docs: networking: convert packet_mmap.txt to ReST

This patch has a big diff, but most are due to whitespaces.

Yet, the conversion is similar to other files under networking:

- add SPDX header;
- add a document title;
- adjust titles and chapters, adding proper markups;
- mark lists as such;
- mark tables as such;
- mark code blocks and literals as such;
- adjust identation, whitespaces and blank lines where needed;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 1084 insertions, 0 deletions

diff --git a/Documentation/networking/packet_mmap.rst b/Documentation/networking/packet_mmap.rst
new file mode 100644
index 000000000000..5f213d17652f
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+++ b/Documentation/networking/packet_mmap.rst
@@ -0,0 +1,1084 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===========
+Packet MMAP
+===========
+
+Abstract
+========
+
+This file documents the mmap() facility available with the PACKET
+socket interface on 2.4/2.6/3.x kernels. This type of sockets is used for
+
+i) capture network traffic with utilities like tcpdump,
+ii) transmit network traffic, or any other that needs raw
+    access to network interface.
+
+Howto can be found at:
+
+    https://sites.google.com/site/packetmmap/
+
+Please send your comments to
+    - Ulisses Alonso Camaró <uaca@i.hate.spam.alumni.uv.es>
+    - Johann Baudy
+
+Why use PACKET_MMAP
+===================
+
+In Linux 2.4/2.6/3.x 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 that can be used to either
+send or receive packets. This way reading packets just needs to wait for them,
+most of the time there is no need to issue a single system call. Concerning
+transmission, multiple packets can be sent through one system call to get the
+highest bandwidth. 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 and
+transmission 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. For transmission, check the MTU (Maximum Transmission Unit) used and
+supported by devices of your network. CPU IRQ pinning of your network interface
+card can also be an advantage.
+
+How to use mmap() to improve capture process
+============================================
+
+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.
+
+Packet MMAP support was integrated into libpcap around the time of version 1.3.0;
+TPACKET_V3 support was added in version 1.5.0
+
+How to use mmap() directly to improve capture process
+=====================================================
+
+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)
+				  option: PACKET_RX_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 = 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).
+
+Similarly as without PACKET_MMAP, it is possible to use one socket
+for capture and transmission. This can be done by mapping the
+allocated RX and TX buffer ring with a single mmap() call.
+See "Mapping and use of the circular buffer (ring)".
+
+Next I will describe PACKET_MMAP settings and its constraints,
+also the mapping of the circular buffer in the user process and
+the use of this buffer.
+
+How to use mmap() directly to improve transmission process
+==========================================================
+Transmission process is similar to capture as shown below::
+
+    [setup]         socket() -------> creation of the transmission socket
+		    setsockopt() ---> allocation of the circular buffer (ring)
+				      option: PACKET_TX_RING
+		    bind() ---------> bind transmission socket with a network interface
+		    mmap() ---------> mapping of the allocated buffer to the
+				      user process
+
+    [transmission]  poll() ---------> wait for free packets (optional)
+		    send() ---------> send all packets that are set as ready in
+				      the ring
+				      The flag MSG_DONTWAIT can be used to return
+				      before end of transfer.
+
+    [shutdown]      close() --------> destruction of the transmission socket and
+				      deallocation of all associated resources.
+
+Socket creation and destruction is also straight forward, and is done
+the same way as in capturing described in the previous paragraph::
+
+ int fd = socket(PF_PACKET, mode, 0);
+
+The protocol can optionally be 0 in case we only want to transmit
+via this socket, which avoids an expensive call to packet_rcv().
+In this case, you also need to bind(2) the TX_RING with sll_protocol = 0
+set. Otherwise, htons(ETH_P_ALL) or any other protocol, for example.
+
+Binding the socket to your network interface is mandatory (with zero copy) to
+know the header size of frames used in the circular buffer.
+
+As capture, each frame contains two parts::
+
+    --------------------
+    | struct tpacket_hdr | Header. It contains the status of
+    |                    | of this frame
+    |--------------------|
+    | data buffer        |
+    .                    .  Data that will be sent over the network interface.
+    .                    .
+    --------------------
+
+ bind() associates the socket to your network interface thanks to
+ sll_ifindex parameter of struct sockaddr_ll.
+
+ Initialization example::
+
+    struct sockaddr_ll my_addr;
+    struct ifreq s_ifr;
+    ...
