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package pcap

import ""

Package pcap allows users of gopacket to read packets off the wire or from pcap files.

This package is meant to be used with its parent,, although it can also be used independently if you just want to get packet data from the wire.

Reading PCAP Files

The following code can be used to read in data from a pcap file.

if handle, err := pcap.OpenOffline("/path/to/my/file"); err != nil {
} else {
  packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
  for packet := range packetSource.Packets() {
    handlePacket(packet)  // Do something with a packet here.

Reading Live Packets

The following code can be used to read in data from a live device, in this case "eth0".

if handle, err := pcap.OpenLive("eth0", 1600, true, pcap.BlockForever); err != nil {
} else if err := handle.SetBPFFilter("tcp and port 80"); err != nil {  // optional
} else {
  packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
  for packet := range packetSource.Packets() {
    handlePacket(packet)  // Do something with a packet here.

Inactive Handles

Newer PCAP functionality requires the concept of an 'inactive' PCAP handle. Instead of constantly adding new arguments to pcap_open_live, users now call pcap_create to create a handle, set it up with a bunch of optional function calls, then call pcap_activate to activate it. This library mirrors that mechanism, for those that want to expose/use these new features:

inactive, err := pcap.NewInactiveHandle(deviceName)
if err != nil {
defer inactive.CleanUp()

// Call various functions on inactive to set it up the way you'd like:
if err = inactive.SetTimeout(time.Minute); err != nil {
} else if err = inactive.SetTimestampSource("foo"); err != nil {

// Finally, create the actual handle by calling Activate:
handle, err := inactive.Activate()  // after this, inactive is no longer valid
if err != nil {
defer handle.Close()

// Now use your handle as you see fit.

PCAP Timeouts

pcap.OpenLive and pcap.SetTimeout both take timeouts. If you don't care about timeouts, just pass in BlockForever, which should do what you expect with minimal fuss.

A timeout of 0 is not recommended. Some platforms, like Macs ( say:

The read timeout is used to arrange that the read not necessarily return
immediately when a packet is seen, but that it wait for some amount of time
to allow more packets to arrive and to read multiple packets from the OS
kernel in one operation.

This means that if you only capture one packet, the kernel might decide to wait 'timeout' for more packets to batch with it before returning. A timeout of 0, then, means 'wait forever for more packets', which is... not good.

To get around this, we've introduced the following behavior: if a negative timeout is passed in, we set the positive timeout in the handle, then loop internally in ReadPacketData/ZeroCopyReadPacketData when we see timeout errors.

PCAP File Writing

This package does not implement PCAP file writing. However, gopacket/pcapgo does! Look there if you'd like to write PCAP files.

Note For Windows 10 Users

If you're trying to use 64-bit winpcap on Windows 10, you might have to do the crazy hijinks detailed at



Package Files

doc.go pcap.go


const BlockForever = -time.Millisecond * 10

BlockForever causes it to block forever waiting for packets, when passed into SetTimeout or OpenLive, while still returning incoming packets to userland relatively quickly.

const MaxBpfInstructions = 4096

MaxBpfInstructions is the maximum number of BPF instructions supported (BPF_MAXINSNS), taken from Linux kernel: include/uapi/linux/bpf_common.h


var CannotSetRFMon = errors.New("Cannot set rfmon for this handle")

CannotSetRFMon is returned by SetRFMon if the handle does not allow setting RFMon because pcap_can_set_rfmon returns 0.

func CompileBPFFilter Uses

func CompileBPFFilter(linkType layers.LinkType, captureLength int, expr string) ([]BPFInstruction, error)

CompileBPFFilter compiles and returns a BPF filter with given a link type and capture length.

func FindAllDevs Uses

func FindAllDevs() (ifs []Interface, err error)

FindAllDevs attempts to enumerate all interfaces on the current machine.

func Version Uses

func Version() string

Version returns pcap_lib_version.

type BPF Uses

type BPF struct {
    // contains filtered or unexported fields

BPF is a compiled filter program, useful for offline packet matching.


