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



NAME
       netlink - Communication between kernel and userspace (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 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 compatibil-
       ity.

       Netlink  is  a datagram-oriented service.  Both SOCK_RAW and SOCK_DGRAM are valid values for socket_type.  How-
       ever, 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  classi-
              fiers (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 mes-
       sage.

           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 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  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 pro-
       cess will no longer have the same view of kernel state.  It is up to the application to detect when  this  hap-
       pens (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;     /* 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 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 con-
       nect(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.

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  compatibil-
       ity 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)  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 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. */
               ...
           }

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"

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-11-11                        NETLINK(7)