+
+    strncpy (s_ifr.ifr_name, "eth0", sizeof(s_ifr.ifr_name));
+
+    /* get interface index of eth0 */
+    ioctl(this->socket, SIOCGIFINDEX, &s_ifr);
+
+    /* fill sockaddr_ll struct to prepare binding */
+    my_addr.sll_family = AF_PACKET;
+    my_addr.sll_protocol = htons(ETH_P_ALL);
+    my_addr.sll_ifindex =  s_ifr.ifr_ifindex;
+
+    /* bind socket to eth0 */
+    bind(this->socket, (struct sockaddr *)&my_addr, sizeof(struct sockaddr_ll));
+
+ A complete tutorial is available at: https://sites.google.com/site/packetmmap/
+
+By default, the user should put data at::
+
+ frame base + TPACKET_HDRLEN - sizeof(struct sockaddr_ll)
+
+So, whatever you choose for the socket mode (SOCK_DGRAM or SOCK_RAW),
+the beginning of the user data will be at::
+
+ frame base + TPACKET_ALIGN(sizeof(struct tpacket_hdr))
+
+If you wish to put user data at a custom offset from the beginning of
+the frame (for payload alignment with SOCK_RAW mode for instance) you
+can set tp_net (with SOCK_DGRAM) or tp_mac (with SOCK_RAW). In order
+to make this work it must be enabled previously with setsockopt()
+and the PACKET_TX_HAS_OFF option.
+
+PACKET_MMAP settings
+====================
+
+To setup PACKET_MMAP from user level code is done with a call like
+
+ - Capture process::
+
+     setsockopt(fd, SOL_PACKET, PACKET_RX_RING, (void *) &req, sizeof(req))
+
+ - Transmission process::
+
+     setsockopt(fd, SOL_PACKET, PACKET_TX_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.
+Being mapped in the capture process allows reading the captured frames and
+related meta-information like timestamps without requiring a system call.
+
+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 across 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
+contiguously 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.
+
+To use one socket for capture and transmission, the mapping of both the
+RX and TX buffer ring has to be done with one call to mmap::
+
+    ...
+    setsockopt(fd, SOL_PACKET, PACKET_RX_RING, &foo, sizeof(foo));
+    setsockopt(fd, SOL_PACKET, PACKET_TX_RING, &bar, sizeof(bar));
+    ...
+    rx_ring = mmap(0, size * 2, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
+    tx_ring = rx_ring + size;
+
+RX must be the first as the kernel maps the TX ring memory right
+after the RX one.
+
+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:
+
+Capture process
+^^^^^^^^^^^^^^^
+
+     from include/linux/if_packet.h
+
+     #define TP_STATUS_COPY          (1 << 1)
+     #define TP_STATUS_LOSING        (1 << 2)
+     #define TP_STATUS_CSUMNOTREADY  (1 << 3)
+     #define TP_STATUS_CSUM_VALID    (1 << 7)
+
+======================  =======================================================
+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 that 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
+			its checksum will be done in hardware. So while
+			reading the packet we should not try to check the
+			checksum.
+
+TP_STATUS_CSUM_VALID	This flag indicates that at least the transport
+			header checksum of the packet has been already
+			validated on the kernel side. If the flag is not set
+			then we are free to check the checksum by ourselves
+			provided that TP_STATUS_CSUMNOTREADY is also not set.
+======================  =======================================================
+
+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.
+
+Transmission process
+^^^^^^^^^^^^^^^^^^^^
+
+Those defines are also used for transmission::
+
+     #define TP_STATUS_AVAILABLE        0 // Frame is available
+     #define TP_STATUS_SEND_REQUEST     1 // Frame will be sent on next send()
+     #define TP_STATUS_SENDING          2 // Frame is currently in transmission
+     #define TP_STATUS_WRONG_FORMAT     4 // Frame format is not correct
+
+First, the kernel initializes all frames to TP_STATUS_AVAILABLE. To send a
+packet, the user fills a data buffer of an available frame, sets tp_len to
+current data buffer size and sets its status field to TP_STATUS_SEND_REQUEST.
+This can be done on multiple frames. Once the user is ready to transmit, it
+calls send(). Then all buffers with status equal to TP_STATUS_SEND_REQUEST are
+forwarded to the network device. The kernel updates each status of sent
+frames with TP_STATUS_SENDING until the end of transfer.
