netlink(7)
NAME
netlink, PF_NETLINK - Communication between kernel and
user.
SYNOPSIS
#include <asm/types.h>
#include <sys/socket.h>
#include <linux/netlink.h>
netlink_socket = socket(PF_NETLINK, socket_type, netlink_family);
DESCRIPTION
Netlink is used to transfer information between kernel
modules and user space processes. It consists of a stan
dard sockets based interface for user processes and an
internal kernel API for kernel modules. The internal ker
nel interface is not documented in this man page. Also
there is an obsolete netlink interface via netlink charac
ter devices, this interface is not documented here and is
only provided for backwards compatibility.
Netlink is a datagram oriented service. Both SOCK_RAW and
SOCK_DGRAM are valid values for socket_type; however the
netlink protocol does not distinguish between datagram and
raw sockets.
netlink_family selects the kernel module or netlink group
to communicate with. The currently assigned netlink fami
lies are:
NETLINK_ROUTE
Receives routing updates and may be used to modify
the IPv4 routing table (see rtnetlink(7)).
NETLINK_FIREWALL
Receives packets sent by the IPv4 firewall code.
NETLINK_ARPD
For managing the arp table in user space.
NETLINK_ROUTE6
Receives and sends IPv6 routing table updates.
NETLINK_IP6_FW
to receive packets that failed the IPv6 firewall
checks (currently not implemented).
NETLINK_TAPBASE...NETLINK_TAPBASE+15
are the instances of the ethertap device. Ethertap
is a pseudo network tunnel device that allows an
ethernet driver to be simulated from user space.
NETLINK_SKIP
Reserved for ENskip.
NETLINK_USERSOCK
is reserved for future user space protocols.
Netlink messages consist of a byte stream with one or mul
tiple nlmsghdr headers and associated payload. For multi
part messages the first and all following headers have the
NLM_F_MULTI flag set, except for the last header which has
the type NLMSG_DONE. The byte stream should only be
accessed with the standard NLMSG_* macros, see netlink(3).
Netlink is not a reliable protocol. It tries its best to
deliver a message to its destination(s), but may drop mes
sages when an out of memory condition or other error
occurs. For reliable transfer the sender can request an
acknowledgement from the receiver by setting the NLM_F_ACK
flag. An acknowledgment is an NLMSG_ERROR packet with the
error field set to 0. The application must generate acks
for received messages itself. The kernel tries to send an
NLMSG_ERROR message for every failed packet. A user pro
cess should follow this convention too.
Each netlink family has a set of 32 multicast groups.
When bind(2) is called on the socket, the nl_groups field
in the sockaddr_nl should be set to a bitmask of the
groups which it wishes to listen to. The default value
for this field is zero which means that no multicasts will
be received. A socket may multicast messages to any of
the multicast groups by setting nl_groups to a bitmask of
the groups it wishes to send to when it calls sendmsg(2)
or does a connect(2). Only users with an effective uid of
0 or the CAP_NET_ADMIN capability may send or listen to a
netlink multicast group. Any replies to a message
received for a multicast group should be sent back to the
sending pid and the multicast group.
struct nlmsghdr
{
__u32 nlmsg_len; /* Length of message including header */
__u16 nlmsg_type; /* Message content */
__u16 nlmsg_flags;/* Additional flags */
__u32 nlmsg_seq; /* Sequence number */
__u32 nlmsg_pid; /* Sending process PID */
};
struct nlmsgerr
{
int error; /* negative errno or 0 for acks. */
struct nlmsghdr msg; /* message header that caused the error */
};
After each nlmsghdr the payload follows. nlmsg_type can
be one of the standard message types: NLMSG_NOOP message
is to be ignored, NLMSG_ERROR the message signals an error
and the payload contains a nlmsgerr structure, NLMSG_DONE
message terminates a multipart message,
A netlink family usually specifies more message types, see
the appropiate man pages for that, e.g. rtnetlink(7) for
NETLINK_ROUTE.
Standard Flag bits in nlmsg_flags
NLM_F_REQUEST set on all request messages
NLM_F_MULTI the message is part of a multipart mes
sage terminated by NLMSG_DONE
NLM_F_ACK reply with an acknowledgment on success
NLM_F_ECHO echo this request
Additional flag bits for GET requests
NLM_F_ROOT Return the complete table instead of a single entry.
NLM_F_MATCH Not implemented yet.
NLM_F_ATOMIC Return an atomic snapshot of the table.
NLM_F_DUMP not documented yet.
Additional flag bits for NEW requests
NLM_F_REPLACE Override existing object.
NLM_F_EXCL Don't replace if the object already exists.
NLM_F_CREATE Create object if it doesn't already exist.
NLM_F_APPEND Add to the end of the object list.
Note that NLM_F_ATOMIC requires CAP_NET_ADMIN or super
user rights.
ADDRESS FORMATS
The sockaddr_nl structure describes a netlink client in
user space or in the kernel. A sockaddr_nl can be either
unicast (only send to one peer) or send to netlink groups
(nl_groups not equal 0).
struct sockaddr_nl
{
sa_family_t nl_family; /* AF_NETLINK */
unsigned short nl_pad; /* zero */
pid_t nl_pid; /* process pid */
__u32 nl_groups; /* multicast groups mask */
};
nl_pid is the pid of the user space netlink, or 0 if the
destination is in the kernel. nl_groups is a bitmask with
every bit representing a netlink group number.
BUGS
This man page is not complete.
NOTES
It is often better to use netlink via libnetlink than via
the low level kernel interface.
VERSIONS
The socket interface to netlink is a new feature of Linux
2.2
Linux 2.0 supported a more primitive device based netlink
interface (which is still available as a compatibility
option). This obsolete interface is not described here.
SEE ALSO
cmsg(3), rtnetlink(7), netlink(3).
ftp://ftp.inr.ac.ru/ip-routing/iproute2* for libnetlink
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