1NETLINK(7) Linux Programmer's Manual NETLINK(7)
2
6 netlink - communication between kernel and user space (AF_NETLINK)
7
9 #include <asm/types.h>
10 #include <sys/socket.h>
11 #include <linux/netlink.h>
12 netlink_socket = socket(AF_NETLINK, socket_type, netlink_family);
14
16 Netlink is used to transfer information between the kernel and user-
17 space processes. It consists of a standard sockets-based interface for
18 user space processes and an internal kernel API for kernel modules.
19 The internal kernel interface is not documented in this manual page.
20 There is also an obsolete netlink interface via netlink character
21 devices; this interface is not documented here and is provided only for
22 backward compatibility.
23 Netlink is a datagram-oriented service. Both SOCK_RAW and SOCK_DGRAM
25 are valid values for socket_type. However, the netlink protocol does
26 not distinguish between datagram and raw sockets.
27 netlink_family selects the kernel module or netlink group to communi‐
29 cate with. The currently assigned netlink families are:
30 NETLINK_ROUTE
32 Receives routing and link updates and may be used to modify the
33 routing tables (both IPv4 and IPv6), IP addresses, link parame‐
34 ters, neighbor setups, queueing disciplines, traffic classes and
35 packet classifiers (see rtnetlink(7)).
36 NETLINK_W1 (Linux 2.6.13 to 2.16.17)
38 Messages from 1-wire subsystem.
39 NETLINK_USERSOCK
41 Reserved for user-mode socket protocols.
42 NETLINK_FIREWALL (up to and including Linux 3.4)
44 Transport IPv4 packets from netfilter to user space. Used by
45 ip_queue kernel module. After a long period of being declared
46 obsolete (in favor of the more advanced nfnetlink_queue fea‐
47 ture), NETLINK_FIREWALL was removed in Linux 3.5.
48 NETLINK_INET_DIAG (since Linux 2.6.14)
50 Query information about sockets of various protocol families
51 from the kernel (see sock_diag(7)).
52 NETLINK_SOCK_DIAG (since Linux 3.3)
54 A synonym for NETLINK_INET_DIAG.
55 NETLINK_NFLOG (up to and including Linux 3.16)
57 Netfilter/iptables ULOG.
58 NETLINK_XFRM
60 IPsec.
61 NETLINK_SELINUX (since Linux 2.6.4)
63 SELinux event notifications.
64 NETLINK_ISCSI (since Linux 2.6.15)
66 Open-iSCSI.
67 NETLINK_AUDIT (since Linux 2.6.6)
69 Auditing.
70 NETLINK_FIB_LOOKUP (since Linux 2.6.13)
72 Access to FIB lookup from user space.
73 NETLINK_CONNECTOR (since Linux 2.6.14)
75 Kernel connector. See Documentation/connector/* in the Linux
76 kernel source tree for further information.
77 NETLINK_NETFILTER (since Linux 2.6.14)
79 Netfilter subsystem.
80 NETLINK_SCSITRANSPORT (since Linux 2.6.19)
82 SCSI Transports.
83 NETLINK_RDMA (since Linux 3.0)
85 Infiniband RDMA.
86 NETLINK_IP6_FW (up to and including Linux 3.4)
88 Transport IPv6 packets from netfilter to user space. Used by
89 ip6_queue kernel module.
90 NETLINK_DNRTMSG
92 DECnet routing messages.
93 NETLINK_KOBJECT_UEVENT (since Linux 2.6.10)
95 Kernel messages to user space.
96 NETLINK_GENERIC (since Linux 2.6.15)
98 Generic netlink family for simplified netlink usage.
99 NETLINK_CRYPTO (since Linux 3.2)
101 Netlink interface to request information about ciphers regis‐
102 tered with the kernel crypto API as well as allow configuration
103 of the kernel crypto API.
104 Netlink messages consist of a byte stream with one or multiple nlmsghdr
106 headers and associated payload. The byte stream should be accessed
107 only with the standard NLMSG_* macros. See netlink(3) for further
108 information.
109 In multipart messages (multiple nlmsghdr headers with associated pay‐
111 load in one byte stream) the first and all following headers have the
112 NLM_F_MULTI flag set, except for the last header which has the type
113 NLMSG_DONE.
114 After each nlmsghdr the payload follows.
116 struct nlmsghdr {
118 __u32 nlmsg_len; /* Length of message including header */
119 __u16 nlmsg_type; /* Type of message content */
120 __u16 nlmsg_flags; /* Additional flags */
121 __u32 nlmsg_seq; /* Sequence number */
122 __u32 nlmsg_pid; /* Sender port ID */
123 };
124 nlmsg_type can be one of the standard message types: NLMSG_NOOP message
126 is to be ignored, NLMSG_ERROR message signals an error and the payload
127 contains an nlmsgerr structure, NLMSG_DONE message terminates a multi‐
128 part message.