+
+At the end of each transfer, buffer status returns to TP_STATUS_AVAILABLE.
+
+::
+
+    header->tp_len = in_i_size;
+    header->tp_status = TP_STATUS_SEND_REQUEST;
+    retval = send(this->socket, NULL, 0, 0);
+
+The user can also use poll() to check if a buffer is available:
+
+(status == TP_STATUS_SENDING)
+
+::
+
+    struct pollfd pfd;
+    pfd.fd = fd;
+    pfd.revents = 0;
+    pfd.events = POLLOUT;
+    retval = poll(&pfd, 1, timeout);
+
+What TPACKET versions are available and when to use them?
+=========================================================
+
+::
+
+ int val = tpacket_version;
+ setsockopt(fd, SOL_PACKET, PACKET_VERSION, &val, sizeof(val));
+ getsockopt(fd, SOL_PACKET, PACKET_VERSION, &val, sizeof(val));
+
+where 'tpacket_version' can be TPACKET_V1 (default), TPACKET_V2, TPACKET_V3.
+
+TPACKET_V1:
+	- Default if not otherwise specified by setsockopt(2)
+	- RX_RING, TX_RING available
+
+TPACKET_V1 --> TPACKET_V2:
+	- Made 64 bit clean due to unsigned long usage in TPACKET_V1
+	  structures, thus this also works on 64 bit kernel with 32 bit
+	  userspace and the like
+	- Timestamp resolution in nanoseconds instead of microseconds
+	- RX_RING, TX_RING available
+	- VLAN metadata information available for packets
+	  (TP_STATUS_VLAN_VALID, TP_STATUS_VLAN_TPID_VALID),
+	  in the tpacket2_hdr structure:
+
+		- TP_STATUS_VLAN_VALID bit being set into the tp_status field indicates
+		  that the tp_vlan_tci field has valid VLAN TCI value
+		- TP_STATUS_VLAN_TPID_VALID bit being set into the tp_status field
+		  indicates that the tp_vlan_tpid field has valid VLAN TPID value
+
+	- How to switch to TPACKET_V2:
+
+		1. Replace struct tpacket_hdr by struct tpacket2_hdr
+		2. Query header len and save
+		3. Set protocol version to 2, set up ring as usual
+		4. For getting the sockaddr_ll,
+		   use ``(void *)hdr + TPACKET_ALIGN(hdrlen)`` instead of
+		   ``(void *)hdr + TPACKET_ALIGN(sizeof(struct tpacket_hdr))``
+
+TPACKET_V2 --> TPACKET_V3:
+	- Flexible buffer implementation for RX_RING:
+		1. Blocks can be configured with non-static frame-size
+		2. Read/poll is at a block-level (as opposed to packet-level)
+		3. Added poll timeout to avoid indefinite user-space wait
+		   on idle links
+		4. Added user-configurable knobs:
+
+			4.1 block::timeout
+			4.2 tpkt_hdr::sk_rxhash
+
+	- RX Hash data available in user space
+	- TX_RING semantics are conceptually similar to TPACKET_V2;
+	  use tpacket3_hdr instead of tpacket2_hdr, and TPACKET3_HDRLEN
+	  instead of TPACKET2_HDRLEN. In the current implementation,
+	  the tp_next_offset field in the tpacket3_hdr MUST be set to
+	  zero, indicating that the ring does not hold variable sized frames.
+	  Packets with non-zero values of tp_next_offset will be dropped.
+
+AF_PACKET fanout mode
+=====================
+
+In the AF_PACKET fanout mode, packet reception can be load balanced among
+processes. This also works in combination with mmap(2) on packet sockets.
