Pcapy Reference

CORE SECURITY TECHNOLOGIES

Copyright © 2014 CORE SECURITY TECHNOLOGIES

Revision History
Revision: 1.2 Date: 2003/10/23 Author: jkohen
Initial revision

Table of Contents

I Pcapy Module Reference
open_live
open_offline
lookupdev
findalldevs
compile
II Reader Object Reference
dispatch
next
setfilter
getnet
datalink
getnonblock
dump_open
III Dumper Object Reference
dump
IV Pkthdr Object Reference
getts
V Bpf Object Reference
filter
Bibliography

Pcapy Module Reference

Table of Contents

open_live
open_offline
lookupdev
findalldevs
compile

open_live

Name

open_live — Obtain a packet capture descriptor to look at packets on the network

Synopsis

Reader open_live(device, snaplen, promisc, to_ms);
string device;
int snaplen;
int promisc;
int to_ms;

DESCRIPTION

open_liveis used to obtain a packet capture descriptor to look at packets on the network. deviceis a string that specifies the network device to open; on Linux systems with 2.2 or later kernels, a device argument of anyor NULLcan be used to capture packets from all interfaces. snaplen specifies the maximum number of bytes to capture. promiscspecifies if the interface is to be put into promiscuous mode. (Note that even if this parameter is false, the interface could well be in promiscuous mode for some other reason.) For now, this doesn't work on the anydevice; if an argument of anyor NULLis supplied, the promiscflag is ignored. to_msspecifies the read timeout in milliseconds. 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. Not all platforms support a read timeout; on platforms that don't, the read timeout is ignored.

open_offline

Name

open_offline — Obtain a packet capture descriptor to look at packets on a savefile

Synopsis

Reader open_offline(filename);
string filename;

DESCRIPTION

open_offlineis called to open a savefile for reading. filename specifies the name of the file to open. The file has the same format as those used by tcpdump(8) and tcpslice(8). The name -is a synonym for stdin.

lookupdev

Name

lookupdev — Return a network device suitable for use with open_live

Synopsis

string lookupdev();

DESCRIPTION

lookupdevreturns the name of a network device suitable for use with open_live.

findalldevs

Name

findalldevs — Obtain the list of available network devices

Synopsis

string[] findalldevs();

DESCRIPTION

findalldevsconstructs a list of network devices that can be opened with open_live. (Note that there may be network devices that cannot be opened with open_live, because, for example, that process might not have sufficient privileges to open them for capturing; if so, those devices will not appear on the list.)

compile

Name

compile — Compile a BPF filter

Synopsis

Bpf compile(linktype, snaplen, filter, optimize, netmask);
int linktype;
int snaplen;
string filter;
int optimize;
int32 netmask;

DESCRIPTION

compileis used to compile the filterinto a filter program. snaplenspecifies the maximum number of bytes to capture. optimizecontrols whether optimization on the resulting code is performed. netmaskspecifies the netmask of the local network.

Reader Object Reference

Table of Contents

dispatch
next
setfilter
getnet
datalink
getnonblock
dump_open

dispatch

Name

dispatch, loop — Collect and process packets

Synopsis

int dispatch(maxcant, (* callback));
int maxcant;
void (* callback) (Pkthdr, string);

int loop(maxcant, (* callback));
int maxcant;
void (* callback) (Pkthdr, string);

DESCRIPTION

dispatchis used to collect and process packets. maxcantspecifies the maximum number of packets to process before returning. This is not a minimum number; when reading a live capture, only one bufferful of packets is read at a time, so fewer than maxcantpackets may be processed. A cntof -1 processes all the packets received in one buffer when reading a live capture, or all the packets in the file when reading a savefile. callback specifies a routine to be called with two arguments: a Pkthdrinstance describing the data passed and the data itself.

The number of packets read is returned. 0 is returned if no packets were read from a live capture (if, for example, they were discarded because they didn't pass the packet filter, or if, on platforms that support a read timeout that starts before any packets arrive, the timeout expires before any packets arrive, or if the file descriptor for the capture device is in non–blocking mode and no packets were available to be read) or if no more packets are available in a savefile.

