400 строки
15 KiB
Plaintext
400 строки
15 KiB
Plaintext
--------------------------------------------------------------------------------
|
|
+ ABSTRACT
|
|
--------------------------------------------------------------------------------
|
|
|
|
This file documents the CONFIG_PACKET_MMAP option available with the PACKET
|
|
socket interface on 2.4 and 2.6 kernels. This type of sockets is used for
|
|
capture network traffic with utilities like tcpdump or any other that uses
|
|
the libpcap library.
|
|
|
|
You can find the latest version of this document at
|
|
|
|
http://pusa.uv.es/~ulisses/packet_mmap/
|
|
|
|
Please send me your comments to
|
|
|
|
Ulisses Alonso Camaró <uaca@i.hate.spam.alumni.uv.es>
|
|
|
|
-------------------------------------------------------------------------------
|
|
+ Why use PACKET_MMAP
|
|
--------------------------------------------------------------------------------
|
|
|
|
In Linux 2.4/2.6 if PACKET_MMAP is not enabled, the capture process is very
|
|
inefficient. It uses very limited buffers and requires one system call
|
|
to capture each packet, it requires two if you want to get packet's
|
|
timestamp (like libpcap always does).
|
|
|
|
In the other hand PACKET_MMAP is very efficient. PACKET_MMAP provides a size
|
|
configurable circular buffer mapped in user space. This way reading packets just
|
|
needs to wait for them, most of the time there is no need to issue a single
|
|
system call. By using a shared buffer between the kernel and the user
|
|
also has the benefit of minimizing packet copies.
|
|
|
|
It's fine to use PACKET_MMAP to improve the performance of the capture process,
|
|
but it isn't everything. At least, if you are capturing at high speeds (this
|
|
is relative to the cpu speed), you should check if the device driver of your
|
|
network interface card supports some sort of interrupt load mitigation or
|
|
(even better) if it supports NAPI, also make sure it is enabled.
|
|
|
|
--------------------------------------------------------------------------------
|
|
+ How to use CONFIG_PACKET_MMAP
|
|
--------------------------------------------------------------------------------
|
|
|
|
From the user standpoint, you should use the higher level libpcap library, which
|
|
is a de facto standard, portable across nearly all operating systems
|
|
including Win32.
|
|
|
|
Said that, at time of this writing, official libpcap 0.8.1 is out and doesn't include
|
|
support for PACKET_MMAP, and also probably the libpcap included in your distribution.
|
|
|
|
I'm aware of two implementations of PACKET_MMAP in libpcap:
|
|
|
|
http://pusa.uv.es/~ulisses/packet_mmap/ (by Simon Patarin, based on libpcap 0.6.2)
|
|
http://public.lanl.gov/cpw/ (by Phil Wood, based on lastest libpcap)
|
|
|
|
The rest of this document is intended for people who want to understand
|
|
the low level details or want to improve libpcap by including PACKET_MMAP
|
|
support.
|
|
|
|
--------------------------------------------------------------------------------
|
|
+ How to use CONFIG_PACKET_MMAP directly
|
|
--------------------------------------------------------------------------------
|
|
|
|
From the system calls stand point, the use of PACKET_MMAP involves
|
|
the following process:
|
|
|
|
|
|
[setup] socket() -------> creation of the capture socket
|
|
setsockopt() ---> allocation of the circular buffer (ring)
|
|
mmap() ---------> mapping of the allocated buffer to the
|
|
user process
|
|
|
|
[capture] poll() ---------> to wait for incoming packets
|
|
|
|
[shutdown] close() --------> destruction of the capture socket and
|
|
deallocation of all associated
|
|
resources.
|
|
|
|
|
|
socket creation and destruction is straight forward, and is done
|
|
the same way with or without PACKET_MMAP:
|
|
|
|
int fd;
|
|
|
|
fd= socket(PF_PACKET, mode, htons(ETH_P_ALL))
|
|
|
|
where mode is SOCK_RAW for the raw interface were link level
|
|
information can be captured or SOCK_DGRAM for the cooked
|
|
interface where link level information capture is not
|
|
supported and a link level pseudo-header is provided
|
|
by the kernel.
|
|
|
|
The destruction of the socket and all associated resources
|
|
is done by a simple call to close(fd).
|
|
|
|
Next I will describe PACKET_MMAP settings and it's constraints,
|
|
also the mapping of the circular buffer in the user process and
|
|
the use of this buffer.