+
+Currently implemented fanout policies are:
+
+  - PACKET_FANOUT_HASH: schedule to socket by skb's packet hash
+  - PACKET_FANOUT_LB: schedule to socket by round-robin
+  - PACKET_FANOUT_CPU: schedule to socket by CPU packet arrives on
+  - PACKET_FANOUT_RND: schedule to socket by random selection
+  - PACKET_FANOUT_ROLLOVER: if one socket is full, rollover to another
+  - PACKET_FANOUT_QM: schedule to socket by skbs recorded queue_mapping
+
+Minimal example code by David S. Miller (try things like "./test eth0 hash",
+"./test eth0 lb", etc.)::
+
+    #include <stddef.h>
+    #include <stdlib.h>
+    #include <stdio.h>
+    #include <string.h>
+
+    #include <sys/types.h>
+    #include <sys/wait.h>
+    #include <sys/socket.h>
+    #include <sys/ioctl.h>
+
+    #include <unistd.h>
+
+    #include <linux/if_ether.h>
+    #include <linux/if_packet.h>
+
+    #include <net/if.h>
+
+    static const char *device_name;
+    static int fanout_type;
+    static int fanout_id;
+
+    #ifndef PACKET_FANOUT
+    # define PACKET_FANOUT			18
+    # define PACKET_FANOUT_HASH		0
+    # define PACKET_FANOUT_LB		1
+    #endif
+
+    static int setup_socket(void)
+    {
+	    int err, fd = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_IP));
+	    struct sockaddr_ll ll;
+	    struct ifreq ifr;
+	    int fanout_arg;
+
+	    if (fd < 0) {
+		    perror("socket");
+		    return EXIT_FAILURE;
+	    }
+
+	    memset(&ifr, 0, sizeof(ifr));
+	    strcpy(ifr.ifr_name, device_name);
+	    err = ioctl(fd, SIOCGIFINDEX, &ifr);
+	    if (err < 0) {
+		    perror("SIOCGIFINDEX");
+		    return EXIT_FAILURE;
+	    }
+
+	    memset(&ll, 0, sizeof(ll));
+	    ll.sll_family = AF_PACKET;
+	    ll.sll_ifindex = ifr.ifr_ifindex;
+	    err = bind(fd, (struct sockaddr *) &ll, sizeof(ll));
+	    if (err < 0) {
+		    perror("bind");
+		    return EXIT_FAILURE;
+	    }
+
+	    fanout_arg = (fanout_id | (fanout_type << 16));
+	    err = setsockopt(fd, SOL_PACKET, PACKET_FANOUT,
+			    &fanout_arg, sizeof(fanout_arg));
+	    if (err) {
+		    perror("setsockopt");
+		    return EXIT_FAILURE;
+	    }
+
+	    return fd;
+    }
+
+    static void fanout_thread(void)
+    {
+	    int fd = setup_socket();
+	    int limit = 10000;
+
+	    if (fd < 0)
+		    exit(fd);
+
+	    while (limit-- > 0) {
+		    char buf[1600];
+		    int err;
+
+		    err = read(fd, buf, sizeof(buf));
+		    if (err < 0) {
+			    perror("read");
+			    exit(EXIT_FAILURE);
+		    }
+		    if ((limit % 10) == 0)
+			    fprintf(stdout, "(%d) \n", getpid());
+	    }
+
+	    fprintf(stdout, "%d: Received 10000 packets\n", getpid());
+
+	    close(fd);
+	    exit(0);
+    }
+
+    int main(int argc, char **argp)
+    {
+	    int fd, err;
+	    int i;
+
+	    if (argc != 3) {
+		    fprintf(stderr, "Usage: %s INTERFACE {hash|lb}\n", argp[0]);
+		    return EXIT_FAILURE;
+	    }
+
+	    if (!strcmp(argp[2], "hash"))
+		    fanout_type = PACKET_FANOUT_HASH;
+	    else if (!strcmp(argp[2], "lb"))
+		    fanout_type = PACKET_FANOUT_LB;
+	    else {
+		    fprintf(stderr, "Unknown fanout type [%s]\n", argp[2]);
+		    exit(EXIT_FAILURE);
+	    }
+
+	    device_name = argp[1];
+	    fanout_id = getpid() & 0xffff;
+
+	    for (i = 0; i < 4; i++) {
+		    pid_t pid = fork();
+
+		    switch (pid) {
+		    case 0:
+			    fanout_thread();
+
+		    case -1:
+			    perror("fork");
+			    exit(EXIT_FAILURE);
+		    }
+	    }
+
+	    for (i = 0; i < 4; i++) {
+		    int status;
+
+		    wait(&status);
+	    }
+
+	    return 0;
+    }
+
+AF_PACKET TPACKET_V3 example
+============================
+
+AF_PACKET's TPACKET_V3 ring buffer can be configured to use non-static frame
+sizes by doing it's own memory management. It is based on blocks where polling
+works on a per block basis instead of per ring as in TPACKET_V2 and predecessor.
