Internet-Draft pcap June 2021
Harris & Richardson Expires 25 December 2021 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-gharris-opsawg-pcap-02
Published:
Intended Status:
Informational
Expires:
Authors:
G. Harris, Ed.
M. Richardson
Sandelman

PCAP Capture File Format

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 December 2021.

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.

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:

                           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|0 0 0 0 0 0 0 0 0 0 0 0|         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 sessions 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 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 (16 bits):

a 16-bit unsigned value that defines the link layer type of packets in the file. This field is defined in the Section 8.2 IANA registry.

Frame Cyclic Sequence (FCS) present (4 bits):

if the "f" bit is set, then the 3 FCS bits provide the number of 16-bit (2 byte) words of FCS that are appended to each packet.

valid values are between 0 and 7, with ethernet typically having a length of 4 bytes, or a value of 2.

The bits marked as zero MUST be set to zero by pcap writers, and MUST be ignored by pcap readers.

5. Packet Record

A Packet Record is the standard container for storing the packets coming from the network.

                          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):

seconds 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 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].

8. IANA Considerations

This document requires the following IANA actions:

8.1. Media-Type Registry

This section registers the 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

8.2. LinkType Registry

IANA is requested to create a new Registry entitled: "The PCAP Registry", and within that Registry to create a table called: "PCAP LinkType List".

The LinkType Registry is a table of 16-bit numbers. The Registry has three sections with different [RFC8126] rules:

  • values from 0 to 32767 are marked as Specification Required.
  • values from 32768 to 65000 are marked as First-Come First-Served.
  • values from 65000 to 65535 are marked as Private Use.

The Registry has four columns: the symbolic name (LINKTYPE_something), the integer value, a very short description, and the document/requestor reference.

The Registry shall be populated as follows in the table below. In each case here, the reference should be http://www.tcpdump.org/linktypes.html, which is not repeated.

The initial value of table is base upon the Link type list maintained by libpcap, and published on the tcpdump.org web site as http://www.tcpdump.org/linktypes.html.

There is often an associated DLT value which are often identical in value, but not universally so. DLT values are associated with specific operation system captures, and are operating system specific.

