PCAP Capture File Format
draft-gharris-opsawg-pcap-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
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|---|---|---|---|
| Authors | Guy Harris , Michael Richardson | ||
| Last updated | 2020-12-21 | ||
| Replaced by | draft-ietf-opsawg-pcap | ||
| RFC stream | (None) | ||
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| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
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draft-gharris-opsawg-pcap-00
Network Working Group G. Harris, Ed.
Internet-Draft
Intended status: Informational M. Richardson
Expires: 25 June 2021 Sandelman
22 December 2020
PCAP Capture File Format
draft-gharris-opsawg-pcap-00
Abstract
This document describes the format used by the libpcap library to
record captured packets to a file. Programs using the libpcap
library to read and write those files, and thus reading and writing
files in that format, include tcpdump.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the OPSAWG Working Group
mailing list (opsawg@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/opsawg/.
Source for this draft and an issue tracker can be found at
https://github.com/pcapng/pcapng.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 25 June 2021.
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Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. General File Structure . . . . . . . . . . . . . . . . . . . 3
4. File Header . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Packet Record . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Recommended File Name Extension: .pcap . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1. Normative References . . . . . . . . . . . . . . . . . . 8
11.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
In the late 1980's, Van Jacobson, Steve McCanne, and others at the
Network Research Group at Lawrence Berkeley National Laboratory
developed the tcpdump program to capture and dissect network traces.
The code to capture traffic, using low-level mechanisms in various
operating systems, and to read and write network traces to a file was
later put into a library named libpcap.
This document describes the format used by tcpdump, and other
programs using libpcap, to read and write network traces.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. General File Structure
A capture file begins with a File Header, followed by zero or more
Packet Records, one per packet.
All fields in the File Header and in Packet Records will always be
saved according to the characteristics (little endian / big endian)
of the capturing machine. This refers to all the fields that are
saved as numbers and that span over two or more octets.
The approach of having the file saved in the native format of the
generating host is more efficient because it avoids translation of
data when reading / writing on the host itself, which is the most
common case when generating/processing capture captures.
The packets are shown in traditional IETF diagram, with the bits
numbered from the left to the right. The bit numbering does not
reflect the binary value position, as IETF protocols are
traditionally in big-endian network-byte order. The most significant
bit is therefore on the left in this diagram as if the file is being
stored on a big-endian system.
4. File Header
The File Header has the following format:
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1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 | Magic Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4 | Major Version | Minor Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8 | Reserved1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 | SnapLen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
20 | FCS |f| LinkType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: File Header
The File Header length is 24 octets.
The meaning of the fields in the File Header is:
Magic Number (32 bits): an unsigned magic number, whose value is
either the hexadecimal number 0xA1B2C3D4 or the hexadecimal number
0xA1B23C4D.
If the value is 0xA1B2C3D4, time stamps in Packet Records (see
Figure 2) are in seconds and microseconds; if it is 0xA1B23C4D,
time stamps in Packet Records are in seconds and nanoseconds.
These numbers can be used to distinguish sections that have been
saved on little-endian machines from the ones saved on big-endian
machines, and to heuristically identify pcap files.
Major Version (16 bits): an unsigned value, giving the number of the
current major version of the format. The value for the current
version of the format is 2. This value should change if the
format changes in such a way that code that reads the new format
could not read the old format (i.e., code to read both formats
would have to check the version number and use different code
paths for the two formats) and code that reads the old format
could not read the new format.
Minor Version (16 bits): an unsigned value, giving the number of the
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current minor version of the format. The value is for the current
version of the format is 4. This value should change if the
format changes in such a way that code that reads the new format
could read the old format without checking the version number but
code that reads the old format could not read all files in the new
format.
Reserved1 (32 bits): not used - SHOULD be filled with 0 by pcap file
writers, and MUST be ignored by pcap file readers. This value was
documented by some older implementations as "gmt to local
correction". Some older pcap file writers stored non-zero values
in this field.
Reserved2 (32 bits): not used - SHOULD be filled with 0 by pcap file
writers, and MUST be ignored by pcap file readers. This value was
documented by some older implementations as "accuracy of
timestamps". Some older pcap file writers stored non-zero values
in this field.