handle, err := OpenOffline("test_ethernet.pcap")
if err != nil {
synack, err := handle.NewBPF("tcp[tcpflags] & (tcp-syn|tcp-ack) == (tcp-syn|tcp-ack)")
if err != nil {
syn, err := handle.NewBPF("tcp[tcpflags] & (tcp-syn|tcp-ack) == tcp-syn")
if err != nil {
for {
    data, ci, err := handle.ReadPacketData()
    switch {
    case err == io.EOF:
    case err != nil:
    case synack.Matches(ci, data):
        fmt.Println("SYN/ACK packet")
    case syn.Matches(ci, data):
        fmt.Println("SYN packet")
        fmt.Println("SYN flag not set")


SYN packet
SYN/ACK packet
SYN flag not set
SYN flag not set
SYN flag not set
SYN flag not set
SYN flag not set
SYN flag not set
SYN flag not set
SYN flag not set
SYN flag not set
SYN flag not set
SYN flag not set
SYN flag not set
SYN flag not set
SYN flag not set

func (*BPF) Matches Uses

func (b *BPF) Matches(ci gopacket.CaptureInfo, data []byte) bool

Matches returns true if the given packet data matches this filter.

func (*BPF) String Uses

func (b *BPF) String() string

String returns the original string this BPF filter was compiled from.

type BPFInstruction Uses

type BPFInstruction struct {
    Code uint16
    Jt   uint8
    Jf   uint8
    K    uint32

BPFInstruction is a byte encoded structure holding a BPF instruction

type Datalink struct {
    Name        string
    Description string

Datalink describes the datalink

type Direction Uses

type Direction uint8

Direction is used by Handle.SetDirection.

const (
    DirectionIn    Direction = C.PCAP_D_IN
    DirectionOut   Direction = C.PCAP_D_OUT
    DirectionInOut Direction = C.PCAP_D_INOUT

Direction values for Handle.SetDirection.

type Handle Uses

type Handle struct {
    // contains filtered or unexported fields

Handle provides a connection to a pcap handle, allowing users to read packets off the wire (Next), inject packets onto the wire (Inject), and perform a number of other functions to affect and understand packet output.

Handles are already pcap_activate'd

func OpenLive Uses

func OpenLive(device string, snaplen int32, promisc bool, timeout time.Duration) (handle *Handle, _ error)

OpenLive opens a device and returns a *Handle. It takes as arguments the name of the device ("eth0"), the maximum size to read for each packet (snaplen), whether to put the interface in promiscuous mode, and a timeout.

See the package documentation for important details regarding 'timeout'.

func OpenOffline Uses

func OpenOffline(file string) (handle *Handle, err error)

OpenOffline opens a file and returns its contents as a *Handle.

func (*Handle) Close Uses

func (p *Handle) Close()

Close closes the underlying pcap handle.

func (*Handle) CompileBPFFilter Uses

func (p *Handle) CompileBPFFilter(expr string) ([]BPFInstruction, error)

CompileBPFFilter compiles and returns a BPF filter for the pcap handle.

func (*Handle) Error Uses

func (p *Handle) Error() error

Error returns the current error associated with a pcap handle (pcap_geterr).

func (*Handle) LinkType Uses

func (p *Handle) LinkType() layers.LinkType

LinkType returns pcap_datalink, as a layers.LinkType.

func (p *Handle) ListDataLinks() (datalinks []Datalink, err error)

ListDataLinks obtains a list of all possible data link types supported for an interface.

func (*Handle) NewBPF Uses

func (p *Handle) NewBPF(expr string) (*BPF, error)

NewBPF compiles the given string into a new filter program.

BPF filters need to be created from activated handles, because they need to know the underlying link type to correctly compile their offsets.

func (*Handle) NewBPFInstructionFilter Uses

func (p *Handle) NewBPFInstructionFilter(bpfInstructions []BPFInstruction) (*BPF, error)

NewBPFInstructionFilter sets the given BPFInstructions as new filter program.