Table 1
LINKTYPE name LINKTYPE value description
LINKTYPE_NULL 0 BSD loopback encapsulation
LINKTYPE_ETHERNET 1 IEEE 802.3 Ethernet
LINKTYPE_EXP_ETHERNET 2 Xerox experimental 3Mb Ethernet
LINKTYPE_AX25 3 AX.25 packet
LINKTYPE_PRONET 4 Reserved for PRONET
LINKTYPE_CHAOS 5 Reserved for MIT CHAOSNET
LINKTYPE_IEEE802_5 6 IEEE 802.5 Token Ring
LINKTYPE_ARCNET_BSD 7 ARCNET Data Packets with BSD encapsulation
LINKTYPE_SLIP 8 SLIP, w/LINKTYPE_SLIP header.
LINKTYPE_PPP 9 PPP, as per RFC 1661/RFC 1662
LINKTYPE_FDDI 10 FDDI: per ANSI INCITS 239-1994.
not to be used 11-49 Do not use these values
LINKTYPE_PPP_HDLC 50 PPP in HDLC-like framing, as per RFC 1662
LINKTYPE_PPP_ETHER 51 PPPoE; per RFC 2516
not to be used 52-98 Do not use these values
LINKTYPE_SYMANTEC_FIREWALL 99 Reserved for Symantec Enterprise Firewall
LINKTYPE_ATM_RFC1483 100 RFC 1483 LLC/SNAP-encapsulated ATM
LINKTYPE_RAW 101 Raw IP; begins with an IPv4 or IPv6 header
LINKTYPE_SLIP_BSDOS 102 Reserved for BSD/OS SLIP BPF header
LINKTYPE_PPP_BSDOS 103 Reserved for BSD/OS PPP BPF header
LINKTYPE_C_HDLC 104 Cisco PPP with HDLC framing, as per section 4.3.1 of RFC 1547
LINKTYPE_IEEE802_11 105 IEEE 802.11 wireless LAN.
LINKTYPE_ATM_CLIP 106 ATM Classical IP, with no header preceding IP
LINKTYPE_FRELAY 107 Frame Relay LAPF frames
LINKTYPE_LOOP 108 OpenBSD loopback encapsulation
LINKTYPE_ENC 109 Reserved for OpenBSD IPSEC encapsulation
LINKTYPE_LANE8023 110 Reserved for ATM LANE + 802.3
LINKTYPE_HIPPI 111 Reserved for NetBSD HIPPI
LINKTYPE_HDLC 112 Reserved for NetBSD HDLC framing
LINKTYPE_LINUX_SLL 113 Linux "cooked" capture encapsulation
LINKTYPE_LTALK 114 Apple LocalTalk
LINKTYPE_ECONET 115 Reserved for Acorn Econet
LINKTYPE_IPFILTER 116 Reserved for OpenBSD ipfilter
LINKTYPE_PFLOG 117 OpenBSD pflog; "struct pfloghdr" structure
LINKTYPE_CISCO_IOS 118 Reserved for Cisco-internal use
LINKTYPE_IEEE802_11_PRISM 119 Prism monitor mode
LINKTYPE_IEEE802_11_AIRONET 120 Reserved for 802.11 + FreeFreeBSD Aironet radio metadata
LINKTYPE_HHDLC 121 Reserved for Siemens HiPath HDLC
LINKTYPE_IP_OVER_FC 122 RFC 2625 IP-over-Fibre Channel
LINKTYPE_SUNATM 123 ATM traffic, / per SunATM devices
LINKTYPE_RIO 124 Reserved for RapidIO
LINKTYPE_PCI_EXP 125 Reserved for PCI Express
LINKTYPE_AURORA 126 Reserved for Xilinx Aurora link layer
LINKTYPE_IEEE802_11_RADIOTAP 127 Radiotap header[Radiotap], followed by an 802.11 header
LINKTYPE_TZSP 128 Reserved for Tazmen Sniffer Protocol
LINKTYPE_ARCNET_LINUX 129 ARCNET Data Packets, per RFC 1051 frames w/variations
LINKTYPE_JUNIPER_MLPPP 130 Reserved for Juniper Networks
LINKTYPE_JUNIPER_MLFR 131 Reserved for Juniper Networks
LINKTYPE_JUNIPER_ES 132 Reserved for Juniper Networks
LINKTYPE_JUNIPER_GGSN 133 Reserved for Juniper Networks
LINKTYPE_JUNIPER_MFR 134 Reserved for Juniper Networks
LINKTYPE_JUNIPER_ATM2 135 Reserved for Juniper Networks
LINKTYPE_JUNIPER_SERVICES 136 Reserved for Juniper Networks
LINKTYPE_JUNIPER_ATM1 137 Reserved for Juniper Networks
LINKTYPE_APPLE_IP_OVER_IEEE1394 138 Apple IP-over-IEEE 1394 cooked header
LINKTYPE_MTP2_WITH_PHDR 139 Signaling System 7 (SS7) Message Transfer Part Level ITU-T Q.703
LINKTYPE_MTP2 140 SS7 Level 2, Q.703
LINKTYPE_MTP3 141 SS7 Level 3, Q.704
LINKTYPE_SCCP 142 SS7 Control Part, ITU-T Q.711/Q.712/Q.713/Q.714
LINKTYPE_DOCSIS 143 DOCSIS MAC frames, DOCSIS 3.1
LINKTYPE_LINUX_IRDA 144 Linux-IrDA packets w/LINKTYPE_LINUX_IRDA header
LINKTYPE_IBM_SP 145 Reserved for IBM SP switch
LINKTYPE_IBM_SN 146 Reserved for IBM Next Federation switch
LINKTYPE_RESERVED_01 147 For private use
LINKTYPE_RESERVED_02 148 For private use
LINKTYPE_RESERVED_03 149 For private use
LINKTYPE_RESERVED_04 150 For private use
LINKTYPE_RESERVED_05 151 For private use
LINKTYPE_RESERVED_06 152 For private use
