Manpage of PACKET

PACKET

Section: Linux Programmer's Manual (7)
Updated: 2015-05-07
Index
 

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_typeis either SOCK_RAWfor raw packets including the link-level header or SOCK_DGRAMfor cooked packets with the link-level header removed. The link-level header information is available in a common format in a sockaddr_llstructure. protocolis 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.

Only processes with the CAP_NET_RAWcapability may open packet sockets.

SOCK_RAWpackets 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_lladdress 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_RAWis similar to but not compatible with the obsolete AF_INET/SOCK_PACKETof Linux 2.0.

SOCK_DGRAMoperates on a slightly higher level. The physical header is removed before the packet is passed to the user. Packets sent through a SOCK_DGRAMpacket socket get a suitable physical-layer header based on the information in the sockaddr_lldestination 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_llto 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_TRUNCflag 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_llstructure 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_protocolis 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_ifindexis the interface index of the interface (see netdevice(7)); 0 matches any interface (only permitted for binding). sll_hatypeis an ARP type as defined in the <linux/if_arp.h>include file.
*
sll_pkttypecontains the packet type. Valid types are PACKET_HOSTfor a packet addressed to the local host, PACKET_BROADCASTfor a physical-layer broadcast packet, PACKET_MULTICASTfor a packet sent to a physical-layer multicast address, PACKET_OTHERHOSTfor a packet to some other host that has been caught by a device driver in promiscuous mode, and PACKET_OUTGOINGfor a packet originating from the local host that is looped back to a packet socket. These types make sense only for receiving.
*
sll_addrand sll_halencontain 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_hatypeand sll_pkttypeare 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_MEMBERSHIPadds a binding and PACKET_DROP_MEMBERSHIPdrops it. They both expect a packet_mreqstructure 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_ifindexcontains the interface index for the interface whose status should be changed. The mr_typefield specifies which action to perform. PACKET_MR_PROMISCenables receiving all packets on a shared medium (often known as "promiscuous mode"), PACKET_MR_MULTICASTbinds the socket to the physical-layer multicast group specified in mr_addressand mr_alen, and PACKET_MR_ALLMULTIsets the socket up to receive all multicast packets arriving at the interface.

In addition, the traditional ioctls SIOCSIFFLAGS, SIOCADDMULTI, SIOCDELMULTIcan 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_padding;
};
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_PACKETand 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_LBimplements a round-robin algorithm.
*
PACKET_FANOUT_CPUselects the socket based on the CPU that the packet arrived on.
*
PACKET_FANOUT_ROLLOVERprocesses all data on a single socket, moving to the next when one becomes backlogged.
*
PACKET_FANOUT_RNDselects 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_ROLLOVERenables 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_statusto TP_STATUS_WRONG_FORMATand 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_statusreset to TP_STATUS_SEND_REQUEST, and the transmission process restarted via send(2). However, if PACKET_LOSSis set, any malformed packet will be skipped, its tp_statusreset 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_netstores the offset to the network layer. If the packet socket is of type SOCK_DGRAM, then tp_macis 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_statusfield. The packet socket owns all slots with tp_statusequal 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_statusvalue has at least the TP_STATUS_USERbit 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_statusequal to TP_STATUS_KERNEL.

Packet sockets implement multiple variants of the packet ring. The implementation details are described in Documentation/networking/packet_mmap.txtin 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.txtin 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_RINGand takes the same arguments. The application writes packets into slots with tp_statusequal to TP_STATUS_AVAILABLEand schedules them for transmission by changing tp_statusto TP_STATUS_SEND_REQUEST. When packets are ready to be transmitted, the application calls send(2) or a variant thereof. The bufand lenfields 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_statusto TP_STATUS_AVAILABLE. It immediately aborts the transmission on error unless PACKET_LOSSis set.
PACKET_VERSION (with PACKET_RX_RING; since Linux 2.6.27)
By default, PACKET_RX_RINGcreates 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

SIOCGSTAMPcan be used to receive the timestamp of the last received packet. Argument is a struct timevalvariable.

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

 

NOTES

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

The SOCK_DGRAMpacket sockets make no attempt to create or parse the IEEE 802.2 LLC header for a IEEE 802.3 frame. When ETH_P_802_3is 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_2with the LLC header prefixed. It is thus not possible to bind to ETH_P_802_3; bind to ETH_P_802_2instead 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_PACKETuses the old struct sockaddr_pktto 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_familycontains the device type, spkt_protocolis the IEEE 802.3 protocol type as defined in <sys/if_ether.h>and spkt_deviceis the device name as a null-terminated string, for example, eth0.

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

BUGS

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

Socket filters are not documented.

The MSG_TRUNCrecvmsg(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.  

SEE ALSO

socket(2), pcap(3), capabilities(7), ip(7), raw(7), socket(7)

RFC 894 for the standard IP Ethernet encapsulation. RFC 1700 for the IEEE 802.3 IP encapsulation.

The <linux/if_ether.h>include file for physical-layer protocols.

The Linux kernel source tree. /Documentation/networking/filter.txtdescribes how to apply Berkeley Packet Filters to packet sockets. /tools/testing/selftests/net/psock_tpacket.ccontains example source code for all available versions of PACKET_RX_RINGand PACKET_TX_RING.


 

Index

NAME
SYNOPSIS
DESCRIPTION
Address types
Socket options
Ioctls
Error handling
ERRORS
VERSIONS
NOTES
Compatibility
BUGS
SEE ALSO

This document was created by man2html, using the manual pages.
Time: 22:28:03 GMT, June 20, 2016