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 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.
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.
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 4.2)
- 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.
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
information
about libnl
RFC 3549 "Linux Netlink as an IP Services Protocol"