1NETLINK(7)                 Linux Programmer's Manual                NETLINK(7)
2
3
4

NAME

6       netlink - communication between kernel and user space (AF_NETLINK)
7

SYNOPSIS

9       #include <asm/types.h>
10       #include <sys/socket.h>
11       #include <linux/netlink.h>
12
13       netlink_socket = socket(AF_NETLINK, socket_type, netlink_family);
14

DESCRIPTION

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 de‐
21       vices; this interface is not documented here and is provided  only  for
22       backward compatibility.
23
24       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
28       netlink_family  selects  the kernel module or netlink group to communi‐
29       cate with.  The currently assigned netlink families are:
30
31       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
37       NETLINK_W1 (Linux 2.6.13 to 2.16.17)
38              Messages from 1-wire subsystem.
39
40       NETLINK_USERSOCK
41              Reserved for user-mode socket protocols.
42
43       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
49       NETLINK_SOCK_DIAG (since Linux 3.3)
50              Query information about sockets  of  various  protocol  families
51              from the kernel (see sock_diag(7)).
52
53       NETLINK_INET_DIAG (since Linux 2.6.14)
54              An obsolete synonym for NETLINK_SOCK_DIAG.
55
56       NETLINK_NFLOG (up to and including Linux 3.16)
57              Netfilter/iptables ULOG.
58
59       NETLINK_XFRM
60              IPsec.
61
62       NETLINK_SELINUX (since Linux 2.6.4)
63              SELinux event notifications.
64
65       NETLINK_ISCSI (since Linux 2.6.15)
66              Open-iSCSI.
67
68       NETLINK_AUDIT (since Linux 2.6.6)
69              Auditing.
70
71       NETLINK_FIB_LOOKUP (since Linux 2.6.13)
72              Access to FIB lookup from user space.
73
74       NETLINK_CONNECTOR (since Linux 2.6.14)
75              Kernel  connector.   See  Documentation/driver-api/connector.rst
76              (or /Documentation/connector/connector.*  in kernel 5.2 and ear‐
77              lier) in the Linux kernel source tree for further information.
78
79       NETLINK_NETFILTER (since Linux 2.6.14)
80              Netfilter subsystem.
81
82       NETLINK_SCSITRANSPORT (since Linux 2.6.19)
83              SCSI Transports.
84
85       NETLINK_RDMA (since Linux 3.0)
86              Infiniband RDMA.
87
88       NETLINK_IP6_FW (up to and including Linux 3.4)
89              Transport  IPv6  packets  from netfilter to user space.  Used by
90              ip6_queue kernel module.
91
92       NETLINK_DNRTMSG
93              DECnet routing messages.
94
95       NETLINK_KOBJECT_UEVENT (since Linux 2.6.10)
96              Kernel messages to user space.
97
98       NETLINK_GENERIC (since Linux 2.6.15)
99              Generic netlink family for simplified netlink usage.
100
101       NETLINK_CRYPTO (since Linux 3.2)
102              Netlink interface to request information  about  ciphers  regis‐
103              tered  with the kernel crypto API as well as allow configuration
104              of the kernel crypto API.
105
106       Netlink messages consist of a byte stream with one or multiple nlmsghdr
107       headers  and  associated  payload.   The byte stream should be accessed
108       only with the standard NLMSG_* macros.  See netlink(3) for further  in‐
109       formation.
110
111       In  multipart  messages (multiple nlmsghdr headers with associated pay‐
112       load in one byte stream) the first and all following headers  have  the
113       NLM_F_MULTI  flag  set,  except  for the last header which has the type
114       NLMSG_DONE.
115
116       After each nlmsghdr the payload follows.
117
118           struct nlmsghdr {
119               __u32 nlmsg_len;    /* Length of message including header */
120               __u16 nlmsg_type;   /* Type of message content */
121               __u16 nlmsg_flags;  /* Additional flags */
122               __u32 nlmsg_seq;    /* Sequence number */
123               __u32 nlmsg_pid;    /* Sender port ID */
124           };
125
126       nlmsg_type can be one of the standard message types: NLMSG_NOOP message
127       is  to be ignored, NLMSG_ERROR message signals an error and the payload
