# netlink

## 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 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/connector/connector.*_  in Linux 5.2 and
                  earlier) 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;    /* Size 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 requested.
    
               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 size 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 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 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 _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 **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 message.  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 interface.
    


## BUGS



           This manual page is not complete.
    


## EXAMPLES



           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 size;
               /* 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 };
               size = recvmsg(fd, &msg, 0);
    
               for (nh = (struct nlmsghdr *) buf; NLMSG_OK (nh, size);
                    nh = NLMSG_NEXT (nh, size)) {
                   /* 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 */
                   ...
               }
    


* * *