Note

When reading a live capture, dispatch will not necessarily return when the read times out; on some platforms, the read timeout isn't supported, and, on other platforms, the timer doesn't start until at least one packet arrives. This means that the read timeout should not be used in, for example, an interactive application, to allow the packet capture loop to poll for user input periodically, as there's no guarantee that dispatchwill return after the timeout expires.

loopis similar to dispatchexcept it keeps reading packets until maxcantpackets are processed or an error occurs. It does not return when live read timeouts occur. Rather, specifying a non–zero read timeout to open_liveand then calling dispatchallows the reception and processing of any packets that arrive when the timeout occurs. A negative maxcantcauses loopto loop forever (or at least until an error occurs). 0 is returned if maxcantis exhausted.

next

Name

next — Collect the next packet

Synopsis

(Pkthdr, string) next();

DESCRIPTION

nextreads the next packet (by calling dispatchwith a maxcantof 1) and returns a tuple (header, data) where headeris a Pkthdrinstance describing the data passed and datais the data itself.

setfilter

Name

setfilter — Specify a filter

Synopsis

setfilter(filter);
string filter;

DESCRIPTION

setfilteris used to specify a filter for this object.

getnet

Name

getnet, getmask — Get the associated network number and mask

Synopsis

int32 getnet();

int32 getmask();

DESCRIPTION

getnetand getmask are used to determine the network number and mask associated with the network device attached to this Reader.

datalink

Name

datalink — Obtain the link layer type

Synopsis

int datalink();

DESCRIPTION

datalinkreturns the link layer type; link layer types it can return include:

DLT_NULL

BSD loopback encapsulation; the link layer header is a 4–byte field, in host byte order, containing a PF_ value from socket.hfor the network–layer protocol of the packet.

Note

“host byte order” is the byte order of the machine on which the packets are captured, and the PF_values are for the OS of the machine on which the packets are captured; if a live capture is being done, “host byte order” is the byte order of the machine capturing the packets, and the PF_values are those of the OS of the machine capturing the packets, but if a savefile is being read, the byte order and PF_ values are not necessarily those of the machine reading the capture file.

DLT_EN10MB

Ethernet (10Mb, 100Mb, 1000Mb, and up)

DLT_IEEE802

IEEE 802.5 Token Ring

DLT_ARCNET

ARCNET

DLT_SLIP

SLIP; the link layer header contains, in order:

  • a 1–byte flag, which is 0for packets received by the machine and 1for packets sent by the machine.

  • a 1–byte field, the upper 4 bits of which indicate the type of packet, as per RFC 1144:

    • 0x40; an unmodified IP datagram (TYPE_IP)

    • 0x70; an uncompressed–TCP/IP datagram (UNCOMPRESSED_TCP), with that byte being the first byte of the raw IP header on the wire, containing the connection number in the protocol field

    • 0x80; a compressed–TCP/IP datagram (COMPRESSED_TCP), with that byte being the first byte of the compressed TCP/IP datagram header

  • for UNCOMPRESSED_TCP, the rest of the modified IP header, and for COMPRESSED_TCP, the compressed TCP/IP datagram header

for a total of 16 bytes; the uncompressed IP datagram follows the header.

DLT_PPP

PPP; if the first 2 bytes are 0xffand 0x03, it's PPP in HDLC–like framing, with the PPP header following those two bytes, otherwise it's PPP without framing, and the packet begins with the PPP header.

DLT_FDDI

FDDI

DLT_ATM_RFC1483

RFC 1483 LLC/SNAP–encapsulated ATM; the packet begins with an IEEE 802.2 LLC header.

DLT_RAW

Raw IP; the packet begins with an IP header.

DLT_PPP_SERIAL

PPP in HDLC–like framing, as per RFC 1662, or Cisco PPP with HDLC framing, as per section §4.3.1 of RFC 1547; the first byte will be 0xFFfor PPP in HDLC–like framing, and will be 0x0For 0x8Ffor Cisco PPP with HDLC framing.

