NAME
netlink - Communication between kernel and userspace (AF_NETLINK)
SYNOPSIS
#include <asm/types.h>
#include <sys/socket.h>
#include <linux/netlink.h>
netlink_socket = socket(AF_NETLINK, socket_type, netlink_family);
DESCRIPTION
Netlink is used to transfer information between kernel and userspace
processes. It consists of a standard sockets-based interface for
userspace processes and an internal kernel API for kernel modules. The
internal kernel interface is not documented in this manual page. There
is also an obsolete netlink interface via netlink character 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 families are:
NETLINK_ROUTE
Receives routing and link updates and may be used to modify the
routing tables (both IPv4 and IPv6), IP addresses, link
parameters, neighbor setups, queueing disciplines, traffic
classes and packet classifiers (see rtnetlink(7)).
NETLINK_W1
Messages from 1-wire subsystem.
NETLINK_USERSOCK
Reserved for user-mode socket protocols.
NETLINK_FIREWALL
Transport IPv4 packets from netfilter to userspace. Used by
ip_queue kernel module.
NETLINK_INET_DIAG
INET socket monitoring.
NETLINK_NFLOG
Netfilter/iptables ULOG.
NETLINK_XFRM
IPsec.
NETLINK_SELINUX
SELinux event notifications.
NETLINK_ISCSI
Open-iSCSI.
NETLINK_AUDIT
Auditing.
NETLINK_FIB_LOOKUP
Access to FIB lookup from userspace.
NETLINK_CONNECTOR
Kernel connector. See Documentation/connector/* in the kernel
source for further information.
NETLINK_NETFILTER
Netfilter subsystem.
NETLINK_IP6_FW
Transport IPv6 packets from netfilter to userspace. Used by
ip6_queue kernel module.
NETLINK_DNRTMSG
DECnet routing messages.
NETLINK_KOBJECT_UEVENT
Kernel messages to userspace.
NETLINK_GENERIC
Generic netlink family for simplified netlink usage.
Netlink messages consist of a byte stream with one or multiple nlmsghdr
headers and associated payload. The byte stream should only be
accessed with the standard NLMSG_* macros. See netlink(3) for further
information.
In multipart messages (multiple nlmsghdr headers with associated
payload in one byte stream) the first and all following headers have
the NLM_F_MULTI flag set, except for the last header which has the type
NLMSG_DONE.
After each nlmsghdr the payload follows.
struct nlmsghdr {
__u32 nlmsg_len; /* Length of message including header. */
__u16 nlmsg_type; /* Type of message content. */
__u16 nlmsg_flags; /* Additional flags. */
__u32 nlmsg_seq; /* Sequence number. */
__u32 nlmsg_pid; /* PID of the sending process. */
};
nlmsg_type can be one of the standard message types: NLMSG_NOOP message
is to be ignored, NLMSG_ERROR message signals an error and the payload
contains an nlmsgerr structure, NLMSG_DONE message terminates a
multipart message.
struct nlmsgerr {
int error; /* Negative errno or 0 for acknowledgements */
struct nlmsghdr msg; /* Message header that caused the error */
};
A netlink family usually specifies more message types, see the
appropriate manual pages for that, for example, rtnetlink(7) for
NETLINK_ROUTE.
Standard flag bits in nlmsg_flags
---------------------------------
NLM_F_REQUEST Must be set on all request messages.
NLM_F_MULTI The message is part of a multipart
message terminated by NLMSG_DONE.
NLM_F_ACK Request for 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 Return all entries matching criteria passed in message
content. Not implemented yet.
NLM_F_ATOMIC Return an atomic snapshot of the table.
NLM_F_DUMP Convenience macro; equivalent to (NLM_F_ROOT|NLM_F_MATCH).
Note that NLM_F_ATOMIC requires the CAP_NET_ADMIN capability or an
effective UID of 0.
Additional flag bits for NEW requests
-------------------------------------
NLM_F_REPLACE Replace existing matching 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.
nlmsg_seq and nlmsg_pid are used to track messages. nlmsg_pid shows
the origin of the message. Note that there isn’t a 1:1 relationship
between nlmsg_pid and the PID of the process if the message originated
from a netlink socket. See the ADDRESS FORMATS section for further
information.
Both nlmsg_seq and nlmsg_pid are opaque to netlink core.
Netlink is not a reliable protocol. It tries its best to deliver a
message to its destination(s), but may drop messages 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 acknowledgements
for received messages itself. The kernel tries to send an NLMSG_ERROR
message for every failed packet. A user process should follow this
convention too.
