netlink(7) - Linux manual page

NETLINK(7)                Linux Programmer's Manual               NETLINK(7)

NAME top

       netlink - communication between kernel and user space (AF_NETLINK)

SYNOPSIS top

       #include <asm/types.h>
       #include <sys/socket.h>
       #include <linux/netlink.h>
       netlink_socket = socket(AF_NETLINK, socket_type, netlink_family);

DESCRIPTION top

Netlink is used to transfer information between the kernel and user-
space processes. It consists of a standard sockets-based interface
for user space 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
provided only for backward 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 (Linux 2.6.13 to 2.16.17)
Messages from 1-wire subsystem.
NETLINK_USERSOCK
Reserved for user-mode socket protocols.
NETLINK_FIREWALL (up to and including Linux 3.4)
Transport IPv4 packets from netfilter to user space. Used by
ip_queue kernel module. After a long period of being declared
obsolete (in favor of the more advanced nfnetlink_queue
feature), NETLINK_FIREWALL was removed in Linux 3.5.
NETLINK_INET_DIAG (since Linux 2.6.14)
Query information about sockets of various protocol families
from the kernel (see sock_diag(7)).
NETLINK_SOCK_DIAG (since Linux 3.3)
A synonym for NETLINK_INET_DIAG.
NETLINK_NFLOG (up to and including Linux 3.16)
Netfilter/iptables ULOG.
NETLINK_XFRM
IPsec.
NETLINK_SELINUX (since Linux 2.6.4)
SELinux event notifications.
NETLINK_ISCSI (since Linux 2.6.15)
Open-iSCSI.
NETLINK_AUDIT (since Linux 2.6.6)
Auditing.
NETLINK_FIB_LOOKUP (since Linux 2.6.13)
Access to FIB lookup from user space.
NETLINK_CONNECTOR (since Linux 2.6.14)
Kernel connector. See Documentation/connector/* in the Linux
kernel source tree for further information.
NETLINK_NETFILTER (since Linux 2.6.14)
Netfilter subsystem.
NETLINK_SCSITRANSPORT (since Linux 2.6.19)
SCSI Transports.
NETLINK_RDMA (since Linux 3.0)
Infiniband RDMA.
NETLINK_IP6_FW (up to and including Linux 3.4)
Transport IPv6 packets from netfilter to user space. Used by
ip6_queue kernel module.
NETLINK_DNRTMSG
DECnet routing messages.
NETLINK_KOBJECT_UEVENT (since Linux 2.6.10)
Kernel messages to user space.
NETLINK_GENERIC (since Linux 2.6.15)
Generic netlink family for simplified netlink usage.
NETLINK_CRYPTO (since Linux 3.2)
Netlink interface to request information about ciphers
registered with the kernel crypto API as well as allow
configuration of the kernel crypto API.
Netlink messages consist of a byte stream with one or multiple
nlmsghdr headers and associated payload. The byte stream should be
accessed only 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; /* Sender port ID */
};
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
user-space 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; /* Port 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 user-space 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 be equal to the
process ID only 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. Since Linux 2.6.13, messages can't be broadcast to multiple
groups. Any replies to a message received for a multicast group
should be sent back to the sending PID and the multicast group. Some
Linux kernel subsystems may additionally allow other users to send
and/or receive messages. As at Linux 3.0, the
NETLINK_KOBJECT_UEVENT, NETLINK_GENERIC, NETLINK_ROUTE, and
NETLINK_SELINUX groups allow other users to receive messages. No
groups allow other users to send messages.
Socket options
To set or get a netlink socket option, call getsockopt(2) to read or
setsockopt(2) to write the option with the option level argument set
to SOL_NETLINK. Unless otherwise noted, optval is a pointer to an
int.
NETLINK_PKTINFO (since Linux 2.6.14)
Enable nl_pktinfo control messages for received packets to get
the extended destination group number.
NETLINK_ADD_MEMBERSHIP, NETLINK_DROP_MEMBERSHIP (since Linux 2.6.14)
Join/leave a group specified by optval.
NETLINK_LIST_MEMBERSHIPS (since Linux 4.2)
Retrieve all groups a socket is a member of. optval is a
pointer to __u32 and optlen is the size of the array. The
array is filled with the full membership set of the socket,
and the required array size is returned in optlen.
NETLINK_BROADCAST_ERROR (since Linux 2.6.30)
When not set, netlink_broadcast() only reports ESRCH errors
and silently ignore NOBUFS errors.
NETLINK_NO_ENOBUFS (since Linux 2.6.30)
This flag can be used by unicast and broadcast listeners to
avoid receiving ENOBUFS errors.
NETLINK_LISTEN_ALL_NSID (since Linux 4.2)
When set, this socket will receive netlink notifications from
all network namespaces that have an nsid assigned into the
network namespace where the socket has been opened. The nsid
is sent to user space via an ancillary data.
NETLINK_CAP_ACK (since Linux 4.2)
The kernel may fail to allocate the necessary room for the
acknowledgment message back to user space. This option trims
off the payload of the original netlink message. The netlink
message header is still included, so the user can guess from
the sequence number which message triggered the
acknowledgment.

VERSIONS top

       The socket interface to netlink first appeared 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.

NOTES top

       It is often better to use netlink via libnetlink or libnl than via
       the low-level kernel interface.

BUGS top

       This manual page is not complete.

EXAMPLE top

       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 the 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 = { nh, nh->nlmsg_len };
           struct msghdr msg;
           msg = { &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 = { &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 */
               ...
           }

COLOPHON top

       This page is part of release 4.12 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.
Linux                            2016-12-12                       NETLINK(7)

Pages that refer to this page: bind(2), socket(2), audit_open(3), libnetlink(3), mq_notify(3), netlink(3), rtnetlink(3), arp(7), ip(7), rtnetlink(7), sock_diag(7), quota_nld(8), ss(8)