bionic (7) netlink.7.gz

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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 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 (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  feature),
              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 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.

   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 network 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) 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[8192];     /* 8192 to avoid message truncation on
                                  platforms with page size > 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

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       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       https://www.kernel.org/doc/man-pages/.