Ubuntu Manpage: packet - packet interface on device level

NAME

       packet - packet interface on device level

SYNOPSIS

       #include <sys/socket.h>
       #include <linux/if_packet.h>
       #include <net/ethernet.h> /* the L2 protocols */

       packet_socket = socket(AF_PACKET, int socket_type, int protocol);

DESCRIPTION

Packet sockets are used to receive or send raw packets at the device driver (OSI Layer 2)
level. They allow the user to implement protocol modules in user space on top of the
physical layer.

The socket_type is either SOCK_RAW for raw packets including the link-level header or
SOCK_DGRAM for cooked packets with the link-level header removed. The link-level header
information is available in a common format in a sockaddr_ll structure. protocol is the
IEEE 802.3 protocol number in network byte order. See the <linux/if_ether.h> include file
for a list of allowed protocols. When protocol is set to htons(ETH_P_ALL), then all
protocols are received. All incoming packets of that protocol type will be passed to the
packet socket before they are passed to the protocols implemented in the kernel. If
protocol is set to zero, no packets are received. bind(2) can optionally be called with a
nonzero sll_protocol to start receiving packets for the protocols specified.

In order to create a packet socket, a process must have the CAP_NET_RAW capability in the
user namespace that governs its network namespace.

SOCK_RAW packets are passed to and from the device driver without any changes in the
packet data. When receiving a packet, the address is still parsed and passed in a
standard sockaddr_ll address structure. When transmitting a packet, the user-supplied
buffer should contain the physical-layer header. That packet is then queued unmodified to
the network driver of the interface defined by the destination address. Some device
drivers always add other headers. SOCK_RAW is similar to but not compatible with the
obsolete AF_INET/SOCK_PACKET of Linux 2.0.

SOCK_DGRAM operates on a slightly higher level. The physical header is removed before the
packet is passed to the user. Packets sent through a SOCK_DGRAM packet socket get a
suitable physical-layer header based on the information in the sockaddr_ll destination
address before they are queued.

By default, all packets of the specified protocol type are passed to a packet socket. To
get packets only from a specific interface use bind(2) specifying an address in a struct
sockaddr_ll to bind the packet socket to an interface. Fields used for binding are
sll_family (should be AF_PACKET), sll_protocol, and sll_ifindex.

The connect(2) operation is not supported on packet sockets.

When the MSG_TRUNC flag is passed to recvmsg(2), recv(2), or recvfrom(2), the real length
of the packet on the wire is always returned, even when it is longer than the buffer.

Address types
The sockaddr_ll structure is a device-independent physical-layer address.

struct sockaddr_ll {
unsigned short sll_family; /* Always AF_PACKET */
unsigned short sll_protocol; /* Physical-layer protocol */
int sll_ifindex; /* Interface number */
unsigned short sll_hatype; /* ARP hardware type */
unsigned char sll_pkttype; /* Packet type */
unsigned char sll_halen; /* Length of address */
unsigned char sll_addr[8]; /* Physical-layer address */
};

The fields of this structure are as follows:

sll_protocol
is the standard ethernet protocol type in network byte order as defined in the
<linux/if_ether.h> include file. It defaults to the socket's protocol.

sll_ifindex
is the interface index of the interface (see netdevice(7)); 0 matches any interface
(only permitted for binding). sll_hatype is an ARP type as defined in the
<linux/if_arp.h> include file.

sll_pkttype
contains the packet type. Valid types are PACKET_HOST for a packet addressed to
the local host, PACKET_BROADCAST for a physical-layer broadcast packet,
PACKET_MULTICAST for a packet sent to a physical-layer multicast address,
PACKET_OTHERHOST for a packet to some other host that has been caught by a device
driver in promiscuous mode, and PACKET_OUTGOING for a packet originating from the
local host that is looped back to a packet socket. These types make sense only for
receiving.

sll_addr
sll_halen
contain the physical-layer (e.g., IEEE 802.3) address and its length. The exact
interpretation depends on the device.

