pcap

package
v1.1.20 Latest Latest
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 Go to latest Published: Nov 10, 2023 License: BSD-3-Clause Imports: 16 Imported by: 0

Documentation

Overview

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, http://github.com/MerlinKodo/gopacket, although it can also be used independently if you just want to get packet data from the wire.

Depending on libpcap version, os support, or file timestamp resolution, nanosecond resolution is used for the internal timestamps. Returned timestamps are always scaled to nanosecond resolution due to the usage of time.Time. libpcap must be at least version 1.5 to support nanosecond timestamps. OpenLive supports only microsecond resolution.

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 {
  panic(err)
} 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". Be aware, that OpenLive only supports microsecond resolution. 

if handle, err := pcap.OpenLive("eth0", 1600, true, pcap.BlockForever); err != nil {
  panic(err)
} else if err := handle.SetBPFFilter("tcp and port 80"); err != nil {  // optional
  panic(err)
} 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 {
  log.Fatal(err)
}
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 {
  log.Fatal(err)
} else if err = inactive.SetTimestampSource("foo"); err != nil {
  log.Fatal(err)
}

// Finally, create the actual handle by calling Activate:
handle, err := inactive.Activate()  // after this, inactive is no longer valid
if err != nil {
  log.Fatal(err)
}
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 (http://www.manpages.info/macosx/pcap.3.html) 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 Users

gopacket can use winpcap or npcap. If both are installed at the same time, npcap is preferred. Make sure the right windows service is loaded (npcap for npcap and npf for winpcap).

Index

Examples

Constants

View Source 
const (
	DirectionIn    = Direction(pcapDIN)
	DirectionOut   = Direction(pcapDOUT)
	DirectionInOut = Direction(pcapDINOUT)
)

Direction values for Handle.SetDirection.

View Source 
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.

View Source 
const ErrNotActive = pcapErrorNotActivated

ErrNotActive is returned if handle is not activated

View Source 
const MaxBpfInstructions = 4096

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

https://github.com/torvalds/linux/blob/master/include/uapi/linux/bpf_common.h

Variables

View Source 
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.

Functions

func DatalinkNameToVal added in v1.1.15 

func DatalinkNameToVal(name string) int

DatalinkNameToVal returns pcap_datalink_name_to_val as int

func DatalinkValToDescription added in v1.1.15 

func DatalinkValToDescription(dlt int) string

DatalinkValToDescription returns pcap_datalink_val_to_description as string

func DatalinkValToName added in v1.1.15 

func DatalinkValToName(dlt int) string

DatalinkValToName returns pcap_datalink_val_to_name as string

func Version 

func Version() string

Version returns pcap_lib_version.

Types

type BPF added in v1.1.7 

type BPF struct {
	// contains filtered or unexported fields
}

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

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

Output:

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

func NewBPF added in v1.1.15 

func NewBPF(linkType layers.LinkType, captureLength int, expr string) (*BPF, error)

NewBPF allows to create a BPF without requiring an existing handle. This allows to match packets obtained from a-non GoPacket capture source to be matched. 

buf := make([]byte, MaxFrameSize)
bpfi, _ := pcap.NewBPF(layers.LinkTypeEthernet, MaxFrameSize, "icmp")
n, _ := someIO.Read(buf)
ci := gopacket.CaptureInfo{CaptureLength: n, Length: n}
if bpfi.Matches(ci, buf) {
	doSomething()
}

func (*BPF) Matches added in v1.1.7 

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

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

func (*BPF) String added in v1.1.7 

func (b *BPF) String() string

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

type BPFInstruction added in v1.1.12 

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

BPFInstruction is a byte encoded structure holding a BPF instruction

func CompileBPFFilter added in v1.1.13 

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. 

type Datalink struct {
	Name        string
	Description string
}

Datalink describes the datalink

type Direction added in v1.1.8 

type Direction uint8

Direction is used by Handle.SetDirection.

type Handle 

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 

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. Warning: this function supports only microsecond timestamps. For nanosecond resolution use an InactiveHandle.