+
+It is said that TPACKET_V3 brings the following benefits:
+
+ * ~15% - 20% reduction in CPU-usage
+ * ~20% increase in packet capture rate
+ * ~2x increase in packet density
+ * Port aggregation analysis
+ * Non static frame size to capture entire packet payload
+
+So it seems to be a good candidate to be used with packet fanout.
+
+Minimal example code by Daniel Borkmann based on Chetan Loke's lolpcap (compile
+it with gcc -Wall -O2 blob.c, and try things like "./a.out eth0", etc.)::
+
+    /* Written from scratch, but kernel-to-user space API usage
+    * dissected from lolpcap:
+    *  Copyright 2011, Chetan Loke <loke.chetan@gmail.com>
+    *  License: GPL, version 2.0
+    */
+
+    #include <stdio.h>
+    #include <stdlib.h>
+    #include <stdint.h>
+    #include <string.h>
+    #include <assert.h>
+    #include <net/if.h>
+    #include <arpa/inet.h>
+    #include <netdb.h>
+    #include <poll.h>
+    #include <unistd.h>
+    #include <signal.h>
+    #include <inttypes.h>
+    #include <sys/socket.h>
+    #include <sys/mman.h>
+    #include <linux/if_packet.h>
+    #include <linux/if_ether.h>
+    #include <linux/ip.h>
+
+    #ifndef likely
+    # define likely(x)		__builtin_expect(!!(x), 1)
+    #endif
+    #ifndef unlikely
+    # define unlikely(x)		__builtin_expect(!!(x), 0)
+    #endif
+
+    struct block_desc {
+	    uint32_t version;
+	    uint32_t offset_to_priv;
+	    struct tpacket_hdr_v1 h1;
+    };
+
+    struct ring {
+	    struct iovec *rd;
+	    uint8_t *map;
+	    struct tpacket_req3 req;
+    };
+
+    static unsigned long packets_total = 0, bytes_total = 0;
+    static sig_atomic_t sigint = 0;
+
+    static void sighandler(int num)
+    {
+	    sigint = 1;
+    }
+
+    static int setup_socket(struct ring *ring, char *netdev)
+    {
+	    int err, i, fd, v = TPACKET_V3;
+	    struct sockaddr_ll ll;
+	    unsigned int blocksiz = 1 << 22, framesiz = 1 << 11;
+	    unsigned int blocknum = 64;
+
+	    fd = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
+	    if (fd < 0) {
+		    perror("socket");
+		    exit(1);
+	    }
+
+	    err = setsockopt(fd, SOL_PACKET, PACKET_VERSION, &v, sizeof(v));
+	    if (err < 0) {
+		    perror("setsockopt");
+		    exit(1);
+	    }
+
+	    memset(&ring->req, 0, sizeof(ring->req));
+	    ring->req.tp_block_size = blocksiz;
+	    ring->req.tp_frame_size = framesiz;
+	    ring->req.tp_block_nr = blocknum;
+	    ring->req.tp_frame_nr = (blocksiz * blocknum) / framesiz;
+	    ring->req.tp_retire_blk_tov = 60;
+	    ring->req.tp_feature_req_word = TP_FT_REQ_FILL_RXHASH;
+
+	    err = setsockopt(fd, SOL_PACKET, PACKET_RX_RING, &ring->req,
+			    sizeof(ring->req));
+	    if (err < 0) {
+		    perror("setsockopt");
+		    exit(1);
+	    }
+
+	    ring->map = mmap(NULL, ring->req.tp_block_size * ring->req.tp_block_nr,
+			    PROT_READ | PROT_WRITE, MAP_SHARED | MAP_LOCKED, fd, 0);
+	    if (ring->map == MAP_FAILED) {
+		    perror("mmap");
+		    exit(1);
+	    }
+
+	    ring->rd = malloc(ring->req.tp_block_nr * sizeof(*ring->rd));
+	    assert(ring->rd);
+	    for (i = 0; i < ring->req.tp_block_nr; ++i) {
+		    ring->rd[i].iov_base = ring->map + (i * ring->req.tp_block_size);
+		    ring->rd[i].iov_len = ring->req.tp_block_size;
+	    }
+
+	    memset(&ll, 0, sizeof(ll));
+	    ll.sll_family = PF_PACKET;
+	    ll.sll_protocol = htons(ETH_P_ALL);
+	    ll.sll_ifindex = if_nametoindex(netdev);