129 struct nlmsgerr {
131 int error; /* Negative errno or 0 for acknowledgements */
132 struct nlmsghdr msg; /* Message header that caused the error */
133 };
134 A netlink family usually specifies more message types, see the appro‐
136 priate manual pages for that, for example, rtnetlink(7) for
137 NETLINK_ROUTE.
138 Standard flag bits in nlmsg_flags
140 ──────────────────────────────────────────────────────────
141 NLM_F_REQUEST Must be set on all request messages.
142 NLM_F_MULTI The message is part of a multipart mes‐
143 sage terminated by NLMSG_DONE.
144 NLM_F_ACK Request for an acknowledgment on success.
145 NLM_F_ECHO Echo this request.
146 Additional flag bits for GET requests
148 ────────────────────────────────────────────────────────────────────
149 NLM_F_ROOT Return the complete table instead of a single entry.
150 NLM_F_MATCH Return all entries matching criteria passed in mes‐
151 sage content. Not implemented yet.
152 NLM_F_ATOMIC Return an atomic snapshot of the table.
153 NLM_F_DUMP Convenience macro; equivalent to
154 (NLM_F_ROOT|NLM_F_MATCH).
155 Note that NLM_F_ATOMIC requires the CAP_NET_ADMIN capability or an
157 effective UID of 0.
158 Additional flag bits for NEW requests
160 ────────────────────────────────────────────────────────────
161 NLM_F_REPLACE Replace existing matching object.
162 NLM_F_EXCL Don't replace if the object already exists.
163 NLM_F_CREATE Create object if it doesn't already exist.
164 NLM_F_APPEND Add to the end of the object list.
165 nlmsg_seq and nlmsg_pid are used to track messages. nlmsg_pid shows
167 the origin of the message. Note that there isn't a 1:1 relationship
168 between nlmsg_pid and the PID of the process if the message originated
169 from a netlink socket. See the ADDRESS FORMATS section for further
170 information.
171 Both nlmsg_seq and nlmsg_pid are opaque to netlink core.
173 Netlink is not a reliable protocol. It tries its best to deliver a
175 message to its destination(s), but may drop messages when an out-of-
176 memory condition or other error occurs. For reliable transfer the
177 sender can request an acknowledgement from the receiver by setting the
178 NLM_F_ACK flag. An acknowledgment is an NLMSG_ERROR packet with the
179 error field set to 0. The application must generate acknowledgements
180 for received messages itself. The kernel tries to send an NLMSG_ERROR
181 message for every failed packet. A user process should follow this
182 convention too.
183 However, reliable transmissions from kernel to user are impossible in
185 any case. The kernel can't send a netlink message if the socket buffer
186 is full: the message will be dropped and the kernel and the user-space
187 process will no longer have the same view of kernel state. It is up to
188 the application to detect when this happens (via the ENOBUFS error
189 returned by recvmsg(2)) and resynchronize.
190 Address formats
192 The sockaddr_nl structure describes a netlink client in user space or
193 in the kernel. A sockaddr_nl can be either unicast (only sent to one
194 peer) or sent to netlink multicast groups (nl_groups not equal 0).
195 struct sockaddr_nl {
197 sa_family_t nl_family; /* AF_NETLINK */
198 unsigned short nl_pad; /* Zero */
199 pid_t nl_pid; /* Port ID */
200 __u32 nl_groups; /* Multicast groups mask */
201 };
202 nl_pid is the unicast address of netlink socket. It's always 0 if the
204 destination is in the kernel. For a user-space process, nl_pid is usu‐
205 ally the PID of the process owning the destination socket. However,
206 nl_pid identifies a netlink socket, not a process. If a process owns
207 several netlink sockets, then nl_pid can be equal to the process ID
208 only for at most one socket. There are two ways to assign nl_pid to a
209 netlink socket. If the application sets nl_pid before calling bind(2),
210 then it is up to the application to make sure that nl_pid is unique.
211 If the application sets it to 0, the kernel takes care of assigning it.
212 The kernel assigns the process ID to the first netlink socket the
213 process opens and assigns a unique nl_pid to every netlink socket that
214 the process subsequently creates.
215 nl_groups is a bit mask with every bit representing a netlink group
217 number. Each netlink family has a set of 32 multicast groups. When
218 bind(2) is called on the socket, the nl_groups field in the sockaddr_nl
219 should be set to a bit mask of the groups which it wishes to listen to.
220 The default value for this field is zero which means that no multicasts
221 will be received. A socket may multicast messages to any of the multi‐
222 cast groups by setting nl_groups to a bit mask of the groups it wishes
223 to send to when it calls sendmsg(2) or does a connect(2). Only pro‐
224 cesses with an effective UID of 0 or the CAP_NET_ADMIN capability may
225 send or listen to a netlink multicast group. Since Linux 2.6.13, mes‐
226 sages can't be broadcast to multiple groups. Any replies to a message
227 received for a multicast group should be sent back to the sending PID
228 and the multicast group. Some Linux kernel subsystems may additionally
229 allow other users to send and/or receive messages. As at Linux 3.0,
230 the NETLINK_KOBJECT_UEVENT, NETLINK_GENERIC, NETLINK_ROUTE, and
231 NETLINK_SELINUX groups allow other users to receive messages. No
232 groups allow other users to send messages.