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

func OpenOffline 

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

OpenOffline opens a file and returns its contents as a *Handle. Depending on libpcap support and on the timestamp resolution used in the file, nanosecond or microsecond resolution is used internally. All returned timestamps are scaled to nanosecond resolution. Resolution() can be used to query the actual resolution used.

func OpenOfflineFile added in v1.1.15 

func OpenOfflineFile(file *os.File) (handle *Handle, err error)

OpenOfflineFile returns contents of input file as a *Handle. Depending on libpcap support and on the timestamp resolution used in the file, nanosecond or microsecond resolution is used internally. All returned timestamps are scaled to nanosecond resolution. Resolution() can be used to query the actual resolution used.

func (*Handle) Close 

func (p *Handle) Close()

Close closes the underlying pcap handle.

func (*Handle) CompileBPFFilter added in v1.1.12 

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

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

func (*Handle) Error 

func (p *Handle) Error() error

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

func (*Handle) LinkType 

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 added in v1.1.7 

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 added in v1.1.12 

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 

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) Resolution added in v1.1.16 

func (p *Handle) Resolution() gopacket.TimestampResolution

Resolution returns the timestamp resolution of acquired timestamps before scaling to NanosecondTimestampResolution.

func (*Handle) SetBPFFilter 

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

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

func (*Handle) SetBPFInstructionFilter added in v1.1.12 

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: https://www.kernel.org/doc/Documentation/networking/filter.txt

To compile the code use bpf_asm from https://github.com/torvalds/linux/tree/master/tools/net

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 added in v1.1.8 

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

SetDirection sets the direction for which packets will be captured.

func (*Handle) SetLinkType 

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

SetLinkType calls pcap_set_datalink on the pcap handle.

func (*Handle) SnapLen added in v1.1.15 

func (p *Handle) SnapLen() int

SnapLen returns the snapshot length

func (*Handle) Stats 

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

Stats returns statistics on the underlying pcap handle.

func (*Handle) WritePacketData 

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

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

func (*Handle) ZeroCopyReadPacketData 

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 added in v1.1.0 

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 added in v1.1.0 

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 added in v1.1.0 

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 added in v1.1.0 

func (p *InactiveHandle) CleanUp()

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

func (*InactiveHandle) Error added in v1.1.15 

func (p *InactiveHandle) Error() error

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

func (*InactiveHandle) SetBufferSize added in v1.1.8 

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

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

func (*InactiveHandle) SetImmediateMode added in v1.1.8 

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 added in v1.1.0 

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 added in v1.1.5 

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 added in v1.1.0 

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

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

func (*InactiveHandle) SetTimeout added in v1.1.0 

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 added in v1.1.0 

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

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

func (*InactiveHandle) SupportedTimestamps added in v1.1.0 

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

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

type Interface 

type Interface struct {
	Name        string
	Description string
	Flags       uint32
	Addresses   []InterfaceAddress
}

Interface describes a single network interface on a machine.

func FindAllDevs 

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

FindAllDevs attempts to enumerate all interfaces on the current machine.

type InterfaceAddress 

type InterfaceAddress struct {
	IP        net.IP
	Netmask   net.IPMask // Netmask may be nil if we were unable to retrieve it.
	Broadaddr net.IP     // Broadcast address for this IP may be nil
	P2P       net.IP     // P2P destination address for this IP may be nil
}

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

type NextError 

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 

func (n NextError) Error() string

NextError implements the error interface.

type Stats 

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 added in v1.0.8 

type TimestampSource int

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

func TimestampSourceFromString added in v1.0.8 

func TimestampSourceFromString(s string) (TimestampSource, error)

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

func (TimestampSource) String added in v1.0.8 

func (t TimestampSource) String() string

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

Source Files

View all Source files

Directories

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

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