More details see func SetBPFInstructionFilter

BPF filters need to be created from activated handles, because they need to know the underlying link type to correctly compile their offsets.

func (*Handle) ReadPacketData Uses

func (p *Handle) ReadPacketData() (data []byte, ci gopacket.CaptureInfo, err error)

ReadPacketData returns the next packet read from the pcap handle, along with an error code associated with that packet. If the packet is read successfully, the returned error is nil.

func (*Handle) SetBPFFilter Uses

func (p *Handle) SetBPFFilter(expr string) (err error)

SetBPFFilter compiles and sets a BPF filter for the pcap handle.

func (*Handle) SetBPFInstructionFilter Uses

func (p *Handle) SetBPFInstructionFilter(bpfInstructions []BPFInstruction) (err error)

SetBPFInstructionFilter may be used to apply a filter in BPF asm byte code format.

Simplest way to generate BPF asm byte code is with tcpdump:

tcpdump -dd 'udp'

The output may be used directly to add a filter, e.g.:

    bpfInstructions := []pcap.BpfInstruction{
			{0x28, 0, 0, 0x0000000c},
			{0x15, 0, 9, 0x00000800},
			{0x30, 0, 0, 0x00000017},
			{0x15, 0, 7, 0x00000006},
			{0x28, 0, 0, 0x00000014},
			{0x45, 5, 0, 0x00001fff},
			{0xb1, 0, 0, 0x0000000e},
			{0x50, 0, 0, 0x0000001b},
			{0x54, 0, 0, 0x00000012},
			{0x15, 0, 1, 0x00000012},
			{0x6, 0, 0, 0x0000ffff},
			{0x6, 0, 0, 0x00000000},

An other posibility is to write the bpf code in bpf asm. Documentation:

To compile the code use bpf_asm from

The following command may be used to convert bpf_asm output to c/go struct, usable for SetBPFFilterByte: bpf_asm -c tcp.bpf

func (*Handle) SetDirection Uses

func (p *Handle) SetDirection(direction Direction) error

SetDirection sets the direction for which packets will be captured.

func (*Handle) SetLinkType Uses

func (p *Handle) SetLinkType(dlt layers.LinkType) error

SetLinkType calls pcap_set_datalink on the pcap handle.

func (*Handle) Stats Uses

func (p *Handle) Stats() (stat *Stats, err error)

Stats returns statistics on the underlying pcap handle.

func (*Handle) WritePacketData Uses

func (p *Handle) WritePacketData(data []byte) (err error)

WritePacketData calls pcap_sendpacket, injecting the given data into the pcap handle.

func (*Handle) ZeroCopyReadPacketData Uses

func (p *Handle) ZeroCopyReadPacketData() (data []byte, ci gopacket.CaptureInfo, err error)

ZeroCopyReadPacketData reads the next packet off the wire, and returns its data. The slice returned by ZeroCopyReadPacketData points to bytes owned by the the Handle. Each call to ZeroCopyReadPacketData invalidates any data previously returned by ZeroCopyReadPacketData. Care must be taken not to keep pointers to old bytes when using ZeroCopyReadPacketData... if you need to keep data past the next time you call ZeroCopyReadPacketData, use ReadPacketData, which copies the bytes into a new buffer for you.

data1, _, _ := handle.ZeroCopyReadPacketData()
// do everything you want with data1 here, copying bytes out of it if you'd like to keep them around.
data2, _, _ := handle.ZeroCopyReadPacketData()  // invalidates bytes in data1

type InactiveHandle Uses

type InactiveHandle struct {
    // contains filtered or unexported fields

InactiveHandle allows you to call pre-pcap_activate functions on your pcap handle to set it up just the way you'd like.

func NewInactiveHandle Uses

func NewInactiveHandle(device string) (*InactiveHandle, error)

NewInactiveHandle creates a new InactiveHandle, which wraps an un-activated PCAP handle. Callers of NewInactiveHandle should immediately defer 'CleanUp', as in:

inactive := NewInactiveHandle("eth0")
defer inactive.CleanUp()

func (*InactiveHandle) Activate Uses

func (p *InactiveHandle) Activate() (*Handle, error)