LINKTYPE_RESERVED_07 153 For private use
LINKTYPE_RESERVED_08 154 For private use
LINKTYPE_RESERVED_09 155 For private use
LINKTYPE_RESERVED_10 156 For private use
LINKTYPE_RESERVED_11 157 For private use
LINKTYPE_RESERVED_12 158 For private use
LINKTYPE_RESERVED_13 159 For private use
LINKTYPE_RESERVED_14 160 For private use
LINKTYPE_RESERVED_15 161 For private use
LINKTYPE_RESERVED_16 162 For private use
LINKTYPE_IEEE802_11_AVS 163 AVS header[AVS], followed by an 802.11 header
LINKTYPE_JUNIPER_MONITOR 164 Reserved for Juniper Networks
LINKTYPE_BACNET_MS_TP 165 BACnet MS/TP frames, per 9.3 MS/TP Frame Format ANSI 135
LINKTYPE_PPP_PPPD 166 PPP in HDLC-like encapsulation, like LINKTYPE_PPP_HDLC, different stuffing
LINKTYPE_JUNIPER_PPPOE 167 Reserved for Juniper Networks
LINKTYPE_JUNIPER_PPPOE_ATM 168 Reserved for Juniper Networks
LINKTYPE_GPRS_LLC 169 General Packet Radio Service Logical Link Control, as per 3GPP TS 04.64
LINKTYPE_GPF_T 170 Transparent-mapped generic framing procedure, as specified by ITU-T Recommendation G.7041/Y.1303
LINKTYPE_GPF_F 171 Frame-mapped generic framing procedure, as specified by ITU-T Recommendation G.7041/Y.1303
LINKTYPE_GCOM_T1E1 172 Reserved for Gcom T1/E1 line monitoring equipment
LINKTYPE_GCOM_SERIAL 173 Reserved for Gcom T1/E1 line monitoring equipment
LINKTYPE_JUNIPER_PIC_PEER 174 Reserved for Juniper Networks
LINKTYPE_ERF_ETH 175 Endace ERF header followed by 802.3 Ethernet
LINKTYPE_ERF_POS 176 Endace ERF header followed by Packet-over-SONET
LINKTYPE_LINUX_LAPD 177 Link Access Procedures on the D Channel (LAPD) frames, as specified by ITU-T Recommendation Q.920 and ITU-T Recommendation Q.921 , captured via vISDN, with a LINKTYPE_LINUX_LAPD header , followed by the Q.921 frame, starting with the address field.
LINKTYPE_JUNIPER_ETHER 178 Reserved for Juniper Networks
LINKTYPE_JUNIPER_PPP 179 Reserved for Juniper Networks
LINKTYPE_JUNIPER_FRELAY 180 Reserved for Juniper Networks
LINKTYPE_JUNIPER_CHDLC 181 Reserved for Juniper Networks
LINKTYPE_MFR 182 FRF.16.1 Multi-Link Frame Relay frames, beginning with an FRF.12 Interface fragmentation format fragmentation header.
LINKTYPE_JUNIPER_VP 182 Reserved for Juniper Networks
LINKTYPE_A653_ICM 185 Reserved for Arinc 653 Interpartition Communication messages
LINKTYPE_USB_FREEBSD 186 USB packets, beginning with a FreeBSD USB header
LINKTYPE_BLUETOOTH_HCI_H4 187 Bluetooth HCI UART transport layer; the frame contains an HCI packet indicator byte, as specified by the UART Transport Layer portion of the most recent Bluetooth Core specification , followed by an HCI packet of the specified packet type, as specified by the Host Controller Interface Functional Specification portion of the most recent Bluetooth Core Specification.
LINKTYPE_IEEE802_16_MAC_CPS 188 Reserved for IEEE 802.16 MAC Common Part Sublayer
LINKTYPE_USB_LINUX 189 USB packets, beginning with a Linux USB header, as specified by the struct usbmon_packet in the Documentation/usb/usbmon.txt file in the Linux source tree. Only the first 48 bytes of that header are present. All fields in the header are in host byte order. When performing a live capture, the host byte order is the byte order of the machine on which the packets are captured. When reading a pcap file, the byte order is the byte order for the file, as specified by the file's magic number; when reading a pcapng file, the byte order is the byte order for the section of the pcapng file, as specified by the Section Header Block.
LINKTYPE_CAN20B 190 Reserved for Controller Area Network (CAN) v. 2.0B packets
LINKTYPE_IEEE802_15_4_LINUX 191 IEEE 802.15.4, with address fields padded, as is done by Linux drivers