When you send packets, it is enough to specify sll_family, sll_addr, sll_halen,
sll_ifindex, and sll_protocol. The other fields should be 0. sll_hatype and sll_pkttype
are set on received packets for your information.

Socket options
Packet socket options are configured by calling setsockopt(2) with level SOL_PACKET.

PACKET_ADD_MEMBERSHIP
PACKET_DROP_MEMBERSHIP
Packet sockets can be used to configure physical-layer multicasting and promiscuous
mode. PACKET_ADD_MEMBERSHIP adds a binding and PACKET_DROP_MEMBERSHIP drops it.
They both expect a packet_mreq structure as argument:

struct packet_mreq {
int mr_ifindex; /* interface index */
unsigned short mr_type; /* action */
unsigned short mr_alen; /* address length */
unsigned char mr_address[8]; /* physical-layer address */
};

mr_ifindex contains the interface index for the interface whose status should be
changed. The mr_type field specifies which action to perform. PACKET_MR_PROMISC
enables receiving all packets on a shared medium (often known as "promiscuous
mode"), PACKET_MR_MULTICAST binds the socket to the physical-layer multicast group
specified in mr_address and mr_alen, and PACKET_MR_ALLMULTI sets the socket up to
receive all multicast packets arriving at the interface.

In addition, the traditional ioctls SIOCSIFFLAGS, SIOCADDMULTI, SIOCDELMULTI can be
used for the same purpose.

PACKET_AUXDATA (since Linux 2.6.21)
If this binary option is enabled, the packet socket passes a metadata structure
along with each packet in the recvmsg(2) control field. The structure can be read
with cmsg(3). It is defined as

struct tpacket_auxdata {
__u32 tp_status;
__u32 tp_len; /* packet length */
__u32 tp_snaplen; /* captured length */
__u16 tp_mac;
__u16 tp_net;
__u16 tp_vlan_tci;
__u16 tp_vlan_tpid; /* Since Linux 3.14; earlier, these
were unused padding bytes */
};

PACKET_FANOUT (since Linux 3.1)
To scale processing across threads, packet sockets can form a fanout group. In
this mode, each matching packet is enqueued onto only one socket in the group. A
socket joins a fanout group by calling setsockopt(2) with level SOL_PACKET and
option PACKET_FANOUT. Each network namespace can have up to 65536 independent
groups. A socket selects a group by encoding the ID in the first 16 bits of the
integer option value. The first packet socket to join a group implicitly creates
it. To successfully join an existing group, subsequent packet sockets must have
the same protocol, device settings, fanout mode, and flags (see below). Packet
sockets can leave a fanout group only by closing the socket. The group is deleted
when the last socket is closed.

Fanout supports multiple algorithms to spread traffic between sockets, as follows:

• The default mode, PACKET_FANOUT_HASH, sends packets from the same flow to the
same socket to maintain per-flow ordering. For each packet, it chooses a socket
by taking the packet flow hash modulo the number of sockets in the group, where
a flow hash is a hash over network-layer address and optional transport-layer
port fields.

• The load-balance mode PACKET_FANOUT_LB implements a round-robin algorithm.

• PACKET_FANOUT_CPU selects the socket based on the CPU that the packet arrived
on.

• PACKET_FANOUT_ROLLOVER processes all data on a single socket, moving to the next
when one becomes backlogged.

• PACKET_FANOUT_RND selects the socket using a pseudo-random number generator.

• PACKET_FANOUT_QM (available since Linux 3.14) selects the socket using the
recorded queue_mapping of the received skb.

Fanout modes can take additional options. IP fragmentation causes packets from the
same flow to have different flow hashes. The flag PACKET_FANOUT_FLAG_DEFRAG, if
set, causes packets to be defragmented before fanout is applied, to preserve order
even in this case. Fanout mode and options are communicated in the second 16 bits
of the integer option value. The flag PACKET_FANOUT_FLAG_ROLLOVER enables the roll
over mechanism as a backup strategy: if the original fanout algorithm selects a
backlogged socket, the packet rolls over to the next available one.