|
|
|
|
--------------------------------------------------------------------------------
|
|
+ PACKET_MMAP settings
|
|
--------------------------------------------------------------------------------
|
|
|
|
|
|
To setup PACKET_MMAP from user level code is done with a call like
|
|
|
|
setsockopt(fd, SOL_PACKET, PACKET_RX_RING, (void *) &req, sizeof(req))
|
|
|
|
The most significant argument in the previous call is the req parameter,
|
|
this parameter must to have the following structure:
|
|
|
|
struct tpacket_req
|
|
{
|
|
unsigned int tp_block_size; /* Minimal size of contiguous block */
|
|
unsigned int tp_block_nr; /* Number of blocks */
|
|
unsigned int tp_frame_size; /* Size of frame */
|
|
unsigned int tp_frame_nr; /* Total number of frames */
|
|
};
|
|
|
|
This structure is defined in /usr/include/linux/if_packet.h and establishes a
|
|
circular buffer (ring) of unswappable memory mapped in the capture process.
|
|
Being mapped in the capture process allows reading the captured frames and
|
|
related meta-information like timestamps without requiring a system call.
|
|
|
|
Captured frames are grouped in blocks. Each block is a physically contiguous
|
|
region of memory and holds tp_block_size/tp_frame_size frames. The total number
|
|
of blocks is tp_block_nr. Note that tp_frame_nr is a redundant parameter because
|
|
|
|
frames_per_block = tp_block_size/tp_frame_size
|
|
|
|
indeed, packet_set_ring checks that the following condition is true
|
|
|
|
frames_per_block * tp_block_nr == tp_frame_nr
|
|
|
|
|
|
Lets see an example, with the following values:
|
|
|
|
tp_block_size= 4096
|
|
tp_frame_size= 2048
|
|
tp_block_nr = 4
|
|
tp_frame_nr = 8
|
|
|
|
we will get the following buffer structure:
|
|
|
|
block #1 block #2
|
|
+---------+---------+ +---------+---------+
|
|
| frame 1 | frame 2 | | frame 3 | frame 4 |
|
|
+---------+---------+ +---------+---------+
|
|
|
|
block #3 block #4
|
|
+---------+---------+ +---------+---------+
|
|
| frame 5 | frame 6 | | frame 7 | frame 8 |
|
|
+---------+---------+ +---------+---------+
|
|
|
|
A frame can be of any size with the only condition it can fit in a block. A block
|
|
can only hold an integer number of frames, or in other words, a frame cannot
|
|
be spawned accross two blocks, so there are some details you have to take into
|
|
account when choosing the frame_size. See "Mapping and use of the circular
|
|
buffer (ring)".
|
|
|
|
|
|
--------------------------------------------------------------------------------
|
|
+ PACKET_MMAP setting constraints
|
|
--------------------------------------------------------------------------------
|
|
|
|
In kernel versions prior to 2.4.26 (for the 2.4 branch) and 2.6.5 (2.6 branch),
|
|
the PACKET_MMAP buffer could hold only 32768 frames in a 32 bit architecture or
|
|
16384 in a 64 bit architecture. For information on these kernel versions
|
|
see http://pusa.uv.es/~ulisses/packet_mmap/packet_mmap.pre-2.4.26_2.6.5.txt
|
|
|
|
Block size limit
|
|
------------------
|
|
|
|
As stated earlier, each block is a contiguous physical region of memory. These
|
|
memory regions are allocated with calls to the __get_free_pages() function. As
|
|
the name indicates, this function allocates pages of memory, and the second
|
|
argument is "order" or a power of two number of pages, that is
|
|
(for PAGE_SIZE == 4096) order=0 ==> 4096 bytes, order=1 ==> 8192 bytes,
|
|
order=2 ==> 16384 bytes, etc. The maximum size of a
|
|
region allocated by __get_free_pages is determined by the MAX_ORDER macro. More
|
|
precisely the limit can be calculated as:
|
|
|
|
PAGE_SIZE << MAX_ORDER
|
|
|
|
In a i386 architecture PAGE_SIZE is 4096 bytes
|
|
In a 2.4/i386 kernel MAX_ORDER is 10
|
|
In a 2.6/i386 kernel MAX_ORDER is 11
|
|
|
|
So get_free_pages can allocate as much as 4MB or 8MB in a 2.4/2.6 kernel
|
|
respectively, with an i386 architecture.