LINKTYPE_PPI 192 Per-Packet Information information, as specified by the Per-Packet Information Header Specification , followed by a packet with the LINKTYPE_ value specified by the pph_dlt field of that header.
LINKTYPE_IEEE802_16_MAC_CPS_RADIO 193 Reserved for 802.16 MAC Common Part Sublayer plus radio header
LINKTYPE_JUNIPER_ISM 194 Reserved for Juniper Networks
LINKTYPE_IEEE802_15_4_WITHFCS 195 IEEE 802.15.4 Low-Rate Wireless Networks, with each packet having the FCS at the end of the frame.
LINKTYPE_SITA 196 Various link-layer types, with a pseudo-header , for SITA
LINKTYPE_ERF 197 Various link-layer types, with a pseudo-header, for Endace DAG cards; encapsulates Endace ERF records.
LINKTYPE_RAIF1 198 Reserved for Ethernet packets captured from a u10 Networks board
LINKTYPE_IPMB_KONTRON 199 Reserved for IPMB packet for IPMI, with a 2-byte header
LINKTYPE_JUNIPER_ST 200 Reserved for Juniper Networks
LINKTYPE_BLUETOOTH_HCI_H4_WITH_PHDR 201 Bluetooth HCI UART transport layer; the frame contains a 4-byte direction field, in network byte order (big-endian), the low-order bit of which is set if the frame was sent from the host to the controller and clear if the frame was received by the host from the controller, followed by an HCI packet indicator byte, as specified by the UART Transport Layer portion of the most recent Bluetooth Core specification , followed by an HCI packet of the specified packet type, as specified by the Host Controller Interface Functional Specification portion of the most recent Bluetooth Core Specification.
LINKTYPE_AX25_KISS 202 AX.25 packet, with a 1-byte KISS header containing a type indicator.
LINKTYPE_LAPD 203 Link Access Procedures on the D Channel (LAPD) frames, as specified by ITU-T Recommendation Q.920 and ITU-T Recommendation Q.921 , starting with the address field, with no pseudo-header.
LINKTYPE_PPP_WITH_DIR 204 PPP, as per RFC 1661 and RFC 1662 , preceded with a one-byte pseudo-header with a zero value meaning received by this host and a non-zero value meaning sent by this host; if the first 2 bytes are 0xff and 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. The data in the frame is not octet-stuffed or bit-stuffed.
LINKTYPE_C_HDLC_WITH_DIR 205 Cisco PPP with HDLC framing, as per section 4.3.1 of RFC 1547 , preceded with a one-byte pseudo-header with a zero value meaning received by this host and a non-zero value meaning sent by this host.
LINKTYPE_FRELAY_WITH_DIR 206 Frame Relay LAPF frames, beginning with a one-byte pseudo-header with a zero value meaning received by this host (DCE->DTE) and a non-zero value meaning sent by this host (DTE->DCE), followed by an ITU-T Recommendation Q.922 LAPF header starting with the address field, and without an FCS at the end of the frame.
LINKTYPE_LAPB_WITH_DIR 207 Link Access Procedure, Balanced (LAPB), as specified by ITU-T Recommendation X.25 , preceded with a one-byte pseudo-header with a zero value meaning received by this host (DCE->DTE) and a non-zero value meaning sent by this host (DTE->DCE).
Reserved 208 Reserved for an unspecified link-layer type
LINKTYPE_IPMB_LINUX 209 IPMB over an I2C circuit, with a Linux-specific pseudo-header
LINKTYPE_FLEXRAY 210 Reserved for FlexRay automotive bus
LINKTYPE_MOST 211 Reserved for Media Oriented Systems Transport (MOST) bus
LINKTYPE_LIN 212 Reserved for Local Interconnect Network (LIN) bus for vehicle networks
LINKTYPE_X2E_SERIAL 213 Reserved for X2E serial line captures
LINKTYPE_X2E_XORAYA 214 Reserved for X2E Xoraya data loggers
LINKTYPE_IEEE802_15_4_NONASK_PHY 215 IEEE 802.15.4 Low-Rate Wireless Networks, with each packet having the FCS at the end of the frame, and with the PHY-level data for the O-QPSK, BPSK, GFSK, MSK, and RCC DSS BPSK PHYs (4 octets of 0 as preamble, one octet of SFD, one octet of frame length + reserved bit) preceding the MAC-layer data (starting with the frame control field).