+	    ll.sll_hatype = 0;
+	    ll.sll_pkttype = 0;
+	    ll.sll_halen = 0;
+
+	    err = bind(fd, (struct sockaddr *) &ll, sizeof(ll));
+	    if (err < 0) {
+		    perror("bind");
+		    exit(1);
+	    }
+
+	    return fd;
+    }
+
+    static void display(struct tpacket3_hdr *ppd)
+    {
+	    struct ethhdr *eth = (struct ethhdr *) ((uint8_t *) ppd + ppd->tp_mac);
+	    struct iphdr *ip = (struct iphdr *) ((uint8_t *) eth + ETH_HLEN);
+
+	    if (eth->h_proto == htons(ETH_P_IP)) {
+		    struct sockaddr_in ss, sd;
+		    char sbuff[NI_MAXHOST], dbuff[NI_MAXHOST];
+
+		    memset(&ss, 0, sizeof(ss));
+		    ss.sin_family = PF_INET;
+		    ss.sin_addr.s_addr = ip->saddr;
+		    getnameinfo((struct sockaddr *) &ss, sizeof(ss),
+				sbuff, sizeof(sbuff), NULL, 0, NI_NUMERICHOST);
+
+		    memset(&sd, 0, sizeof(sd));
+		    sd.sin_family = PF_INET;
+		    sd.sin_addr.s_addr = ip->daddr;
+		    getnameinfo((struct sockaddr *) &sd, sizeof(sd),
+				dbuff, sizeof(dbuff), NULL, 0, NI_NUMERICHOST);
+
+		    printf("%s -> %s, ", sbuff, dbuff);
+	    }
+
+	    printf("rxhash: 0x%x\n", ppd->hv1.tp_rxhash);
+    }
+
+    static void walk_block(struct block_desc *pbd, const int block_num)
+    {
+	    int num_pkts = pbd->h1.num_pkts, i;
+	    unsigned long bytes = 0;
+	    struct tpacket3_hdr *ppd;
+
+	    ppd = (struct tpacket3_hdr *) ((uint8_t *) pbd +
+					pbd->h1.offset_to_first_pkt);
+	    for (i = 0; i < num_pkts; ++i) {
+		    bytes += ppd->tp_snaplen;
+		    display(ppd);
+
+		    ppd = (struct tpacket3_hdr *) ((uint8_t *) ppd +
+						ppd->tp_next_offset);
+	    }
+
+	    packets_total += num_pkts;
+	    bytes_total += bytes;
+    }
+
+    static void flush_block(struct block_desc *pbd)
+    {
+	    pbd->h1.block_status = TP_STATUS_KERNEL;
+    }
+
+    static void teardown_socket(struct ring *ring, int fd)
+    {
+	    munmap(ring->map, ring->req.tp_block_size * ring->req.tp_block_nr);
+	    free(ring->rd);
+	    close(fd);
+    }
+
+    int main(int argc, char **argp)
+    {
+	    int fd, err;
+	    socklen_t len;
+	    struct ring ring;
+	    struct pollfd pfd;
+	    unsigned int block_num = 0, blocks = 64;
+	    struct block_desc *pbd;
+	    struct tpacket_stats_v3 stats;
+
+	    if (argc != 2) {
+		    fprintf(stderr, "Usage: %s INTERFACE\n", argp[0]);
+		    return EXIT_FAILURE;
+	    }
+
+	    signal(SIGINT, sighandler);
+
+	    memset(&ring, 0, sizeof(ring));
+	    fd = setup_socket(&ring, argp[argc - 1]);
+	    assert(fd > 0);
+
+	    memset(&pfd, 0, sizeof(pfd));
+	    pfd.fd = fd;
+	    pfd.events = POLLIN | POLLERR;
+	    pfd.revents = 0;
+
+	    while (likely(!sigint)) {
+		    pbd = (struct block_desc *) ring.rd[block_num].iov_base;
+
+		    if ((pbd->h1.block_status & TP_STATUS_USER) == 0) {
+			    poll(&pfd, 1, -1);
+			    continue;
+		    }
+
+		    walk_block(pbd, block_num);
+		    flush_block(pbd);
+		    block_num = (block_num + 1) % blocks;
+	    }
+
+	    len = sizeof(stats);
+	    err = getsockopt(fd, SOL_PACKET, PACKET_STATISTICS, &stats, &len);
+	    if (err < 0) {
+		    perror("getsockopt");
+		    exit(1);
+	    }
+
+	    fflush(stdout);
+	    printf("\nReceived %u packets, %lu bytes, %u dropped, freeze_q_cnt: %u\n",
+		stats.tp_packets, bytes_total, stats.tp_drops,
+		stats.tp_freeze_q_cnt);
+
+	    teardown_socket(&ring, fd);
+	    return 0;
+    }
+
+PACKET_QDISC_BYPASS
+===================
+