128       contains an nlmsgerr structure, NLMSG_DONE message terminates a  multi‐
129       part message.
130
131           struct nlmsgerr {
132               int error;        /* Negative errno or 0 for acknowledgements */
133               struct nlmsghdr msg;  /* Message header that caused the error */
134           };
135
136       A  netlink  family usually specifies more message types, see the appro‐
137       priate  manual  pages  for  that,   for   example,   rtnetlink(7)   for
138       NETLINK_ROUTE.
139
140       Standard flag bits in nlmsg_flags
141       ──────────────────────────────────────────────────────────
142       NLM_F_REQUEST   Must be set on all request messages.
143       NLM_F_MULTI     The  message  is part of a multipart mes‐
144                       sage terminated by NLMSG_DONE.
145       NLM_F_ACK       Request for an acknowledgment on success.
146       NLM_F_ECHO      Echo this request.
147
148       Additional flag bits for GET requests
149       ────────────────────────────────────────────────────────────────────
150       NLM_F_ROOT     Return the complete table instead of a single entry.
151       NLM_F_MATCH    Return all entries matching criteria passed in  mes‐
152                      sage content.  Not implemented yet.
153       NLM_F_ATOMIC   Return an atomic snapshot of the table.
154       NLM_F_DUMP     Convenience macro; equivalent to
155                      (NLM_F_ROOT|NLM_F_MATCH).
156
157       Note  that NLM_F_ATOMIC requires the CAP_NET_ADMIN capability or an ef‐
158       fective UID of 0.
159
160       Additional flag bits for NEW requests
161       ────────────────────────────────────────────────────────────
162       NLM_F_REPLACE   Replace existing matching object.
163       NLM_F_EXCL      Don't replace if the object already exists.
164       NLM_F_CREATE    Create object if it doesn't already exist.
165       NLM_F_APPEND    Add to the end of the object list.
166
167       nlmsg_seq and nlmsg_pid are used to track  messages.   nlmsg_pid  shows
168       the  origin  of  the message.  Note that there isn't a 1:1 relationship
169       between nlmsg_pid and the PID of the process if the message  originated
170       from a netlink socket.  See the ADDRESS FORMATS section for further in‐
171       formation.
172
173       Both nlmsg_seq and nlmsg_pid are opaque to netlink core.
174
175       Netlink is not a reliable protocol.  It tries its  best  to  deliver  a
176       message  to  its  destination(s), but may drop messages when an out-of-
177       memory condition or other error  occurs.   For  reliable  transfer  the
178       sender  can request an acknowledgement from the receiver by setting the
179       NLM_F_ACK flag.  An acknowledgment is an NLMSG_ERROR  packet  with  the
180       error  field  set to 0.  The application must generate acknowledgements
181       for received messages itself.  The kernel tries to send an  NLMSG_ERROR
182       message  for  every  failed  packet.  A user process should follow this
183       convention too.
184
185       However, reliable transmissions from kernel to user are  impossible  in
186       any case.  The kernel can't send a netlink message if the socket buffer
187       is full: the message will be dropped and the kernel and the  user-space
188       process will no longer have the same view of kernel state.  It is up to
189       the application to detect when this happens (via the ENOBUFS error  re‐
190       turned by recvmsg(2)) and resynchronize.
191
192   Address formats
193       The  sockaddr_nl  structure describes a netlink client in user space or
194       in the kernel.  A sockaddr_nl can be either unicast (only sent  to  one
195       peer) or sent to netlink multicast groups (nl_groups not equal 0).
196
197           struct sockaddr_nl {
198               sa_family_t     nl_family;  /* AF_NETLINK */
199               unsigned short  nl_pad;     /* Zero */
200               pid_t           nl_pid;     /* Port ID */
201               __u32           nl_groups;  /* Multicast groups mask */
202           };
203
204       nl_pid  is the unicast address of netlink socket.  It's always 0 if the
205       destination is in the kernel.  For a user-space process, nl_pid is usu‐