LINKTYPE_LINUX_EVDEV 216 Reserved for Linux evdev messages
LINKTYPE_GSMTAP_UM 217 Reserved for GSM Um interface, with gsmtap header
LINKTYPE_GSMTAP_ABIS 218 Reserved for GSM Abis interface, with gsmtap header
LINKTYPE_MPLS 219 MPLS packets with MPLS label as the header
LINKTYPE_USB_LINUX_MMAPPED 220 USB packets, beginning with a Linux USB header, as specified by the struct usbmon_packet in the Documentation/usb/usbmon.txt file in the Linux source tree. All 64 bytes of the header are present. All fields in the header are in host byte order. When performing a live capture, the host byte order is the byte order of the machine on which the packets are captured. When reading a pcap file, the byte order is the byte order for the file, as specified by the file's magic number; when reading a pcapng file, the byte order is the byte order for the section of the pcapng file, as specified by the Section Header Block. For isochronous transfers, the ndesc field specifies the number of isochronous descriptors that follow.
LINKTYPE_DECT 221 Reserved for DECT packets, with a pseudo-header
LINKTYPE_AOS 222 Reserved for OS Space Data Link Protocol
LINKTYPE_WIHART 223 Reserved for Wireless HART (Highway Addressable Remote Transducer)
LINKTYPE_FC_2 224 Fibre Channel FC-2 frames, beginning with a Frame_Header.
LINKTYPE_FC_2_WITH_FRAME_DELIMS 225 Fibre Channel FC-2 frames, beginning an encoding of the SOF, followed by a Frame_Header, and ending with an encoding of the SOF. The encodings represent the frame delimiters as 4-byte sequences representing the corresponding ordered sets, with K28.5 represented as 0xBC, and the D symbols as the corresponding byte values; for example, SOFi2, which is K28.5 - D21.5 - D1.2 - D21.2, is represented as 0xBC 0xB5 0x55 0x55.
LINKTYPE_IPNET 226 Solaris ipnet pseudo-header , followed by an IPv4 or IPv6 datagram.
LINKTYPE_CAN_SOCKETCAN 227 CAN (Controller Area Network) frames, with a pseudo-header followed by the frame payload.
LINKTYPE_IPV4 228 Raw IPv4; the packet begins with an IPv4 header.
LINKTYPE_IPV6 229 Raw IPv6; the packet begins with an IPv6 header.
LINKTYPE_IEEE802_15_4_NOFCS 230 IEEE 802.15.4 Low-Rate Wireless Network, without the FCS at the end of the frame.
LINKTYPE_DBUS 231 Raw D-Bus messages , starting with the endianness flag, followed by the message type, etc., but without the authentication handshake before the message sequence.
LINKTYPE_JUNIPER_VS 232 Reserved for Juniper Networks
LINKTYPE_JUNIPER_SRX_E2E 233 Reserved for Juniper Networks
LINKTYPE_JUNIPER_FIBRECHANNEL 234 Reserved for Juniper Networks
LINKTYPE_DVB_CI 235 DVB-CI (DVB Common Interface for communication between a PC Card module and a DVB receiver), with the message format specified by the PCAP format for DVB-CI specification
LINKTYPE_MUX27010 236 Variant of 3GPP TS 27.010 multiplexing protocol (similar to, but not the same as, 27.010).
LINKTYPE_STANAG_5066_D_PDU 237 D_PDUs as described by NATO standard STANAG 5066, starting with the synchronization sequence, and including both header and data CRCs. The current version of STANAG 5066 is backwards-compatible with the 1.0.2 version , although newer versions are classified.
LINKTYPE_JUNIPER_ATM_CEMIC 238 Reserved for Juniper Networks
LINKTYPE_NFLOG 239 Linux netlink NETLINK NFLOG socket log messages.
LINKTYPE_NETANALYZER 240 Pseudo-header for Hilscher Gesellschaft fuer Systemautomation mbH netANALYZER devices , followed by an Ethernet frame, beginning with the MAC header and ending with the FCS.
LINKTYPE_NETANALYZER_TRANSPARENT 241 Pseudo-header for Hilscher Gesellschaft fuer Systemautomation mbH netANALYZER devices , followed by an Ethernet frame, beginning with the preamble, SFD, and MAC header, and ending with the FCS.
