.\" This man page is Copyright (C) 1999 Andi Kleen . .\" Permission is granted to distribute possibly modified copies .\" of this page provided the header is included verbatim, .\" and in case of nontrivial modification author and date .\" of the modification is added to the header. .\" $Id: packet.7,v 1.13 2000/08/14 08:03:45 ak Exp $ .TH PACKET 7 1999-04-29 "Linux Man Page" "Linux Programmer's Manual" .SH NAME packet, PF_PACKET \- packet interface on device level. .SH SYNOPSIS .nf .B #include .br .B #include .br .B #include /* the L2 protocols */ .sp .BI "packet_socket = socket(PF_PACKET, int " socket_type ", int "protocol ); .fi .SH 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 .I socket_type is either .B SOCK_RAW for raw packets including the link level header or .B SOCK_DGRAM for cooked packets with the link level header removed. The link level header information is available in a common format in a .BR sockaddr_ll . .I protocol is the IEEE 802.3 protocol number in network order. See the .I include file for a list of allowed protocols. When protocol is set to .B 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 .B CAP_NET_RAW capability may open packet sockets. .B 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 .B 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. .B SOCK_RAW is similar to but not compatible with the obsolete .B PF_INET/SOCK_PACKET of Linux 2.0. .B 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 .B SOCK_DGRAM packet socket get a suitable physical layer header based on the information in the .B sockaddr_ll destination address before they are queued. By default all packets of the specified protocol type are passed to a packet socket. To only get packets from a specific interface use .BR bind (2) specifying an address in a .I struct sockaddr_ll to bind the packet socket to an interface. Only the .B sll_protocol and the .B sll_ifindex address fields are used for purposes of binding. The .BR connect (2) operation is not supported on packet sockets. When the .B MSG_TRUNC flag is passed to .BR recvmsg (2), .BR recv (2), .BR recvfrom (2) the real length of the packet on the wire is always returned, even when it is longer than the buffer. .SH "ADDRESS TYPES" The sockaddr_ll is a device independent physical layer address. .in +0.25i .nf 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; /* Header type */ unsigned char sll_pkttype; /* Packet type */ unsigned char sll_halen; /* Length of address */ unsigned char sll_addr[8]; /* Physical layer address */ }; .fi .in -0.25i .B sll_protocol is the standard ethernet protocol type in network order as defined in the .I include file. It defaults to the socket's protocol. .B sll_ifindex is the interface index of the interface (see .BR netdevice (7)); 0 matches any interface (only legal for binding). .B sll_hatype is a ARP type as defined in the .I include file. .B sll_pkttype contains the packet type. Valid types are .B PACKET_HOST for a packet addressed to the local host, .B PACKET_BROADCAST for a physical layer broadcast packet, .B PACKET_MULTICAST for a packet sent to a physical layer multicast address, .B PACKET_OTHERHOST for a packet to some other host that has been caught by a device driver in promiscuous mode, and .B PACKET_OUTGOING for a packet originated from the local host that is looped back to a packet socket. These types make only sense for receiving. .B sll_addr and .B 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 .BR sll_family , .BR sll_addr , .BR sll_halen , .BR sll_ifindex . The other fields should be 0. .B sll_hatype and .B sll_pkttype are set on received packets for your information. For bind only .B sll_protocol and .B sll_ifindex are used. .SH "SOCKET OPTIONS" Packet sockets can be used to configure physical layer multicasting and promiscuous mode. It works by calling .BR setsockopt (2) on a packet socket for SOL_PACKET and one of the options .B PACKET_ADD_MEMBERSHIP to add a binding or .B PACKET_DROP_MEMBERSHIP to drop it. They both expect a .B packet_mreq structure as argument: .in +0.25i .nf 