--- Begin Message ---
Hello list.
Below you can find the next draft revision. It incorporates some
feedback received from Guy and Michael, also it reorders the TLVs and
adds SnapLen TLV, Netmask TLV and EOF TLV. Also the text has been
converted to a man page, so it could live next to pcap-savefile(5) when
it is ready enough.
One thing that does not look quite right to me in this revision is
that, for example, LinkTypeValue TLV could be not an optional TLV, but a
part of the fixed header because DLT is a meaningful bit of information
and could (and possibly should) be checked against the DLT where the
bytecode is applied. (This is exactly what
https://github.com/the-tcpdump-group/libpcap/issues/211 does.)
tc-bpf(8) on Linux says: "Since libpcap does not support all Linux'
specific cBPF extensions in its compiler..."
If that's true, then the header would need another field to indicate
the cBPF dialect, so if anybody/anything was to validate the bytecode,
the result would always be conclusive. Could anybody tell an example
of such cBPF differences if those indeed exist?
If you see an issue with terminology or style, let me know.
----------------------------------------------------------------------
CBPF-SAVEFILE(5) File Formats Manual CBPF-SAVEFILE(5)
NAME
cbpf-savefile - cBPF savefile format (DRAFT revision 3)
DESCRIPTION
This man page discusses a file format for cBPF, which is the
"classic" (and for a long time the only) Berkeley Packet Fil-
ter. It does NOT apply to the newer eBPF variety of BPF.
The main purpose of this file format is to store cBPF bytecode,
most commonly compiled from a BPF filter expression using libp-
cap. Besides that, the format allows to encode some informa-
tion about the context in which the compilation was done. This
meta-data can make it easier to reproduce the compilation later
if required.
In the following specification integer fields are big-endian
unsigned, String fields do not use NUL character for termina-
tion or padding.
FILE FORMAT
A cBPF savefile consists of a fixed-size header and a variable-
size body as follows:
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 | MinorVer | InstructionCount=n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| instruction 1 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| instruction 2 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| instruction n |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| optional trailing TLV space |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first four bytes contain a fixed signature, also known as a
magic number, to make it easy to identify the file type auto-
matically. The next four bytes contain the ASCII string "cBPF"
to provide a hint for manual identification.
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) increment the
minor version only. Other format changes increment the major
version and reset the minor version to 0.
InstructionCount is the last field of the fixed header, it con-
tains the number of bytecode instructions following the header.
By convention, valid BPF bytecode must consist of at least one
instruction, so in a valid cBPF savefile this field value is at
least 1.
The file format thus far minimizes the overhead for software
that only writes or reads cBPF 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 con-
tains TLVs in the format specified below.
INSTRUCTION FORMAT
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This is the traditional encoding of a cBPF instruction. Note
that usually its endianness depends on the machine, but in this
format it is fixed.
TLV FORMAT
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 bytes) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All TLVs are optional. Every TLV may appear in the same cBPF
savefile at most once. Length value does not include Type and
Length. Code points for Type and the associated Length con-
straints are defined below.
EOF TLV
Allows to mark the end of TLV space (hence of the file) explic-
itly 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.
Type is 0, Length is 0, Value is empty.
LinkTypeValue TLV
Allows to record the link-layer header type value used for the
compilation, usually this is either the linktype input argument
to pcap_open_dead(3PCAP) or the dlt input argument to
pcap_set_datalink(3PCAP). By convention link-layer header type
values are limited to 16 bits.
Type is 1, Length is 2, Value contains an integer.
LinkTypeName TLV
Allows to record the input argument to
pcap_datalink_name_to_val(3PCAP) if the latter was used to
translate a DLT name into the DLT value (the same name can
sometimes produce different values in different contexts).
Type is 2, Length is variable, Value contains an ASCII string.
SnapLen TLV
Allows to record the snapshot length used for the compilation,
usually this is the snaplen input argument to pcap_open_dead()
or pcap_set_snaplen(3PCAP).
Type is 3, Length is 4, Value contains an integer.
Filter TLV
Allows to record the filter expression that was compiled into
the bytecode, usually this is the str input argument to
pcap_compile(3PCAP).
Type is 4, Length is variable, Value contains an ASCII string.
OptReq TLV
Allows to record whether optimization was requested for the
compilation or not, usually this is the optimize input argument
to pcap_compile(). Note that some link-layer header types and
filter keywords disable the optimization automatically in libp-
cap.
Type is 5, Length is 1, Value contains 1 or 0.
Netmask TLV
Allows to record the value of netmask input argument to
pcap_compile().
Type is 6, Length is 4, Value contains a 32-bit IPv4 netmask.
Comment TLV
Allows to record a free-form text, for example, the name and
version of the program that generated the file.
Type is 7, Length is variable, Value contains a UTF-8 string.
Timestamp TLV
Allows to record when the compilation was performed.
Type is 8, Length is 8, Value contains a 64-bit Unix timestamp.
SOFTWARE SUPPORT
None at the time of this writing.
SEE ALSO
pcap-savefile(5)
24 June 2022 CBPF-SAVEFILE(5)
--
Denis Ovsienko
--- End Message ---
_______________________________________________
tcpdump-workers mailing list
tcpdump-workers@lists.tcpdump.org
https://lists.sandelman.ca/mailman/listinfo/tcpdump-workers