LINKTYPE_IPOIB 242 IP-over-InfiniBand, as specified by RFC 4391 section 6
LINKTYPE_MPEG_2_TS 243 MPEG-2 Transport Stream transport packets, as specified by ISO 13818-1/ ITU-T Recommendation H.222.0 (see table 2-2 of section 2.4.3.2 Transport Stream packet layer).
LINKTYPE_NG40 244 Pseudo-header for ng4T GmbH's UMTS Iub/Iur-over-ATM and Iub/Iur-over-IP format as used by their ng40 protocol tester , followed by frames for the Frame Protocol as specified by 3GPP TS 25.427 for dedicated channels and 3GPP TS 25.435 for common/shared channels in the case of ATM AAL2 or UDP traffic, by SSCOP packets as specified by ITU-T Recommendation Q.2110 for ATM AAL5 traffic, and by NBAP packets for SCTP traffic.
LINKTYPE_NFC_LLCP 245 Pseudo-header for NFC LLCP packet captures , followed by frame data for the LLCP Protocol as specified by NFCForum-TS-LLCP_1.1
LINKTYPE_PFSYNC 246 Reserved for pfsync output
LINKTYPE_INFINIBAND 247 Raw InfiniBand frames, starting with the Local Routing Header, as specified in Chapter 5 Data packet format of InfiniBand[TM] Architectural Specification Release 1.2.1 Volume 1 - General Specifications
LINKTYPE_SCTP 248 SCTP packets, as defined by RFC 4960 , with no lower-level protocols such as IPv4 or IPv6.
LINKTYPE_USBPCAP 249 USB packets, beginning with a USBPcap header
LINKTYPE_RTAC_SERIAL 250 Serial-line packet header for the Schweitzer Engineering Laboratories RTAC product , followed by a payload for one of a number of industrial control protocols.
LINKTYPE_BLUETOOTH_LE_LL 251 Bluetooth Low Energy air interface Link Layer packets, in the format described in section 2.1 PACKET FORMAT of volume 6 of the Bluetooth Specification Version 4.0 (see PDF page 2200), but without the Preamble.
LINKTYPE_WIRESHARK_UPPER_PDU 252 Reserved for Wireshark
LINKTYPE_NETLINK 253 Linux Netlink capture encapsulation
LINKTYPE_BLUETOOTH_LINUX_MONITOR 254 Bluetooth Linux Monitor encapsulation of traffic for the BlueZ stack
LINKTYPE_BLUETOOTH_BREDR_BB 255 Bluetooth Basic Rate and Enhanced Data Rate baseband packets
LINKTYPE_BLUETOOTH_LE_LL_WITH_PHDR 256 Bluetooth Low Energy link-layer packets
LINKTYPE_PROFIBUS_DL 257 PROFIBUS data link layer packets, as specified by IEC standard 61158-4-3, beginning with the start delimiter, ending with the end delimiter, and including all octets between them.
LINKTYPE_PKTAP 258 Apple PKTAP capture encapsulation
LINKTYPE_EPON 259 Ethernet-over-passive-optical-network packets, starting with the last 6 octets of the modified preamble as specified by 65.1.3.2 Transmit in Clause 65 of Section 5 of IEEE 802.3 , followed immediately by an Ethernet frame.
LINKTYPE_IPMI_HPM_2 260 IPMI trace packets, as specified by Table 3-20 Trace Data Block Format in the PICMG HPM.2 specification The time stamps for packets in this format must match the time stamps in the Trace Data Blocks.
LINKTYPE_ZWAVE_R1_R2 261 Z-Wave RF profile R1 and R2 packets , as specified by ITU-T Recommendation G.9959 , with some MAC layer fields moved.
LINKTYPE_ZWAVE_R3 262 Z-Wave RF profile R3 packets , as specified by ITU-T Recommendation G.9959 , with some MAC layer fields moved.
LINKTYPE_WATTSTOPPER_DLM 263 Formats for WattStopper Digital Lighting Management (DLM) and Legrand Nitoo Open protocol common packet structure captures.
LINKTYPE_ISO_14443 264 Messages between ISO 14443 contactless smartcards (Proximity Integrated Circuit Card, PICC) and card readers (Proximity Coupling Device, PCD), with the message format specified by the PCAP format for ISO14443 specification
LINKTYPE_RDS 265 Radio data system (RDS) groups, as per IEC 62106, encapsulated in this form
LINKTYPE_USB_DARWIN 266 USB packets, beginning with a Darwin (macOS, etc.) USB header
