NETLINK(7)

NAME
netlink - Communication between kernel and userspace (PF_NETLINK)

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

netlink_socket = socket(PF_NETLINK, socket_type, netlink_family);

DESCRIPTION
Netlink is used to transfer information between kernel and userspace processes. It consists of a standard sockets-based interface for userspace 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 only provided for backwards 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, neighbour setups, queueing disciplines, traffic classes and packet classifiers (see rtnetlink(7)).

NETLINK_W1
    Messages from 1-wire subsystem.

NETLINK_USERSOCK
    Reserved for user-mode socket protocols.

NETLINK_FIREWALL
    Transport IPv4 packets from netfilter to userspace. Used by ip_queue kernel module.

NETLINK_INET_DIAG
    INET socket monitoring.

NETLINK_NFLOG
    Netfilter/iptables ULOG.

NETLINK_XFRM
    IPsec.

NETLINK_SELINUX
    SELinux event notifications.

NETLINK_ISCSI
    Open-iSCSI.

NETLINK_AUDIT
    Auditing.

NETLINK_FIB_LOOKUP
    Access to FIB lookup from userspace.

NETLINK_CONNECTOR
    Kernel connector. See Documentation/connector/* in the kernel source for further information.

NETLINK_NETFILTER
    Netfilter subsystem.

NETLINK_IP6_FW
    Transport IPv6 packets from netfilter to userspace. Used by ip6_queue kernel module.

NETLINK_DNRTMSG
    DECnet routing messages.

NETLINK_KOBJECT_UEVENT
    Kernel messages to userspace.

NETLINK_GENERIC
    Generic netlink family for simplified netlink usage.

Netlink messages consist of a byte stream with one or multiple nlmsghdr headers and associated payload. The byte stream should only be accessed 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;    /* PID of the sending process. */
};

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, e.g. 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
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 userspace 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 resynchronise.

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;     /* Process 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 userspace 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 only be equal to the process ID 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 bitmask 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 bitmask 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 bitmask 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. Any replies to a message received for a multicast group should be sent back to the sending PID and the multicast group.

EXAMPLE
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));
snl.nl_family = AF_NETLINK;
snl.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 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 = { (void *) nh, nh->nlmsg_len };
struct msghdr msg;

msg = { (void *)&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;
char buf[4096];
struct iovec iov = { buf, sizeof(buf) };
struct sockaddr_nl sa;
struct msghdr msg;
struct nlmsghdr *nh;

msg = { (void *)&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. */
    ...
}

BUGS
This manual page is not complete.

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

VERSIONS
The socket interface to netlink is a new feature of 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.

NETLINK_SELINUX appeared in Linux 2.6.4.

NETLINK_AUDIT appeared in Linux 2.6.6.

NETLINK_KOBJECT_UEVENT appeared in Linux 2.6.10.

NETLINK_W1 and NETLINK_FIB_LOOKUP appeared in Linux 2.6.13.

NETLINK_INET_DIAG, NETLINK_CONNECTOR and NETLINK_NETFILTER appeared in Linux 2.6.14.

NETLINK_GENERIC and NETLINK_ISCSI appeared in Linux 2.6.15.

SEE ALSO
cmsg(3), netlink(3), capabilities(7), rtnetlink(7)

ftp://ftp.inr.ac.ru/ip-routing/iproute2* for information about libnetlink.

http://people.suug.ch/~tgr/libnl/ for information about libnl.

RFC 3549 "Linux Netlink as an IP Services Protocol"