|
|
|
|
User space programs can include /usr/include/sys/user.h and
|
|
/usr/include/linux/mmzone.h to get PAGE_SIZE MAX_ORDER declarations.
|
|
|
|
The pagesize can also be determined dynamically with the getpagesize (2)
|
|
system call.
|
|
|
|
|
|
Block number limit
|
|
--------------------
|
|
|
|
To understand the constraints of PACKET_MMAP, we have to see the structure
|
|
used to hold the pointers to each block.
|
|
|
|
Currently, this structure is a dynamically allocated vector with kmalloc
|
|
called pg_vec, its size limits the number of blocks that can be allocated.
|
|
|
|
+---+---+---+---+
|
|
| x | x | x | x |
|
|
+---+---+---+---+
|
|
| | | |
|
|
| | | v
|
|
| | v block #4
|
|
| v block #3
|
|
v block #2
|
|
block #1
|
|
|
|
|
|
kmalloc allocates any number of bytes of physically contiguous memory from
|
|
a pool of pre-determined sizes. This pool of memory is maintained by the slab
|
|
allocator which is at the end the responsible for doing the allocation and
|
|
hence which imposes the maximum memory that kmalloc can allocate.
|
|
|
|
In a 2.4/2.6 kernel and the i386 architecture, the limit is 131072 bytes. The
|
|
predetermined sizes that kmalloc uses can be checked in the "size-<bytes>"
|
|
entries of /proc/slabinfo
|
|
|
|
In a 32 bit architecture, pointers are 4 bytes long, so the total number of
|
|
pointers to blocks is
|
|
|
|
131072/4 = 32768 blocks
|
|
|
|
|
|
PACKET_MMAP buffer size calculator
|
|
------------------------------------
|
|
|
|
Definitions:
|
|
|
|
<size-max> : is the maximum size of allocable with kmalloc (see /proc/slabinfo)
|
|
<pointer size>: depends on the architecture -- sizeof(void *)
|
|
<page size> : depends on the architecture -- PAGE_SIZE or getpagesize (2)
|
|
<max-order> : is the value defined with MAX_ORDER
|
|
<frame size> : it's an upper bound of frame's capture size (more on this later)
|
|
|
|
from these definitions we will derive
|
|
|
|
<block number> = <size-max>/<pointer size>
|
|
<block size> = <pagesize> << <max-order>
|
|
|
|
so, the max buffer size is
|
|
|
|
<block number> * <block size>
|
|
|
|
and, the number of frames be
|
|
|
|
<block number> * <block size> / <frame size>
|
|
|
|
Suppose the following parameters, which apply for 2.6 kernel and an
|
|
i386 architecture:
|
|
|
|
<size-max> = 131072 bytes
|
|
<pointer size> = 4 bytes
|
|
<pagesize> = 4096 bytes
|
|
<max-order> = 11
|
|
|
|
and a value for <frame size> of 2048 bytes. These parameters will yield
|
|
|
|
<block number> = 131072/4 = 32768 blocks
|
|
<block size> = 4096 << 11 = 8 MiB.
|
|
|
|
and hence the buffer will have a 262144 MiB size. So it can hold
|
|
262144 MiB / 2048 bytes = 134217728 frames
|
|
|
|
|
|
Actually, this buffer size is not possible with an i386 architecture.
|
|
Remember that the memory is allocated in kernel space, in the case of
|
|
an i386 kernel's memory size is limited to 1GiB.
|
|
|
|
All memory allocations are not freed until the socket is closed. The memory
|
|
allocations are done with GFP_KERNEL priority, this basically means that
|
|
the allocation can wait and swap other process' memory in order to allocate
|
|
the necessary memory, so normally limits can be reached.
|
|
|
|
Other constraints
|
|
-------------------
|
|
|
|
If you check the source code you will see that what I draw here as a frame
|
|
is not only the link level frame. At the beginning of each frame there is a
|
|
header called struct tpacket_hdr used in PACKET_MMAP to hold link level's frame
|
|
meta information like timestamp. So what we draw here a frame it's really
|
|
the following (from include/linux/if_packet.h):
|
|
|
|
/*
|
|
Frame structure:
|
|
|
|
- Start. Frame must be aligned to TPACKET_ALIGNMENT=16
|
|
- struct tpacket_hdr
|
|
- pad to TPACKET_ALIGNMENT=16
|
|
- struct sockaddr_ll
|
|
- Gap, chosen so that packet data (Start+tp_net) aligns to
|
|
TPACKET_ALIGNMENT=16
|
|
- Start+tp_mac: [ Optional MAC header ]
|
|
- Start+tp_net: Packet data, aligned to TPACKET_ALIGNMENT=16.