LINKTYPE_OPENFLOW 267 Reserved for OpenBSD DLT_OPENFLOW
LINKTYPE_SDLC 268 SDLC packets, as specified by Chapter 1, DLC Links, section Synchronous Data Link Control (SDLC) of Systems Network Architecture Formats, GA27-3136-20 , without the flag fields, zero-bit insertion, or Frame Check Sequence field, containing SNA path information units (PIUs) as the payload.
LINKTYPE_TI_LLN_SNIFFER 269 Reserved for Texas Instruments protocol sniffer
LINKTYPE_LORATAP 270 LoRaTap pseudo-header , followed by the payload, which is typically the PHYPayload from the LoRaWan specification
LINKTYPE_VSOCK 271 Protocol for communication between host and guest machines in VMware and KVM hypervisors.
LINKTYPE_NORDIC_BLE 272 Messages to and from a Nordic Semiconductor nRF Sniffer for Bluetooth LE packets, beginning with a pseudo-header
LINKTYPE_DOCSIS31_XRA31 273 DOCSIS packets and bursts, preceded by a pseudo-header giving metadata about the packet
LINKTYPE_ETHERNET_MPACKET 274 mPackets, as specified by IEEE 802.3br Figure 99-4, starting with the preamble and always ending with a CRC field.
LINKTYPE_DISPLAYPORT_AUX 275 DisplayPort AUX channel monitoring data as specified by VESA DisplayPort(DP) Standard preceded by a pseudo-header
LINKTYPE_LINUX_SLL2 276 Linux cooked capture encapsulation v2
LINKTYPE_SERCOS_MONITOR 277 Reserved for Sercos Monitor
LINKTYPE_OPENVIZSLA 278 Openvizsla FPGA-based USB sniffer
LINKTYPE_EBHSCR 279 Elektrobit High Speed Capture and Replay (EBHSCR) format
LINKTYPE_VPP_DISPATCH 280 Records in traces from the http://fd.io VPP graph dispatch tracer, in the the graph dispatcher trace format
LINKTYPE_DSA_TAG_BRCM 281 Ethernet frames, with a switch tag inserted between the source address field and the type/length field in the Ethernet header.
LINKTYPE_DSA_TAG_BRCM_PREPEND 282 Ethernet frames, with a switch tag inserted before the destination address in the Ethernet header.
LINKTYPE_IEEE802_15_4_TAP 283 IEEE 802.15.4 Low-Rate Wireless Networks, with a pseudo-header containing TLVs with metadata preceding the 802.15.4 header.
LINKTYPE_DSA_TAG_DSA 284 Ethernet frames, with a switch tag inserted between the source address field and the type/length field in the Ethernet header.
LINKTYPE_DSA_TAG_EDSA 285 Ethernet frames, with a programmable Ethernet type switch tag inserted between the source address field and the type/length field in the Ethernet header.
LINKTYPE_ELEE 286 Payload of lawful intercept packets using the ELEE protocol The packet begins with the ELEE header; it does not include any transport-layer or lower-layer headers for protcols used to transport ELEE packets.
LINKTYPE_Z_WAVE_SERIAL 287 Serial frames transmitted between a host and a Z-Wave chip over an RS-232 or USB serial connection, as described in section 5 of the Z-Wave Serial API Host Application Programming Guide
LINKTYPE_USB_2_0 288 USB 2.0, 1.1, or 1.0 packet, beginning with a PID, as described by Chapter 8 Protocol Layer of the the Universal Serial Bus Specification Revision 2.0
LINKTYPE_ATSC_ALP 289 ATSC Link-Layer Protocol frames, as described in section 5 of the A/330 Link-Layer Protocol specification, found at the ATSC 3.0 standards page , beginning with a Base Header

9. Contributors

[Insert pcap developers etc. here].

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, , <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, , <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, , <https://www.ietf.org/archive/id/draft-tuexen-opsawg-pcapng-02.txt>.
[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, , <https://www.rfc-editor.org/info/rfc8126>.
[Radiotap]
radiotap.org, "Radiotap Web site", n.d., <http://www.radiotap.org/>.
[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)
Michael C. Richardson
Sandelman Software Works Inc