man > netlink(7)

netlink(7)                        Miscellaneous Information 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 con‐
       sists  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 Linux 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_SOCK_DIAG (since Linux 3.3)
              Query  information  about  sockets  of  various protocol families from the kernel (see
              sock_diag(7)).

       NETLINK_INET_DIAG (since Linux 2.6.14)
              An obsolete synonym for NETLINK_SOCK_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/driver-api/connector.rst (or  /Documentation/con‐
              nector/connector.*  in Linux 5.2 and earlier) in the Linux kernel source tree for fur‐
              ther 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  associ‐
       ated 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.  Error messages get the  original  request
       appended,  unless the user requests to cap the error message, and get extra error data if re‐
       quested.

           struct nlmsgerr {
               int error;        /* Negative errno or 0 for acknowledgements */
               struct nlmsghdr msg;  /* Message header that caused the error */
               /*
                * followed by the message contents
                * unless NETLINK_CAP_ACK was set
                * or the ACK indicates success (error == 0).
                * For example Generic Netlink message with attributes.
                * message length is aligned with NLMSG_ALIGN()
                */
               /*
                * followed by TLVs defined in enum nlmsgerr_attrs
                * if NETLINK_EXT_ACK was set
                */
           };

       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 acknowledgement 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  mes‐
       sage.   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  destina‐
       tion(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 acknowledgement 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 sock‐
       addr_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 desti‐
       nation 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 re‐
       ceived.  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.  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_KOB‐
       JECT_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  destina‐
              tion 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
              ENOBUFS 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 4.3)
              The  kernel  may  fail  to allocate the necessary room for the acknowledgement message
              back to user space.  This option trims off the payload of the  original  netlink  mes‐
              sage.   The  netlink  message header is still included, so the user can guess from the
              sequence number which message triggered the acknowledgement.

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  in‐
       terface.

BUGS
       This manual page is not complete.

EXAMPLES
       The  following  example  creates a NETLINK_ROUTE netlink socket which will listen to the RTM‐
       GRP_LINK (network interface create/delete/up/down events) and  RTMGRP_IPV4_IFADDR  (IPv4  ad‐
       dresses 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 ac‐
       knowledgements.

           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;
           /* 8192 to avoid message truncation on platforms with
              page size > 4096 */
           struct nlmsghdr buf[8192/sizeof(struct nlmsghdr)];
           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)

       ]8;;ftp://ftp.inr.ac.ru/ip-routing/iproute2*\information about libnetlink]8;;\

       ]8;;http://www.infradead.org/~tgr/libnl/\information about libnl]8;;\

       RFC 3549 "Linux Netlink as an IP Services Protocol"

Linux man-pages 6.7                          2023-10-31                                   netlink(7)