cbpf-savefile.manfile.in

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.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 BPF_MOD, BPF_XOR,
BPF_COP and BPF_COPX respectively are valid instructions.  For the current
list of OSes that support BPF_MOD and BPF_XOR in the kernel see the
.B %
and
.B ^
binary operators in
.BR \%pcap-filter (7).
BPF_COP and 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
.I 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 "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@)