netlink(7)



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

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 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 (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 fea-
              ture), 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 regis-
              tered 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  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_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 mes-
                      sage 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 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
       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 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.

   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 NOBUFS 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  net-
              work  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.

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.

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);
           bind(fd, (struct sockaddr *) &sa, sizeof(sa));

       The next example demonstrates how to send a netlink message to the ker-
       nel  (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
       cmsg(3), netlink(3), capabilities(7), rtnetlink(7), sock_diag(7)

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

       information about libnl <http://www.infradead.org/~tgr/libnl/>

       RFC 3549 "Linux Netlink as an IP Services Protocol"

COLOPHON
       This page is part of release 4.11 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                             2016-12-12                        NETLINK(7)

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