206       ally  the  PID  of the process owning the destination socket.  However,
207       nl_pid identifies a netlink socket, not a process.  If a  process  owns
208       several  netlink  sockets,  then  nl_pid can be equal to the process ID
209       only for at most one socket.  There are two ways to assign nl_pid to  a
210       netlink socket.  If the application sets nl_pid before calling bind(2),
211       then it is up to the application to make sure that  nl_pid  is  unique.
212       If the application sets it to 0, the kernel takes care of assigning it.
213       The kernel assigns the process ID  to  the  first  netlink  socket  the
214       process  opens and assigns a unique nl_pid to every netlink socket that
215       the process subsequently creates.
216
217       nl_groups is a bit mask with every bit  representing  a  netlink  group
218       number.   Each  netlink  family has a set of 32 multicast groups.  When
219       bind(2) is called on the socket, the nl_groups field in the sockaddr_nl
220       should be set to a bit mask of the groups which it wishes to listen to.
221       The default value for this field is zero which means that no multicasts
222       will be received.  A socket may multicast messages to any of the multi‐
223       cast groups by setting nl_groups to a bit mask of the groups it  wishes
224       to  send  to  when it calls sendmsg(2) or does a connect(2).  Only pro‐
225       cesses with an effective UID of 0 or the CAP_NET_ADMIN  capability  may
226       send  or listen to a netlink multicast group.  Since Linux 2.6.13, mes‐
227       sages can't be broadcast to multiple groups.  Any replies to a  message
228       received  for  a multicast group should be sent back to the sending PID
229       and the multicast group.  Some Linux kernel subsystems may additionally
230       allow  other  users  to send and/or receive messages.  As at Linux 3.0,
231       the   NETLINK_KOBJECT_UEVENT,   NETLINK_GENERIC,   NETLINK_ROUTE,   and
232       NETLINK_SELINUX  groups  allow  other  users  to  receive messages.  No
233       groups allow other users to send messages.
234
235   Socket options
236       To set or get a netlink socket option, call getsockopt(2)  to  read  or
237       setsockopt(2) to write the option with the option level argument set to
238       SOL_NETLINK.  Unless otherwise noted, optval is a pointer to an int.
239
240       NETLINK_PKTINFO (since Linux 2.6.14)
241              Enable nl_pktinfo control messages for received packets  to  get
242              the extended destination group number.
243
244       NETLINK_ADD_MEMBERSHIP, NETLINK_DROP_MEMBERSHIP (since Linux 2.6.14)
245              Join/leave a group specified by optval.
246
247       NETLINK_LIST_MEMBERSHIPS (since Linux 4.2)
248              Retrieve  all  groups  a  socket  is  a  member of.  optval is a
249              pointer to __u32 and optlen is the size of the array.  The array
250              is  filled  with  the full membership set of the socket, and the
251              required array size is returned in optlen.
252
253       NETLINK_BROADCAST_ERROR (since Linux 2.6.30)
254              When not set, netlink_broadcast() only reports ESRCH errors  and
255              silently ignore ENOBUFS errors.
256
257       NETLINK_NO_ENOBUFS (since Linux 2.6.30)
258              This  flag  can  be  used  by unicast and broadcast listeners to
259              avoid receiving ENOBUFS errors.
260
261       NETLINK_LISTEN_ALL_NSID (since Linux 4.2)
262              When set, this socket will receive  netlink  notifications  from
263              all  network namespaces that have an nsid assigned into the net‐
264              work namespace where the socket has been opened.   The  nsid  is
265              sent to user space via an ancillary data.
266
267       NETLINK_CAP_ACK (since Linux 4.2)
268              The  kernel  may fail to allocate the necessary room for the ac‐
269              knowledgment message back to user space.  This option trims  off
270              the  payload  of the original netlink message.  The netlink mes‐
271              sage header is still included, so the user can  guess  from  the
272              sequence number which message triggered the acknowledgment.
273

