Internet DRAFT - draft-ietf-opsawg-pcap
draft-ietf-opsawg-pcap
Network Working Group G. Harris, Ed.
Internet-Draft
Intended status: Informational M. Richardson
Expires: 24 January 2024 Sandelman
23 July 2023
PCAP Capture File Format
draft-ietf-opsawg-pcap-03
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.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-opsawg-pcap/.
Discussion of this document takes place on the opsawg Working Group
mailing list (mailto:opsawg@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/opsawg/. Subscribe at
https://www.ietf.org/mailman/listinfo/opsawg/.
Source for this draft and an issue tracker can be found at
https://github.com/IETF-OPSAWG-WG/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
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This Internet-Draft will expire on 24 January 2024.
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Copyright Notice
Copyright (c) 2023 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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. General File Structure . . . . . . . . . . . . . . . . . . . 3
4. File Header . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Packet Record . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Recommended File Name Extension: .pcap . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8.1. Media-Type Registry . . . . . . . . . . . . . . . . . . . 8
8.1.1. application/pcap . . . . . . . . . . . . . . . . . . 8
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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 the headers of Packet Records
will always be written according to the characteristics (little
endian / big endian) of the machine that is writing the file. This
refers to all the fields that are written as numbers and that span
over two or more octets.
The approach of having the file written in the native format of the
host writing the file is more efficient because it avoids translation
of data when writing the file or reading the file on the host that
wrote the file, which is the most common case when generating or
processing capture captures.
4. File Header
The File Header has the following format, with the octet offset of
fields shown to the left of the field:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 | Magic Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4 | Major Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6 | Minor Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8 | Reserved1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 | SnapLen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
20 | LinkType and additional information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: File Header
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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 sessions that have been
written on little-endian machines from the ones written 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
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" or "time zone offset". 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
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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 and additional information (32 bits): a 32-bit unsigned
value that contains the link-layer type of packets in the file and
may contain additional information.
The LinkType and additional information field is in the form
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|FCS len|R|P| Reserved3 | Link-layer type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: LinkType and additional information
The field is shown as if it were in the byte order of the host
reading or writing the file, with bit 0 being the most-significant
bit of the field and bit 31 being the least-significant bit of the
field.
Link-layer type (16 bits): a 16-bit value indicating link-layer type
for packets in the file; it is a value as defined in the PCAP
LinkType list registry, as defined in
[I-D.ietf-opsawg-pcaplinktype].
Reserved3 (10 bits): not used - MUST be set to zero by pcap writers,
and MUST NOT be interpreted by pcap readers; a reader SHOULD treat
a non-zero value as an error.
P (1 bit): a bit that, if set, indicates that the Frame Check
Sequence (FCS) length value is present and, if not set, indicates
that the FCS value is not present.
R (1 bit): not used - MUST be set to zero by pcap writers, and MUST
NOT be interpreted by pcap readers; a reader SHOULD treat a non-
zero value as an error.
FCS len (4 bits): a 4-bit unsigned value indicating the number of
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16-bit (2-octet) words of FCS that are appended to each packet, if
the P bit is set; if the P bit is not set, and the FCS length is
not indicated by the link-layer type value, the FCS length is
unknown. The valid values of the FCS len field are between 0 and
15; Ethernet, for example, would have an FCS length value of 2,
corresponding to a 4-octet FCS.
5. Packet Record
A Packet Record is the standard container for storing the packets
coming from the network.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 | Timestamp (Seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4 | Timestamp (Microseconds or nanoseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8 | Captured Packet Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 | Original Packet Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 / /
/ Packet Data /
/ variable length, not padded /
/ /
Figure 3: Packet Record
The Packet Record begins with a 16-octet header, followed by data
from the packet.
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
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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; it SHOULD
NOT be less than the Captured Packet Length.
A pcap file writer MAY write an Original Packet Length that is
less than the Captured Packet Length if both the Captured Packet
Length and the Original Packet length came from a file in which a
packet had an Original Packet Length less than the Captured Packet
Length; otherwise, it MUST write an Original Packet Length that is
greater than or equal to the Captured Packet Length.
A pcap file reader MAY convert an Original Packet Length that is
less than the Captured Packet Length to a value that is greater
than or equal to the Captured Packet Length.
Packet Data: the data coming from the network, including link-layer
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 [I-D.ietf-opsawg-pcaplinktype].
Packet Records are not padded to a 4-octet boundary; if the number of
octets of packet data is not a multiple of 4, there are no padding
octets following it, so Packet Records are not guaranteed to begin on
a 4-octet boundary within a file.
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.
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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.ietf-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
This document requires the following IANA actions:
8.1. Media-Type Registry
This section registers the 'application/pcap' in the "Media Types"
registry. These media types are used to indicate that the content is
packet capture as described in this document.
8.1.1. application/pcap
Type name: application
Subtype name: pcap
Required parameters: none
Optional parameters: none
Encoding considerations: PCAP files contain network packets
Security considerations: See Security Considerations, Section
Interoperability considerations: The format is designed to be broadly interoperable.
Published specification: THIS RFC.
Applications that use this media type: tcpdump, wireshark, others.
Additional information:
Magic number(s): 0xA1B2C3D4, and 0xA1B23C4D in both endian orders
File extension(s): .pcap
Macintosh file type code(s): none
Person & email address to contact for further information: The Tcpdump Group, www.tcpdump.org
Intended usage: LIMITED
Restrictions on usage: NONE
Author: Guy Harris and Michael Richardson
Change controller: The Tcpdump Group
Provisional registration? (standards tree only): NO
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9. Contributors
Insert pcap developers etc. here
10. Acknowledgments
The authors wish to thank (many reviewers) and many others for their
invaluable comments.
11. References
11.1. Normative References
[I-D.ietf-opsawg-pcaplinktype]
Harris, G. and M. Richardson, "Link-Layer Types for PCAP
and PCAPNG Capture File Formats", Work in Progress,
Internet-Draft, draft-ietf-opsawg-pcaplinktype-00, 22
January 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-opsawg-pcaplinktype-00>.
[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/rfc/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/rfc/rfc8174>.
11.2. Informative References
[I-D.ietf-opsawg-pcapng]
Tüxen, M., Risso, F., Bongertz, J., Combs, G., Harris, G.,
Chaudron, E., and M. Richardson, "PCAP Next Generation
(pcapng) Capture File Format", Work in Progress, Internet-
Draft, draft-ietf-opsawg-pcapng-00, 24 January 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-opsawg-
pcapng-00>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
[Radiotap] radiotap.org, "Radiotap Web site", n.d.,
<http://www.radiotap.org/>.
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[AVS] Peachy, S., "Archived AVS specification", n.d.,
<http://web.archive.org/web/20040803232023/
http://www.shaftnet.org/~pizza/software/capturefrm.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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