#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);
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. 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.
Only processes with effective UID 0 or the CAP_NET_RAW capability may open packet sockets.
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. Only the sll_protocol and the sll_ifindex address fields are used for purposes of binding.
The connect(2) operation is not supported on packet sockets.
When the MSG_TRUNC flag is passed to recvmsg(2), recv(2), recvfrom(2) the real length of the packet on the wire is always returned, even when it is longer than the buffer.
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 */
};
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 originated from the local host that is looped back to a packet socket. These types make sense only for receiving. sll_addr and 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. The other fields should be 0. sll_hatype and sll_pkttype are set on received packets for your information. For bind only sll_protocol and sll_ifindex are used.
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 parameter 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.
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;
};
Fanout supports multiple algorithms to spread traffic between sockets. 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, moves 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 packet 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.
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.
In addition, all standard ioctls defined in netdevice(7) and socket(7) are valid on packet sockets.
In addition, other errors may be generated by the low-level driver.
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.
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.
#ifndef SOL_PACKET #define SOL_PACKET 263 #endifThis is fixed in later glibc versions.
The IEEE 802.2/803.3 LLC handling could be considered as a bug.
Socket filters are not documented.
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.
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.txt describes how to apply Berkeley Packet Filters to packet sockets. /tools/testing/selftests/net/psock_tpacket.c contains example source code for all available versions of PACKET_RX_RING and PACKET_TX_RING.