DLT_PPP_ETHER

PPPoE; the packet begins with a PPPoE header, as per RFC 2516.

DLT_C_HDLC

Cisco PPP with HDLC framing, as per section § 4.3.1 of RFC 1547.

DLT_IEEE802_11

IEEE 802.11 wireless LAN.

DLT_LOOP

OpenBSD loopback encapsulation; the link layer header is a 4–byte field, in network byte order, containing a PF_value from OpenBSD's socket.hfor the network–layer protocol of the packet.

Note

Note that, if a savefile is being read, those PF_values are not necessarily those of the machine reading the capture file.

DLT_LINUX_SLL

Linux cooked capture encapsulation; the link layer header contains, in order:

  • a 2–byte "packet type", in network byte order, which is one of:

    • 0; packet was sent to us by somebody else.

    • 1; packet was broadcast by somebody else.

    • 2; packet was multicast, but not broadcast, by somebody else.

    • 3; packet was sent by somebody else to somebody else.

    • 4; packet was sent by us.

  • a 2–byte field, in network byte order, containing a Linux ARPHRD_value for the link layer device type.

  • a 2–byte field, in network byte order, containing the length of the link layer address of the sender of the packet (which could be 0).

  • an 8–byte field containing that number of bytes of the link layer header (if there are more than 8 bytes, only the first 8 are present).

  • a 2–byte field containing an Ethernet protocol type, in network byte order, or containing 1for Novell 802.3 frames without an 802.2 LLC header or 4for frames beginning with an 802.2 LLC header.

DLT_LTALK

Apple LocalTalk; the packet begins with an AppleTalk LLAP header.

getnonblock

Name

getnonblock, setnonblock — Manipulate the non–blocking flag

Synopsis

int getnonblock();

setnonblock(state);
int state;

DESCRIPTION

getnonblockreturns the current non–blocking state of the capture descriptor; it always returns 0 on savefiles.

DESCRIPTION

setnonblockputs a capture descriptor, opened with open_live, into non–blocking mode, or takes it out of non–blocking mode, depending on whether the stateargument is non–zero or zero. It has no effect on savefiles. In non–blocking mode, an attempt to read from the capture descriptor with dispatchwill, if no packets are currently available to be read, return 0 immediately rather than blocking waiting for packets to arrive. loopand nextwill not work in non–blocking mode.

dump_open

Name

dump_open — Create a Dumper object

Synopsis

Dumper dump_open(filename);
string filename;

DESCRIPTION

dump_openis called to open a savefile for writing and associate it to a newly created Dumperinstance. The name -is a synonym for stdout. filenamespecifies the name of the file to open.

Dumper Object Reference

Table of Contents

dump

dump

Name

dump — Dump a packet to a savefile

Synopsis

dump(header, data);
Pkthdr header;
string data;

DESCRIPTION

dumpoutputs a packet to the savefile opened with dump_openfrom type Reader.

Pkthdr Object Reference

Table of Contents

getts

getts

Name

getts, getcaplen, getlen — Obtain packet header information

Synopsis

(long, long) getts();

long getcaplen();

long getlen();

DESCRIPTION

getts, getcaplen and getlenreturn the timestamp, capture length and total length fields of the packet header, respectively.

Timestamp is a tuple with two elements: the number of seconds since the Epoch, and the amount of microseconds past the current second. The capture length is the number of bytes of the packet that are available from the capture. Finally, total length gives the length of the packet, in bytes (which might be more than the number of bytes available from the capture, if the length of the packet is larger than the maximum number of bytes to capture).

Bpf Object Reference

Table of Contents

filter

filter

Name

filter — Test a packet against a compiled filter

Synopsis

int filter(packet);
string packet;

DESCRIPTION

filtertests a packet against a compiled filter as returned by pcapy's compile. If the packet is allowed to pass through -1 is returned, otherwise filterreturns 0.

Bibliography

Sources

Portions of this work based on pcap(3) by the Lawrence Berkeley National Laboratory, University of California, Berkeley, CA. .