|
|
- Pad to align to TPACKET_ALIGNMENT=16
|
|
*/
|
|
|
|
|
|
The following are conditions that are checked in packet_set_ring
|
|
|
|
tp_block_size must be a multiple of PAGE_SIZE (1)
|
|
tp_frame_size must be greater than TPACKET_HDRLEN (obvious)
|
|
tp_frame_size must be a multiple of TPACKET_ALIGNMENT
|
|
tp_frame_nr must be exactly frames_per_block*tp_block_nr
|
|
|
|
Note that tp_block_size should be chosen to be a power of two or there will
|
|
be a waste of memory.
|
|
|
|
--------------------------------------------------------------------------------
|
|
+ Mapping and use of the circular buffer (ring)
|
|
--------------------------------------------------------------------------------
|
|
|
|
The mapping of the buffer in the user process is done with the conventional
|
|
mmap function. Even the circular buffer is compound of several physically
|
|
discontiguous blocks of memory, they are contiguous to the user space, hence
|
|
just one call to mmap is needed:
|
|
|
|
mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
|
|
|
|
If tp_frame_size is a divisor of tp_block_size frames will be
|
|
contiguosly spaced by tp_frame_size bytes. If not, each
|
|
tp_block_size/tp_frame_size frames there will be a gap between
|
|
the frames. This is because a frame cannot be spawn across two
|
|
blocks.
|
|
|
|
At the beginning of each frame there is an status field (see
|
|
struct tpacket_hdr). If this field is 0 means that the frame is ready
|
|
to be used for the kernel, If not, there is a frame the user can read
|
|
and the following flags apply:
|
|
|
|
from include/linux/if_packet.h
|
|
|
|
#define TP_STATUS_COPY 2
|
|
#define TP_STATUS_LOSING 4
|
|
#define TP_STATUS_CSUMNOTREADY 8
|
|
|
|
|
|
TP_STATUS_COPY : This flag indicates that the frame (and associated
|
|
meta information) has been truncated because it's
|
|
larger than tp_frame_size. This packet can be
|
|
read entirely with recvfrom().
|
|
|
|
In order to make this work it must to be
|
|
enabled previously with setsockopt() and
|
|
the PACKET_COPY_THRESH option.
|
|
|
|
The number of frames than can be buffered to
|
|
be read with recvfrom is limited like a normal socket.
|
|
See the SO_RCVBUF option in the socket (7) man page.
|
|
|
|
TP_STATUS_LOSING : indicates there were packet drops from last time
|
|
statistics where checked with getsockopt() and
|
|
the PACKET_STATISTICS option.
|
|
|
|
TP_STATUS_CSUMNOTREADY: currently it's used for outgoing IP packets which
|
|
it's checksum will be done in hardware. So while
|
|
reading the packet we should not try to check the
|
|
checksum.
|
|
|
|
for convenience there are also the following defines:
|
|
|
|
#define TP_STATUS_KERNEL 0
|
|
#define TP_STATUS_USER 1
|
|
|
|
The kernel initializes all frames to TP_STATUS_KERNEL, when the kernel
|
|
receives a packet it puts in the buffer and updates the status with
|
|
at least the TP_STATUS_USER flag. Then the user can read the packet,
|
|
once the packet is read the user must zero the status field, so the kernel
|
|
can use again that frame buffer.
|
|
|
|
The user can use poll (any other variant should apply too) to check if new
|
|
packets are in the ring:
|
|
|
|
struct pollfd pfd;
|
|
|
|
pfd.fd = fd;
|
|
pfd.revents = 0;
|
|
pfd.events = POLLIN|POLLRDNORM|POLLERR;
|
|
|
|
if (status == TP_STATUS_KERNEL)
|
|
retval = poll(&pfd, 1, timeout);
|
|
|
|
It doesn't incur in a race condition to first check the status value and
|
|
then poll for frames.
|
|
|
|
--------------------------------------------------------------------------------
|
|
+ THANKS
|
|
--------------------------------------------------------------------------------
|
|
|
|
Jesse Brandeburg, for fixing my grammathical/spelling errors
|
|
|