.\" Copyright (c) 2022 .\" The TCPDUMP project. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that: (1) source code distributions .\" retain the above copyright notice and this paragraph in its entirety, (2) .\" distributions including binary code include the above copyright notice and .\" this paragraph in its entirety in the documentation or other materials .\" provided with the distribution. .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. .\" .TH CBPF-SAVEFILE @MAN_FILE_FORMATS@ "17 July 2022" .SH NAME cbpf-savefile \- cBPF savefile format (work in progress, DRAFT revision 6) .SH DESCRIPTION This man page discusses a file format for cBPF, which is the "classic" (and for a long time the only) Berkeley Packet Filter. It does .B NOT apply to the newer "extended" variety of BPF (eBPF). .LP The main purpose of this file format is to store BPF bytecode, most commonly compiled from a BPF filter expression (see .BR \%pcap-filter (7) for the filter syntax description) using libpcap. Besides that, the format allows to encode some information about the context in which the compilation was done. This meta-data can make it easier to reproduce the compilation later if required. cBPF savefile design is based on the file format proposed by C.S. Peron in 2005. .LP Unless stated otherwise, in the following specification integer fields are big-endian unsigned, string fields do not use NUL character for termination or padding. .SH FILE FORMAT A savefile consists of a fixed-size header and a variable-size body as follows: .LP .nf 0 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0xA1 | 0xB2 | 0xC3 | 0xCB | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 'c' | 'B' | 'P' | 'F' | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MajorVer=1 | MinorVer | Flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SnapLen | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LinkTypeValue | InstructionCount=n | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | instruction 1 | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | instruction 2 | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | instruction n | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | optional trailing TLV space | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .fi .LP The first four octets contain a fixed signature, also known as a magic number, to make it easy to identify the file type automatically. The next four octets contain the ASCII string "cBPF" to provide a hint for manual identification. .SS MajorVer and MinorVer Contain the major and the minor version numbers of this format respectively. The current major version is 1 and the current minor version is 0. Format changes that do not impact compatibility (e.g., new TLV types or flags) increment the minor version only. Other format changes increment the major version and reset the minor version to 0. All format versions have the first part of the header up to and including MinorVer identical. .SS Flags .nf b15 b14 b13 b12 b11 b10 b09 b08 b07 b06 b05 b04 b03 b02 b01 b00 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | reserved |CPX|COP|XOR|MOD| +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ .fi MOD, XOR, COP and CPX: if set to 1, in this BPF dialect .BR BPF_MOD , .BR BPF_XOR , .B BPF_COP and .B BPF_COPX respectively are valid instructions. For the current list of OSes that support .B BPF_MOD and .B BPF_XOR in the kernel see the .B % and .B ^ binary operators in .BR \%pcap-filter (7). .B BPF_COP and .B BPF_COPX are implemented in NetBSD kernel. .LP Note that the fact an instruction is valid in a BPF dialect does not always mean the compiled bytecode in the savefile contains the instruction. In other words, the purpose of the flags above is not to provide a digest of the file contents, but to enable conclusive automatic verification of the bytecode if required by the use case. .SS SnapLen Contains the snapshot length used for the compilation, usually this is the .I snaplen input argument to .BR pcap_open_dead (3PCAP) or .BR pcap_set_snaplen (3PCAP). .SS LinkTypeValue Contains the link-layer header type value used for the compilation, usually this is either the .I linktype input argument to .BR pcap_open_dead () or the .I dlt input argument to .BR pcap_set_datalink (3PCAP) or the value(s) returned by .BR pcap_datalink (3PCAP) and .BR pcap_list_datalinks (3PCAP). Although these functions take and return link-layer header type values via the .B int C type, by convention only the low 16 bits are in use. .SS InstructionCount This is the last field of the fixed header in major version 1, it contains the number of bytecode instructions following the header. By convention, valid BPF bytecode always ends with a .B ret instruction, so in a valid savefile this field value is at least 1. .PP The file format thus far minimizes the overhead for software that uses the BPF bytecode. If there is any data after the last instruction, it is the trailing TLV space, which mostly contains meta-data for human interpretation. It contains TLVs in the format specified below. .SH INSTRUCTION FORMAT .LP .nf 0 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | opcode | jt | jf | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | k | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .fi .LP This is the traditional encoding of a BPF instruction (a 4-tuple of integers). Note that usually the endianness depends on the machine, but in this format it is fixed. Some opcodes interpret k as a signed integer. .SH TLV FORMAT .LP .nf 0 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length=m | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Value (m octets) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .fi .LP All TLVs are optional. Every TLV may appear in the same savefile at most once. Length value does not include Type and Length. Code points for Type and the associated Length constraints are defined below. .SS EOF TLV Allows to mark the end of TLV space (hence of the savefile) explicitly to make it clear that the file is not truncated. If this TLV is present in the TLV space, it may appear the last only. .LP Type is 0, Length is 0, Value is empty. .SS LinkTypeName TLV Allows to record the input argument to .BR pcap_datalink_name_to_val (3PCAP) if the latter was used to translate a DLT name into LinkTypeValue (the same name can sometimes produce different values in different contexts). .LP Type is 1, Length is variable, Value contains an ASCII string. .SS Filter TLV Allows to record the filter expression that was compiled into the bytecode, usually this is the .I str input argument to .BR pcap_compile (3PCAP). .LP Type is 2, Length is variable, Value contains an ASCII string. .SS OptReq TLV Allows to record whether optimization was requested for the compilation or not, usually this is the .I optimize input argument to .BR pcap_compile (). Note that some link-layer header types and filter keywords disable the optimization automatically in libpcap. .LP Type is 3, Length is 1, Value contains 1 or 0. .SS Netmask TLV Allows to record the value of .I netmask input argument to .BR pcap_compile (). .LP Type is 4, Length is 4, Value contains a 32-bit IPv4 netmask. .SS Comment TLV Allows to record a free-form text, for example, the name and version of the program that generated the file. .LP Type is 5, Length is variable, Value contains a UTF-8 string. .SS Timestamp TLV Allows to record when the compilation was performed. .LP Type is 6, Length is 8, Value contains a 64-bit Unix timestamp. .SH SOFTWARE SUPPORT BPF Exam at .I \%https://www.tcpdump.org/bpfexam/ can produce a cBPF savefile from user input. .SH SEE ALSO .BR \%pcap-savefile (@MAN_FILE_FORMATS@)