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 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 communi-
       cate 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 parame-
              ters, 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.
              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 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 pay-
       load 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  multi-
       part 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 appro-
       priate  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 mes-
                       sage terminated by NLMSG_DONE.

                      (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 usu-
       ally 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
       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 multi-
       cast  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  pro-
       cesses  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,  mes-
       sages  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
       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
       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.

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) mul-
       ticast 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);
           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
       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
       This page is part of release 3.54 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2013-03-15                        NETLINK(7)
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