VERSIONS

275       The socket interface to netlink first appeared Linux 2.2.
276
277       Linux  2.0  supported  a  more primitive device-based netlink interface
278       (which is still available as a compatibility  option).   This  obsolete
279       interface is not described here.
280

NOTES

282       It  is often better to use netlink via libnetlink or libnl than via the
283       low-level kernel interface.
284

BUGS

286       This manual page is not complete.
287

EXAMPLES

289       The following example creates a NETLINK_ROUTE netlink socket which will
290       listen  to  the  RTMGRP_LINK  (network  interface create/delete/up/down
291       events) and RTMGRP_IPV4_IFADDR (IPv4 addresses add/delete events)  mul‐
292       ticast groups.
293
294           struct sockaddr_nl sa;
295
296           memset(&sa, 0, sizeof(sa));
297           sa.nl_family = AF_NETLINK;
298           sa.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR;
299
300           fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
301           bind(fd, (struct sockaddr *) &sa, sizeof(sa));
302
303       The next example demonstrates how to send a netlink message to the ker‐
304       nel (pid 0).  Note that the application must take care of  message  se‐
305       quence numbers in order to reliably track acknowledgements.
306
307           struct nlmsghdr *nh;    /* The nlmsghdr with payload to send */
308           struct sockaddr_nl sa;
309           struct iovec iov = { nh, nh->nlmsg_len };
310           struct msghdr msg;
311
312           msg = { &sa, sizeof(sa), &iov, 1, NULL, 0, 0 };
313           memset(&sa, 0, sizeof(sa));
314           sa.nl_family = AF_NETLINK;
315           nh->nlmsg_pid = 0;
316           nh->nlmsg_seq = ++sequence_number;
317           /* Request an ack from kernel by setting NLM_F_ACK */
318           nh->nlmsg_flags |= NLM_F_ACK;
319
320           sendmsg(fd, &msg, 0);
321
322       And the last example is about reading netlink message.
323
324           int len;
325           /* 8192 to avoid message truncation on platforms with
326              page size > 4096 */
327           struct nlmsghdr buf[8192/sizeof(struct nlmsghdr)];
328           struct iovec iov = { buf, sizeof(buf) };
329           struct sockaddr_nl sa;
330           struct msghdr msg;
331           struct nlmsghdr *nh;
332
333           msg = { &sa, sizeof(sa), &iov, 1, NULL, 0, 0 };
334           len = recvmsg(fd, &msg, 0);
335
336           for (nh = (struct nlmsghdr *) buf; NLMSG_OK (nh, len);
337                nh = NLMSG_NEXT (nh, len)) {
338               /* The end of multipart message */
339               if (nh->nlmsg_type == NLMSG_DONE)
340                   return;
341
342               if (nh->nlmsg_type == NLMSG_ERROR)
343                   /* Do some error handling */
344               ...
345
346               /* Continue with parsing payload */
347               ...
348           }
349

SEE ALSO

351       cmsg(3), netlink(3), capabilities(7), rtnetlink(7), sock_diag(7)
352
353       information about libnetlink ⟨ftp://ftp.inr.ac.ru/ip-routing/iproute2*⟩
354
355       information about libnl ⟨http://www.infradead.org/~tgr/libnl/⟩
356
357       RFC 3549 "Linux Netlink as an IP Services Protocol"
358

COLOPHON

360       This  page  is  part of release 5.10 of the Linux man-pages project.  A
361       description of the project, information about reporting bugs,  and  the
362       latest     version     of     this    page,    can    be    found    at
363       https://www.kernel.org/doc/man-pages/.
364
365
366
367Linux                             2020-06-09                        NETLINK(7)
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