PACKET_LOSS (with PACKET_TX_RING)
When a malformed packet is encountered on a transmit ring, the default is to reset
its tp_status to TP_STATUS_WRONG_FORMAT and abort the transmission immediately.
The malformed packet blocks itself and subsequently enqueued packets from being
sent. The format error must be fixed, the associated tp_status reset to
TP_STATUS_SEND_REQUEST, and the transmission process restarted via send(2).
However, if PACKET_LOSS is set, any malformed packet will be skipped, its tp_status
reset to TP_STATUS_AVAILABLE, and the transmission process continued.

PACKET_RESERVE (with PACKET_RX_RING)
By default, a packet receive ring writes packets immediately following the metadata
structure and alignment padding. This integer option reserves additional headroom.

PACKET_RX_RING
Create a memory-mapped ring buffer for asynchronous packet reception. The packet
socket reserves a contiguous region of application address space, lays it out into
an array of packet slots and copies packets (up to tp_snaplen) into subsequent
slots. Each packet is preceded by a metadata structure similar to tpacket_auxdata.
The protocol fields encode the offset to the data from the start of the metadata
header. tp_net stores the offset to the network layer. If the packet socket is of
type SOCK_DGRAM, then tp_mac is the same. If it is of type SOCK_RAW, then that
field stores the offset to the link-layer frame. Packet socket and application
communicate the head and tail of the ring through the tp_status field. The packet
socket owns all slots with tp_status equal to TP_STATUS_KERNEL. After filling a
slot, it changes the status of the slot to transfer ownership to the application.
During normal operation, the new tp_status value has at least the TP_STATUS_USER
bit set to signal that a received packet has been stored. When the application has
finished processing a packet, it transfers ownership of the slot back to the socket
by setting tp_status equal to TP_STATUS_KERNEL.

Packet sockets implement multiple variants of the packet ring. The implementation
details are described in Documentation/networking/packet_mmap.rst in the Linux
kernel source tree.

PACKET_STATISTICS
Retrieve packet socket statistics in the form of a structure

struct tpacket_stats {
unsigned int tp_packets; /* Total packet count */
unsigned int tp_drops; /* Dropped packet count */
};

Receiving statistics resets the internal counters. The statistics structure
differs when using a ring of variant TPACKET_V3.

PACKET_TIMESTAMP (with PACKET_RX_RING; since Linux 2.6.36)
The packet receive ring always stores a timestamp in the metadata header. By
default, this is a software generated timestamp generated when the packet is copied
into the ring. This integer option selects the type of timestamp. Besides the
default, it support the two hardware formats described in
Documentation/networking/timestamping.rst in the Linux kernel source tree.

PACKET_TX_RING (since Linux 2.6.31)
Create a memory-mapped ring buffer for packet transmission. This option is similar
to PACKET_RX_RING and takes the same arguments. The application writes packets
into slots with tp_status equal to TP_STATUS_AVAILABLE and schedules them for
transmission by changing tp_status to TP_STATUS_SEND_REQUEST. When packets are
ready to be transmitted, the application calls send(2) or a variant thereof. The
buf and len fields of this call are ignored. If an address is passed using
sendto(2) or sendmsg(2), then that overrides the socket default. On successful
transmission, the socket resets tp_status to TP_STATUS_AVAILABLE. It immediately
aborts the transmission on error unless PACKET_LOSS is set.

PACKET_VERSION (with PACKET_RX_RING; since Linux 2.6.27)
By default, PACKET_RX_RING creates a packet receive ring of variant TPACKET_V1. To
create another variant, configure the desired variant by setting this integer
option before creating the ring.

PACKET_QDISC_BYPASS (since Linux 3.14)
By default, packets sent through packet sockets pass through the kernel's qdisc
(traffic control) layer, which is fine for the vast majority of use cases. For
traffic generator appliances using packet sockets that intend to brute-force flood
the network—for example, to test devices under load in a similar fashion to pktgen—
this layer can be bypassed by setting this integer option to 1. A side effect is
that packet buffering in the qdisc layer is avoided, which will lead to increased
drops when network device transmit queues are busy; therefore, use at your own
risk.