However, reliable transmissions from kernel to user are impossible in
any case. The kernel can’t send a netlink message if the socket buffer
is full: the message will be dropped and the kernel and the userspace
process will no longer have the same view of kernel state. It is up to
the application to detect when this happens (via the ENOBUFS error
returned by recvmsg(2)) and resynchronize.
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 sent to one
peer) or sent to netlink multicast 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 ID. */
__u32 nl_groups; /* Multicast groups mask. */
};
nl_pid is the unicast address of netlink socket. It’s always 0 if the
destination is in the kernel. For a userspace process, nl_pid is
usually the PID of the process owning the destination socket. However,
nl_pid identifies a netlink socket, not a process. If a process owns
several netlink sockets, then nl_pid can only be equal to the process
ID for at most one socket. There are two ways to assign nl_pid to a
netlink socket. If the application sets nl_pid before calling bind(2),
then it is up to the application to make sure that nl_pid is unique.
If the application sets it to 0, the kernel takes care of assigning it.
The kernel assigns the process ID to the first netlink socket the
process opens and assigns a unique nl_pid to every netlink socket that
the process subsequently creates.
nl_groups is a bit mask with every bit representing a netlink group
number. 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 bit mask 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 bit mask of the groups it
wishes to send to when it calls sendmsg(2) or does a connect(2). Only
processes 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.
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.
NETLINK_SELINUX appeared in Linux 2.6.4.
NETLINK_AUDIT appeared in Linux 2.6.6.
NETLINK_KOBJECT_UEVENT appeared in Linux 2.6.10.
NETLINK_W1 and NETLINK_FIB_LOOKUP appeared in Linux 2.6.13.
NETLINK_INET_DIAG, NETLINK_CONNECTOR and NETLINK_NETFILTER appeared in
Linux 2.6.14.
NETLINK_GENERIC and NETLINK_ISCSI appeared in Linux 2.6.15.
NOTES
It is often better to use netlink via libnetlink or libnl than via the
low-level kernel interface.
BUGS
This manual page is not complete.
EXAMPLE
The following example creates a NETLINK_ROUTE netlink socket which will
listen to the RTMGRP_LINK (network interface create/delete/up/down
events) and RTMGRP_IPV4_IFADDR (IPv4 addresses add/delete events)
multicast groups.
struct sockaddr_nl sa;
memset(&sa, 0, sizeof(sa));
sa.nl_family = AF_NETLINK;
sa.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR;
fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
bind(fd, (struct sockaddr *) &sa, sizeof(sa));
The next example demonstrates how to send a netlink message to the
kernel (pid 0). Note that application must take care of message
sequence numbers in order to reliably track acknowledgements.
struct nlmsghdr *nh; /* The nlmsghdr with payload to send. */
struct sockaddr_nl sa;
struct iovec iov = { (void *) nh, nh->nlmsg_len };
struct msghdr msg;
msg = { (void *)&sa, sizeof(sa), &iov, 1, NULL, 0, 0 };
memset(&sa, 0, sizeof(sa));
sa.nl_family = AF_NETLINK;
nh->nlmsg_pid = 0;
nh->nlmsg_seq = ++sequence_number;
/* Request an ack from kernel by setting NLM_F_ACK. */
nh->nlmsg_flags |= NLM_F_ACK;
sendmsg(fd, &msg, 0);
And the last example is about reading netlink message.
int len;
char buf[4096];
struct iovec iov = { buf, sizeof(buf) };
struct sockaddr_nl sa;
struct msghdr msg;
struct nlmsghdr *nh;
msg = { (void *)&sa, sizeof(sa), &iov, 1, NULL, 0, 0 };
len = recvmsg(fd, &msg, 0);
for (nh = (struct nlmsghdr *) buf; NLMSG_OK (nh, len);
nh = NLMSG_NEXT (nh, len)) {
/* The end of multipart message. */
if (nh->nlmsg_type == NLMSG_DONE)
return;
if (nh->nlmsg_type == NLMSG_ERROR)
/* Do some error handling. */
...
/* Continue with parsing payload. */
...
}
SEE ALSO
cmsg(3), netlink(3), capabilities(7), rtnetlink(7)
ftp://ftp.inr.ac.ru/ip-routing/iproute2* for information about
libnetlink.
http://people.suug.ch/~tgr/libnl/ for information about libnl.
RFC 3549 "Linux Netlink as an IP Services Protocol"
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.