+If there is a requirement to load the network with many packets in a similar
+fashion as pktgen does, you might set the following option after socket
+creation::
+
+    int one = 1;
+    setsockopt(fd, SOL_PACKET, PACKET_QDISC_BYPASS, &one, sizeof(one));
+
+This has the side-effect, that packets sent through PF_PACKET will bypass the
+kernel's qdisc layer and are forcedly pushed to the driver directly. Meaning,
+packet are not buffered, tc disciplines are ignored, increased loss can occur
+and such packets are also not visible to other PF_PACKET sockets anymore. So,
+you have been warned; generally, this can be useful for stress testing various
+components of a system.
+
+On default, PACKET_QDISC_BYPASS is disabled and needs to be explicitly enabled
+on PF_PACKET sockets.
+
+PACKET_TIMESTAMP
+================
+
+The PACKET_TIMESTAMP setting determines the source of the timestamp in
+the packet meta information for mmap(2)ed RX_RING and TX_RINGs.  If your
+NIC is capable of timestamping packets in hardware, you can request those
+hardware timestamps to be used. Note: you may need to enable the generation
+of hardware timestamps with SIOCSHWTSTAMP (see related information from
+Documentation/networking/timestamping.txt).
+
+PACKET_TIMESTAMP accepts the same integer bit field as SO_TIMESTAMPING::
+
+    int req = SOF_TIMESTAMPING_RAW_HARDWARE;
+    setsockopt(fd, SOL_PACKET, PACKET_TIMESTAMP, (void *) &req, sizeof(req))
+
+For the mmap(2)ed ring buffers, such timestamps are stored in the
+``tpacket{,2,3}_hdr`` structure's tp_sec and ``tp_{n,u}sec`` members.
+To determine what kind of timestamp has been reported, the tp_status field
+is binary or'ed with the following possible bits ...
+
+::
+
+    TP_STATUS_TS_RAW_HARDWARE
+    TP_STATUS_TS_SOFTWARE
+
+... that are equivalent to its ``SOF_TIMESTAMPING_*`` counterparts. For the
+RX_RING, if neither is set (i.e. PACKET_TIMESTAMP is not set), then a
+software fallback was invoked *within* PF_PACKET's processing code (less
+precise).
+
+Getting timestamps for the TX_RING works as follows: i) fill the ring frames,
+ii) call sendto() e.g. in blocking mode, iii) wait for status of relevant
+frames to be updated resp. the frame handed over to the application, iv) walk
+through the frames to pick up the individual hw/sw timestamps.
+
+Only (!) if transmit timestamping is enabled, then these bits are combined
+with binary | with TP_STATUS_AVAILABLE, so you must check for that in your
+application (e.g. !(tp_status & (TP_STATUS_SEND_REQUEST | TP_STATUS_SENDING))
+in a first step to see if the frame belongs to the application, and then
+one can extract the type of timestamp in a second step from tp_status)!
+
+If you don't care about them, thus having it disabled, checking for
+TP_STATUS_AVAILABLE resp. TP_STATUS_WRONG_FORMAT is sufficient. If in the
+TX_RING part only TP_STATUS_AVAILABLE is set, then the tp_sec and tp_{n,u}sec
+members do not contain a valid value. For TX_RINGs, by default no timestamp
+is generated!
+
+See include/linux/net_tstamp.h and Documentation/networking/timestamping.txt
+for more information on hardware timestamps.
+
+Miscellaneous bits
+==================
+
+- Packet sockets work well together with Linux socket filters, thus you also
+  might want to have a look at Documentation/networking/filter.txt
+
+THANKS
+======
+
+   Jesse Brandeburg, for fixing my grammathical/spelling errors