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 */ }; .fi .in -0.25i .B mr_ifindex contains the interface index for the interface whose status should be changed. The .B mr_type parameter specifies which action to perform. .B PACKET_MR_PROMISC enables receiving all packets on a shared medium (often known as ``promiscuous mode''), .B PACKET_MR_MULTICAST binds the socket to the physical layer multicast group specified in .B mr_address and .BR mr_alen , and .B PACKET_MR_ALLMULTI sets the socket up to receive all multicast packets arriving at the interface. In addition the traditional ioctls .BR SIOCSIFFLAGS , .BR SIOCADDMULTI , .B SIOCDELMULTI can be used for the same purpose. .SH IOCTLS .B SIOCGSTAMP can be used to receive the time stamp of the last received packet. Argument is a .I struct timeval. In addition all standard ioctls defined in .BR netdevice (7) and .BR socket (7) are valid on packet sockets. .SH "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. .SH COMPATIBILITY In Linux 2.0, the only way to get a packet socket was by calling .BI "socket(PF_INET, SOCK_PACKET, " protocol )\fR. This is still supported but strongly deprecated. The main difference between the two methods is that .B SOCK_PACKET uses the old .I struct sockaddr_pkt to specify an interface, which doesn't provide physical layer independence. .in +0.25i .nf struct sockaddr_pkt { unsigned short spkt_family; unsigned char spkt_device[14]; unsigned short spkt_protocol; }; .fi .in -0.25i .B spkt_family contains the device type, .B spkt_protocol is the IEEE 802.3 protocol type as defined in .I and .B spkt_device is the device name as a null terminated string, e.g. eth0. This structure is obsolete and should not be used in new code. .SH NOTES For portable programs it is suggested to use .B PF_PACKET via .BR pcap (3); although this only covers a subset of the .B PF_PACKET features. The .B SOCK_DGRAM packet sockets make no attempt to create or parse the IEEE 802.2 LLC header for a IEEE 802.3 frame. When .B 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 .B ETH_P_802_2 with the LLC header prepended. It is thus not possible to bind to .BR ETH_P_802_3 ; bind to .B 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. .SH ERRORS .TP .B ENETDOWN Interface is not up. .TP .B ENOTCONN No interface address passed. .TP .B ENODEV Unknown device name or interface index specified in interface address. .TP .B EMSGSIZE Packet is bigger than interface MTU. .TP .B ENOBUFS Not enough memory to allocate the packet. .TP .B EFAULT User passed invalid memory address. .TP .B EINVAL Invalid argument. .TP .B ENXIO Interface address contained illegal interface index. .TP .B EPERM User has insufficient privileges to carry out this operation. .TP .B EADDRNOTAVAIL Unknown multicast group address passed. .TP .B ENOENT No packet received. In addition other errors may be generated by the low-level driver. .SH VERSIONS .B PF_PACKET is a new feature in Linux 2.2. Earlier Linux versions supported only .BR SOCK_PACKET . .SH BUGS glibc 2.1 does not have a define for .BR SOL_PACKET . The suggested workaround is to use: .in +0.5i .nf #ifndef SOL_PACKET #define SOL_PACKET 263 #endif .fi .in -0.5i This is fixed in later glibc versions and also does not occur on libc5 systems. The IEEE 802.2/803.3 LLC handling could be considered as a bug. Socket filters are not documented. The .I MSG_TRUNC .BR recvmsg () 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. .SH HISTORICAL NOTE The include file .I is present since glibc2.1. Older systems need .sp .nf .B #include .br .B #include .br .B #include /* The L2 protocols */ .br .fi .\" .SH CREDITS .\" This man page was written by Andi Kleen with help from Matthew Wilcox. .\" PF_PACKET in Linux 2.2 was implemented .\" by Alexey Kuznetsov, based on code by Alan Cox and others. .SH "SEE ALSO" .BR socket (2), .BR pcap (3), .BR capabilities (7), .BR ip (7), .BR raw (7), .BR socket (7) RFC\ 894 for the standard IP Ethernet encapsulation. RFC\ 1700 for the IEEE 802.3 IP encapsulation. The .I include file for physical layer protocols.