NAME | SYNOPSIS | DESCRIPTION | VERSIONS | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

NETLINK(7)                Linux Programmer's Manual               NETLINK(7)

NAME         top

       netlink - communication between kernel and user space (AF_NETLINK)

SYNOPSIS         top

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

       netlink_socket = socket(AF_NETLINK, socket_type, netlink_family);

DESCRIPTION         top

       Netlink is used to transfer information between 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
              Messages from 1-wire subsystem.

       NETLINK_USERSOCK
              Reserved for user-mode socket protocols.

       NETLINK_FIREWALL
              Transport IPv4 packets from netfilter to user space.  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 user space.

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

       NETLINK_NETFILTER
              Netfilter subsystem.

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

       NETLINK_DNRTMSG
              DECnet routing messages.

       NETLINK_KOBJECT_UEVENT
              Kernel messages to user space.

       NETLINK_GENERIC
              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.

   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.

VERSIONS         top

       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.

NOTES         top

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

BUGS         top

       This manual page is not complete.

EXAMPLE         top

       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;
           char buf[4096];
           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. */
               ...
           }

SEE ALSO         top

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

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

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

       RFC 3549 "Linux Netlink as an IP Services Protocol"

COLOPHON         top

       This page is part of release 4.06 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                            2015-01-10                       NETLINK(7)