SnapLen (32 bits): an unsigned value indicating the maximum number
of octets captured from each packet. The portion of each packet
that exceeds this value will not be stored in the file. This
value MUST NOT be zero; if no limit was specified, the value
should be a number greater than or equal to the largest packet
length in the file.
LinkType (32 bits): an unsigned value that defines, in the lower 28
bits, the link layer type of packets in the file.
Frame Cyclic Sequence present (4 bits): if the "f" bit is set, then
the FCS bits provide the number of bytes of FCS that are appended
to each packet.
valid values are between 0 and 7, with ethernet typically having a
length of 4 bytes.
5. Packet Record
A Packet Record is the standard container for storing the packets
coming from the network.
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1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 | Timestamp (Seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4 | Timestamp (Microseconds or nanoseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8 | Captured Packet Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 | Original Packet Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 / /
/ Packet Data /
/ variable length /
/ /
+---------------------------------------------------------------+
Figure 2: Packet Record
The Packet Header length is 16 octets.
The meaning of the fields in the Packet Record is:
Timestamp (Seconds) and Timestamp (Microseconds or nanoseconds): sec
onds and fraction of a seconds values of a timestamp.
The seconds value is a 32-bit unsigned integer that represents the
number of seconds that have elapsed since 1970-01-01 00:00:00 UTC,
and the microseconds or nanoseconds value represents the number of
microseconds or nanoseconds that have elapsed since that seconds.
Whether the value represents microseconds or nanoseconds is
specified by the magic number in the File Header.
Captured Packet Length (32 bits): an unsigned value that indicates
the number of octets captured from the packet (i.e. the length of
the Packet Data field). It will be the minimum value among the
Original Packet Length and the snapshot length for the interface
(SnapLen, defined in Figure 1).
Original Packet Length (32 bits): an unsigned value that indicates
the actual length of the packet when it was transmitted on the
network. It can be different from the Captured Packet Length if
the packet has been truncated by the capture process.
Packet Data: the data coming from the network, including link-layer
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headers. The actual length of this field is Captured Packet
Length. The format of the link-layer headers depends on the
LinkType field specified in the file header (see Figure 1) and it
is specified in the entry for that format in [LINKTYPES].
6. Recommended File Name Extension: .pcap
The recommended file name extension for the "PCAP Capture File
Format" specified in this document is ".pcap".
On Windows and macOS, files are distinguished by an extension to
their filename. Such an extension is technically not actually
required, as applications should be able to automatically detect the
pcap file format through the "magic bytes" at the beginning of the
file, as some other UN*X desktop environments do. However, using
name extensions makes it easier to work with files (e.g. visually
distinguish file formats) so it is recommended - though not required
- to use .pcap as the name extension for files following this
specification.
Please note: To avoid confusion (such as the current usage of .cap
for a plethora of different capture file formats) file name
extensions other than .pcap should be avoided.
There is new work to create the PCAP Next Generation capture File
Format (see [I-D.tuexen-opsawg-pcapng]). The new file format is not
compatible with this specification, but many programs read both
transparently. Files of that type will usually start with a
Section Header Block, with a magic number of 0x0A0D0D0A.
7. Security Considerations
TBD.
8. IANA Considerations
TBD.
[Open issue: decide whether the LinkType values should be IANA
registries. And if so, what the IANA policy for each should be (see
RFC 5226)]
9. Contributors
[Insert pcap developers etc. here].
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10. Acknowledgments
The authors wish to thank [insert list here] and many others for
their invaluable comments.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
11.2. Informative References
[I-D.tuexen-opsawg-pcapng]
Tuexen, M., Risso, F., Bongertz, J., Combs, G., Harris,
G., and M. Richardson, "PCAP Next Generation (pcapng)
Capture File Format", Work in Progress, Internet-Draft,
draft-tuexen-opsawg-pcapng-02, 28 September 2020,
<http://www.ietf.org/internet-drafts/draft-tuexen-opsawg-
pcapng-02.txt>.
Authors' Addresses
Guy Harris (editor)
Email: gharris@sonic.net
Michael C. Richardson
Sandelman Software Works Inc
Email: mcr+ietf@sandelman.ca
URI: http://www.sandelman.ca/
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