Ubuntu Manpage: netlink - communication between kernel and user space (AF

NAME

       netlink - communication between kernel and user space (AF_NETLINK)

SYNOPSIS

       #include <asm/types.h>
       #include <sys/socket.h>
       #include <linux/netlink.h>

       netlink_socket = socket(AF_NETLINK, socket_type, netlink_family);

DESCRIPTION

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.

VERSIONS

       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

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

BUGS

       This manual page is not complete.

EXAMPLES

       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 */
               ...
           }