path: root/man7/packet.7

.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
.\"
.\" %%%LICENSE_START(VERBATIM_ONE_PARA)
.\" Permission is granted to distribute possibly modified copies
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.\" and in case of nontrivial modification author and date
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.\" $Id: packet.7,v 1.13 2000/08/14 08:03:45 ak Exp $
.\"
.TH PACKET  7 2012-05-10 "Linux" "Linux Programmer's Manual"
.SH NAME
packet \- packet interface on device level.
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.br
.B #include <netpacket/packet.h>
.br
.B #include <net/ethernet.h>     /* the L2 protocols */
.sp
.BI "packet_socket = socket(AF_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
.IR sockaddr_ll .
.I protocol
is the IEEE 802.3 protocol number in network order.
See the
.I <linux/if_ether.h>
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
.I 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 AF_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
.I 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
.BR bind (2)
specifying an address in a
.I struct sockaddr_ll
to bind the packet socket to an interface.
Only the
.I sll_protocol
and the
.I 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.
.SS Address types
The sockaddr_ll is a device independent physical layer address.

.in +4n
.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;   /* ARP hardware type */
    unsigned char  sll_pkttype;  /* Packet type */
    unsigned char  sll_halen;    /* Length of address */
    unsigned char  sll_addr[8];  /* Physical layer address */
};
.fi
.in

.I sll_protocol
is the standard ethernet protocol type in network order as defined
in the
.I <linux/if_ether.h>
include file.
It defaults to the socket's protocol.
.I sll_ifindex
is the interface index of the interface
(see
.BR netdevice (7));
0 matches any interface (only permitted for binding).
.I sll_hatype
is an ARP type as defined in the
.I <linux/if_arp.h>
include file.
.I 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 sense only for receiving.
.I sll_addr
and
.I 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
.IR sll_family ,
.IR sll_addr ,
.IR sll_halen ,
.IR sll_ifindex .
The other fields should be 0.
.I sll_hatype
and
.I sll_pkttype
are set on received packets for your information.
For bind only
.I sll_protocol
and
.I sll_ifindex
are used.
.SS 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
.B 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 +4n
.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

.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.
.SS Ioctls
.B SIOCGSTAMP
can be used to receive the timestamp of the last received packet.
Argument is a
.I struct timeval.
.\" FIXME Document SIOCGSTAMPNS

In addition all standard ioctls defined in
.BR netdevice (7)
and
.BR socket (7)
are valid on packet sockets.
.SS 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 ERRORS
.TP
.B EADDRNOTAVAIL
Unknown multicast group address passed.
.TP
.B EFAULT
User passed invalid memory address.
.TP
.B EINVAL
Invalid argument.
.TP
.B EMSGSIZE
Packet is bigger than interface MTU.
.TP
.B ENETDOWN
Interface is not up.
.TP
.B ENOBUFS
Not enough memory to allocate the packet.
.TP
.B ENODEV
Unknown device name or interface index specified in interface address.
.TP
.B ENOENT
No packet received.
.TP
.B ENOTCONN
No interface address passed.
.TP
.B ENXIO
Interface address contained an invalid interface index.
.TP
.B EPERM
User has insufficient privileges to carry out this operation.

In addition other errors may be generated by the low-level driver.
.SH VERSIONS
.B AF_PACKET
is a new feature in Linux 2.2.
Earlier Linux versions supported only
.BR SOCK_PACKET .
.PP
The include file
.I <netpacket/packet.h>
is present since glibc 2.1.
Older systems need:
.sp
.in +4n
.nf
#include <asm/types.h>
#include <linux/if_packet.h>
#include <linux/if_ether.h>  /* The L2 protocols */
.fi
.in
.SH NOTES
For portable programs it is suggested to use
.B AF_PACKET
via
.BR pcap (3);
although this covers only a subset of the
.B AF_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.
.SS Compatibility
In Linux 2.0, the only way to get a packet socket was by calling
.BI "socket(AF_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 +4n
.nf
struct sockaddr_pkt {
    unsigned short spkt_family;
    unsigned char  spkt_device[14];
    unsigned short spkt_protocol;
};
.fi
.in

.I spkt_family
contains
the device type,
.I spkt_protocol
is the IEEE 802.3 protocol type as defined in
.I <sys/if_ether.h>
and
.I 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.
.SH BUGS
glibc 2.1 does not have a define for
.BR SOL_PACKET .
The suggested workaround is to use:
.in +4n
.nf

#ifndef SOL_PACKET
#define SOL_PACKET 263
#endif

.fi
.in
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
.B MSG_TRUNC
.BR 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
.BR SOCK_DGRAM .
.\" .SH CREDITS
.\" This man page was written by Andi Kleen with help from Matthew Wilcox.
.\" AF_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 <linux/if_ether.h>
include file for physical layer protocols.