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NAME | SYNOPSIS | DESCRIPTION | VERSIONS | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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netlink(7) Miscellaneous Information Manual netlink(7)
netlink - communication between kernel and user space
(AF_NETLINK)
#include <asm/types.h>
#include <sys/socket.h>
#include <linux/netlink.h>
netlink_socket = socket(AF_NETLINK, socket_type, netlink_family);
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 Linux 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_SOCK_DIAG (since Linux 3.3)
Query information about sockets of various protocol
families from the kernel (see sock_diag(7)).
NETLINK_INET_DIAG (since Linux 2.6.14)
An obsolete synonym for NETLINK_SOCK_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/driver-api/connector.rst (or
/Documentation/connector/connector.* in Linux 5.2 and
earlier) 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. Error messages get the
original request appended, unless the user requests to cap the
error message, and get extra error data if requested.
struct nlmsgerr {
int error; /* Negative errno or 0 for acknowledgements */
struct nlmsghdr msg; /* Message header that caused the error */
/*
* followed by the message contents
* unless NETLINK_CAP_ACK was set
* or the ACK indicates success (error == 0).
* For example Generic Netlink message with attributes.
* message length is aligned with NLMSG_ALIGN()
*/
/*
* followed by TLVs defined in enum nlmsgerr_attrs
* if NETLINK_EXT_ACK was set
*/
};
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 acknowledgement 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 acknowledgement 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 ENOBUFS 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 4.3)
The kernel may fail to allocate the necessary room for the
acknowledgement 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 acknowledgement.
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.
It is often better to use netlink via libnetlink or libnl than
via the low-level kernel interface.
This manual page is not complete.
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;
/* 8192 to avoid message truncation on platforms with
page size > 4096 */
struct nlmsghdr buf[8192/sizeof(struct nlmsghdr)];
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 */
...
}
cmsg(3), netlink(3), capabilities(7), rtnetlink(7), sock_diag(7)
information about libnetlink
⟨ftp://ftp.inr.ac.ru/ip-routing/iproute2*⟩
information about libnl ⟨http://www.infradead.org/~tgr/libnl/⟩
RFC 3549 "Linux Netlink as an IP Services Protocol"
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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Linux man-pages 6.9.1 2024-05-02 netlink(7)
Pages that refer to this page: bind(2), socket(2), audit_close(3), audit_open(3), libnetlink(3), mq_notify(3), netlink(3), rtnetlink(3), address_families(7), arp(7), ip(7), rtnetlink(7), sock_diag(7), quota_nld(8), ss(8), systemd-journald.service(8)
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