Activate activates the handle. The current InactiveHandle becomes invalid and all future function calls on it will fail.

func (*InactiveHandle) CleanUp Uses

func (p *InactiveHandle) CleanUp()

CleanUp cleans up any stuff left over from a successful or failed building of a handle.

func (*InactiveHandle) SetBufferSize Uses

func (p *InactiveHandle) SetBufferSize(bufferSize int) error

SetBufferSize sets the buffer size (in bytes) of the handle.

func (*InactiveHandle) SetImmediateMode Uses

func (p *InactiveHandle) SetImmediateMode(mode bool) error

SetImmediateMode sets (or unsets) the immediate mode of the handle. In immediate mode, packets are delivered to the application as soon as they arrive. In other words, this overrides SetTimeout.

func (*InactiveHandle) SetPromisc Uses

func (p *InactiveHandle) SetPromisc(promisc bool) error

SetPromisc sets the handle to either be promiscuous (capture packets unrelated to this host) or not.

func (*InactiveHandle) SetRFMon Uses

func (p *InactiveHandle) SetRFMon(monitor bool) error

SetRFMon turns on radio monitoring mode, similar to promiscuous mode but for wireless networks. If this mode is enabled, the interface will not need to associate with an access point before it can receive traffic.

func (*InactiveHandle) SetSnapLen Uses

func (p *InactiveHandle) SetSnapLen(snaplen int) error

SetSnapLen sets the snap length (max bytes per packet to capture).

func (*InactiveHandle) SetTimeout Uses

func (p *InactiveHandle) SetTimeout(timeout time.Duration) error

SetTimeout sets the read timeout for the handle.

See the package documentation for important details regarding 'timeout'.

func (*InactiveHandle) SetTimestampSource Uses

func (p *InactiveHandle) SetTimestampSource(t TimestampSource) error

SetTimestampSource sets the type of timestamp generator PCAP uses when attaching timestamps to packets.

func (*InactiveHandle) SupportedTimestamps Uses

func (p *InactiveHandle) SupportedTimestamps() (out []TimestampSource)

SupportedTimestamps returns a list of supported timstamp types for this handle.

type Interface Uses

type Interface struct {
    Name        string
    Description string
    Addresses   []InterfaceAddress

Interface describes a single network interface on a machine.

type InterfaceAddress Uses

type InterfaceAddress struct {
    IP      net.IP
    Netmask net.IPMask // Netmask may be nil if we were unable to retrieve it.


InterfaceAddress describes an address associated with an Interface. Currently, it's IPv4/6 specific.

type NextError Uses

type NextError int32

NextError is the return code from a call to Next.

const (
    NextErrorOk             NextError = 1
    NextErrorTimeoutExpired NextError = 0
    NextErrorReadError      NextError = -1
    // NextErrorNoMorePackets is returned when reading from a file (OpenOffline) and
    // EOF is reached.  When this happens, Next() returns io.EOF instead of this.
    NextErrorNoMorePackets NextError = -2
    NextErrorNotActivated  NextError = -3

NextError values.

func (NextError) Error Uses

func (n NextError) Error() string

NextError implements the error interface.

type Stats Uses

type Stats struct {
    PacketsReceived  int
    PacketsDropped   int
    PacketsIfDropped int

Stats contains statistics on how many packets were handled by a pcap handle, and what was done with those packets.

type TimestampSource Uses

type TimestampSource

TimestampSource tells PCAP which type of timestamp to use for packets.

func TimestampSourceFromString Uses

func TimestampSourceFromString(s string) (TimestampSource, error)

TimestampSourceFromString translates a string into a timestamp type, case insensitive.

func (TimestampSource) String Uses

func (t TimestampSource) String() string

String returns the timestamp type as a human-readable string.


gopacket_benchmarkThis benchmark reads in file <tempdir>/gopacket_benchmark.pcap and measures the time it takes to decode all packets from that file.

Package pcap imports 14 packages (graph) and is imported by 63 packages. Updated 2017-02-18. Refresh now. Tools for package owners.