233 Socket options
235 To set or get a netlink socket option, call getsockopt(2) to read or
236 setsockopt(2) to write the option with the option level argument set to
237 SOL_NETLINK. Unless otherwise noted, optval is a pointer to an int.
238 NETLINK_PKTINFO (since Linux 2.6.14)
240 Enable nl_pktinfo control messages for received packets to get
241 the extended destination group number.
242 NETLINK_ADD_MEMBERSHIP, NETLINK_DROP_MEMBERSHIP (since Linux 2.6.14)
244 Join/leave a group specified by optval.
245 NETLINK_LIST_MEMBERSHIPS (since Linux 4.2)
247 Retrieve all groups a socket is a member of. optval is a
248 pointer to __u32 and optlen is the size of the array. The array
249 is filled with the full membership set of the socket, and the
250 required array size is returned in optlen.
251 NETLINK_BROADCAST_ERROR (since Linux 2.6.30)
253 When not set, netlink_broadcast() only reports ESRCH errors and
254 silently ignore ENOBUFS errors.
255 NETLINK_NO_ENOBUFS (since Linux 2.6.30)
257 This flag can be used by unicast and broadcast listeners to
258 avoid receiving ENOBUFS errors.
259 NETLINK_LISTEN_ALL_NSID (since Linux 4.2)
261 When set, this socket will receive netlink notifications from
262 all network namespaces that have an nsid assigned into the net‐
263 work namespace where the socket has been opened. The nsid is
264 sent to user space via an ancillary data.
265 NETLINK_CAP_ACK (since Linux 4.2)
267 The kernel may fail to allocate the necessary room for the
268 acknowledgment message back to user space. This option trims
269 off the payload of the original netlink message. The netlink
270 message header is still included, so the user can guess from the
271 sequence number which message triggered the acknowledgment.
272
274 The socket interface to netlink first appeared Linux 2.2.
275 Linux 2.0 supported a more primitive device-based netlink interface
277 (which is still available as a compatibility option). This obsolete
278 interface is not described here.
279
281 It is often better to use netlink via libnetlink or libnl than via the
282 low-level kernel interface.
283
285 This manual page is not complete.
286
288 The following example creates a NETLINK_ROUTE netlink socket which will
289 listen to the RTMGRP_LINK (network interface create/delete/up/down
290 events) and RTMGRP_IPV4_IFADDR (IPv4 addresses add/delete events) mul‐
291 ticast groups.
292 struct sockaddr_nl sa;
294 memset(&sa, 0, sizeof(sa));
296 sa.nl_family = AF_NETLINK;
297 sa.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR;
298 fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
300 bind(fd, (struct sockaddr *) &sa, sizeof(sa));
301 The next example demonstrates how to send a netlink message to the ker‐
303 nel (pid 0). Note that the application must take care of message
304 sequence numbers in order to reliably track acknowledgements.
305 struct nlmsghdr *nh; /* The nlmsghdr with payload to send */
307 struct sockaddr_nl sa;
308 struct iovec iov = { nh, nh->nlmsg_len };
309 struct msghdr msg;
310 msg = { &sa, sizeof(sa), &iov, 1, NULL, 0, 0 };
312 memset(&sa, 0, sizeof(sa));
313 sa.nl_family = AF_NETLINK;
314 nh->nlmsg_pid = 0;
315 nh->nlmsg_seq = ++sequence_number;
316 /* Request an ack from kernel by setting NLM_F_ACK */
317 nh->nlmsg_flags |= NLM_F_ACK;
318 sendmsg(fd, &msg, 0);
320 And the last example is about reading netlink message.
322 int len;
324 char buf[8192]; /* 8192 to avoid message truncation on
325 platforms with page size > 4096 */
326 struct iovec iov = { buf, sizeof(buf) };
327 struct sockaddr_nl sa;
328 struct msghdr msg;
329 struct nlmsghdr *nh;
330 msg = { &sa, sizeof(sa), &iov, 1, NULL, 0, 0 };
332 len = recvmsg(fd, &msg, 0);
333 for (nh = (struct nlmsghdr *) buf; NLMSG_OK (nh, len);
335 nh = NLMSG_NEXT (nh, len)) {
336 /* The end of multipart message */
337 if (nh->nlmsg_type == NLMSG_DONE)
338 return;
339 if (nh->nlmsg_type == NLMSG_ERROR)
341 /* Do some error handling */
342 ...
343 /* Continue with parsing payload */
345 ...
346 }
347
349 cmsg(3), netlink(3), capabilities(7), rtnetlink(7), sock_diag(7)
350 information about libnetlink ⟨ftp://ftp.inr.ac.ru/ip-routing/iproute2*⟩
352 information about libnl ⟨http://www.infradead.org/~tgr/libnl/⟩
354 RFC 3549 "Linux Netlink as an IP Services Protocol"
356
358 This page is part of release 5.04 of the Linux man-pages project. A
359 description of the project, information about reporting bugs, and the
360 latest version of this page, can be found at
361 https://www.kernel.org/doc/man-pages/.
362Linux 2017-09-15 NETLINK(7)