convert arguments to appropriate C types */
	src_cptr := C.CString(ifname)
	defer C.free(unsafe.Pointer(src_cptr))

	/* Create the handle */
	result := new(captureSource)
	result.cptr = C.pcap_create(src_cptr, errbuf)
	if result.cptr == nil {
		return nil, errors.New(C.GoString(errbuf))
	}

	/* Set a finalier */
	runtime.SetFinalizer(result, func(source *captureSource) {
		source.Close()
	})

	return result, nil
}

/*
 * Find a specific interface. Returns nil if it can not be found,
 * and an error if an error occurs iterating the available interfaces.
 */
func FindInterface(ifname string) (*Interface, error) {
	if ifaces, err := FindAllInterfaces(); err != nil {
		return nil, err
	} else {
		for _, iface := range ifaces {
			if iface.Name() == ifname {
				return iface, nil
			}
		}
	}

	return nil, nil
}

/*
 * Find all interfaces available for capture.
 */
func FindAllInterfaces() ([]*Interface, error) {
	var devices *C.pcap_if_t

	/* Set up our error buffer */
	errbuf := (*C.char)(C.calloc(C.PCAP_ERRBUF_SIZE, 1))
	defer C.free(unsafe.Pointer(errbuf))

	/* Request the interfaces */
	if res := C.pcap_findalldevs(&devices, errbuf); res != 0 {
		return nil, errors.New(C.GoString(errbuf))
	}
	defer C.pcap_freealldevs(devices)

	/* Prepare the result */
	var ifaces = make([]*Interface, 0)
	for ifp := devices; ifp != nil; ifp = (*C.pcap_if_t)(ifp.next) {
		iface := new(Interface)
		ifaces = append(ifaces, iface)

		if ifp.name == nil {
			continue
		}

		iface.name = C.GoString(ifp.name)

		if ifp.description != nil {
			iface.description = C.GoString(ifp.description)
		}

		if ifp.flags&C.PCAP_IF_LOOPBACK != 0 {
			iface.loopback = true
		}

		for ap := ifp.addresses; ap != nil; ap = (ap.next) {
			addr := new(InterfaceAddress)
			addr.source = newSocketAddress(iface.name, ap.addr)
			addr.netmask = newSocketAddress(iface.name, ap.netmask)
			addr.broadcast = newSocketAddress(iface.name, ap.broadaddr)
			addr.destination = newSocketAddress(iface.name, ap.dstaddr)

			iface.addresses = append(iface.addresses, addr)
		}
	}

	return ifaces, nil
}

// Call a pcap_ function that may return PCAP_ERROR_ACTIVATED.
func (source *captureSource) requireInactive(impl func() C.int) error {
	if ret := impl(); ret == C.PCAP_ERROR_ACTIVATED {
		return errors.New("Called on a capture source that has already been actived")
	} else if ret != 0 {
		return fmt.Errorf("An unexpected error occured: %d", ret)
	}

	return nil
}

// from CaptureSource
func (source *captureSource) SetSnapshotLength(snaplen int) error {
	return source.requireInactive(func() C.int {
		return C.pcap_set_snaplen(source.cptr, C.int(snaplen))
	})
}

// from CaptureSource
func (source *captureSource) SetBufferSize(bufsize int) error {
	return source.requireInactive(func() C.int {
		return C.pcap_set_buffer_size(source.cptr, C.int(bufsize))
	})
}

// from CaptureSource
func (source *captureSource) Activate() error {
	if ret := C.pcap_activate(source.cptr); ret != 0 {
		switch ret {
		case C.PCAP_ERROR_ACTIVATED:
			return source.requireInactive(func() C.int { return ret })

		case C.PCAP_ERROR_RFMON_NOTSUP:
			return errors.New("Monitor mode is not supported")

		case C.PCAP_ERROR_IFACE_NOT_UP:
			return errors.New("Interface is not up")

			source.Close()
			return errors.New("Could not enable promiscuous mode on the interface")

		case C.PCAP_WARNING:
			// TODO - Should we log here?

		case C.PCAP_WARNING_PROMISC_NOTSUP:
			fallthrough
		case C.PCAP_ERROR_NO_SUCH_DEVICE:
			fallthrough
		case C.PCAP_ERROR_PERM_DENIED:
			fallthrough
		case C.PCAP_ERROR:
			return errors.New(C.GoString(C.pcap_geterr(source.cptr)))
		}
	}

	return nil
}

// from CaptureSource
func (source *captureSource) SendPacket(data []byte) error {
	buf := (*C.uchar)(C.malloc((C.size_t)(len(data))))
	defer C.free(unsafe.Pointer(buf))

	for i, b := range data {
		*(*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(buf)) + uintptr(i))) = b
	}

	if ret := C.pcap_sendpacket(source.cptr, (*C.u_char)(buf), C.int(len(data))); ret != 0 {
		return errors.New(C.GoString(C.pcap_geterr(source.cptr)))
	}

	return nil
}

// from CaptureSource
func (source *captureSource) Close() {
	if source.cptr != nil {
		C.pcap_close(source.cptr)
		source.cptr = nil
	}
}