Ioctls
SIOCGSTAMP can be used to receive the timestamp of the last received packet. Argument is
a struct timeval variable.

In addition, all standard ioctls defined in netdevice(7) and socket(7) are valid on packet
sockets.

Error handling
Packet sockets do no error handling other than errors occurred while passing the packet to
the device driver. They don't have the concept of a pending error.

ERRORS

       EADDRNOTAVAIL
              Unknown multicast group address passed.

       EFAULT User passed invalid memory address.

       EINVAL Invalid argument.

       EMSGSIZE
              Packet is bigger than interface MTU.

       ENETDOWN
              Interface is not up.

       ENOBUFS
              Not enough memory to allocate the packet.

       ENODEV Unknown device name or interface index specified in interface address.

       ENOENT No packet received.

       ENOTCONN
              No interface address passed.

       ENXIO  Interface address contained an invalid interface index.

       EPERM  User has insufficient privileges to carry out this operation.

       In addition, other errors may be generated by the low-level driver.

VERSIONS

       AF_PACKET  is  a  new  feature  in  Linux  2.2.   Earlier  Linux  versions  supported only
       SOCK_PACKET.

NOTES

       For portable programs it is suggested to use AF_PACKET via pcap(3); although  this  covers
       only a subset of the AF_PACKET features.

       The SOCK_DGRAM packet sockets make no attempt to create or parse the IEEE 802.2 LLC header
       for a IEEE 802.3 frame.  When ETH_P_802_3 is specified as protocol for sending the  kernel
       creates  the  802.3  frame  and fills out the length field; the user has to supply the LLC
       header to get a fully conforming packet.  Incoming 802.3 packets are  not  multiplexed  on
       the  DSAP/SSAP  protocol  fields;  instead  they  are  supplied  to  the  user as protocol
       ETH_P_802_2 with the LLC header prefixed.  It is thus not possible to bind to ETH_P_802_3;
       bind  to  ETH_P_802_2  instead  and  do  the protocol multiplex yourself.  The default for
       sending is the standard Ethernet DIX encapsulation with the protocol filled in.

       Packet sockets are not subject to the input or output firewall chains.

   Compatibility
       In Linux 2.0, the only way to get a packet socket was with the call:

           socket(AF_INET, SOCK_PACKET, protocol)

       This is still supported, but deprecated and strongly  discouraged.   The  main  difference
       between the two methods is that SOCK_PACKET uses the old struct sockaddr_pkt to specify an
       interface, which doesn't provide physical-layer independence.

           struct sockaddr_pkt {
               unsigned short spkt_family;
               unsigned char  spkt_device[14];
               unsigned short spkt_protocol;
           };

       spkt_family contains the device type, spkt_protocol is the IEEE  802.3  protocol  type  as
       defined  in  <sys/if_ether.h>  and  spkt_device  is  the  device name as a null-terminated
       string, for example, eth0.

       This structure is obsolete and should not be used in new code.

BUGS

   LLC header handling
       The IEEE 802.2/803.3 LLC handling could be considered as a bug.

   MSG_TRUNC issues
       The MSG_TRUNC recvmsg(2) extension is an ugly hack and should be  replaced  by  a  control
       message.  There is currently no way to get the original destination address of packets via
       SOCK_DGRAM.

   spkt_device device name truncation
       The spkt_device field of sockaddr_pkt has a size of 14  bytes,  which  is  less  than  the
       constant  IFNAMSIZ  defined in <net/if.h> which is 16 bytes and describes the system limit
       for a network interface name.  This means the names of  network  devices  longer  than  14
       bytes  will  be  truncated  to  fit  into  spkt_device.   All  these  lengths  include the
       terminating null byte ('\0')).

       Issues from this with old code typically show up with very long interface  names  used  by
       the  Predictable  Network  Interface Names feature enabled by default in many modern Linux
       distributions.

       The preferred solution is to rewrite code to avoid SOCK_PACKET.  Possible  user  solutions
       are to disable Predictable Network Interface Names or to rename the interface to a name of
       at most 13 bytes, for example using the ip(8) tool.

   Documentation issues
       Socket filters are not documented.