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netlink (4)
  • netlink (3) ( Русские man: Библиотечные вызовы )
  • netlink (3) ( Linux man: Библиотечные вызовы )
  • >> netlink (4) ( Linux man: Специальные файлы /dev/* )
  • netlink (7) ( Русские man: Макропакеты и соглашения )
  • netlink (7) ( Linux man: Макропакеты и соглашения )


    netlink, PF_NETLINK - Communication between kernel and user.


    #include <asm/types.h>
    #include <sys/socket.h>
    #include <linux/netlink.h>
    netlink_socket = socket(PF_NETLINK, socket_type, netlink_family);

    int NLMSG_ALIGN(size_t len);
    int NLMSG_LENGTH(size_t len);
    int NLMSG_SPACE(size_t len);
    void *NLMSG_DATA(struct nlmsghdr *);
    struct nlmsghdr *NLMSG_NEXT(struct nlmsghdr *, int length);
    int NLMSG_OK(struct nlmsghdr *, int length);
    int NLMSG_PAYLOAD(struct nlmsghdr *, int length);  


    Netlink is used to transfer information between kernel and user space modules. It consists of a standard sockets based interface for user processes and an internal kernel API for kernel modules. The internal kernel interface is not documented in this man page.

    Netlink is a datagram oriented service. Valid socket types are SOCK_RAW and SOCK_DGRAM.

    netlink_family selects the kernel module or netlink group to communicate with. Currently assigned netlink families are: NETLINK_ROUTE to receive routing updates and to modify the IP routing table. NETLINK_FIREWALL to receive packets sent by the IPv4 firewall code. NETLINK_ARPD to manage the arp table in user space. NETLINK_ROUTE6 to receive and send IPv6 routing table updates. NETLINK_IP6_FW to receive packets that failed the IPv6 firewall checks (currently not implemented). NETLINK_TAPBASE ... NETLINK_TAPBASE+15 are the instances of the ethertap device. Ethertap is a pseudo network tunnel device that allows to simulate an ethernet driver from user space. Reserved are NETLINK_SKIP for ENskip and NETLINK_USERSOCK for future user space protocols.

    Netlink messages consist of a byte stream with one or multiple nlmsghdr headers and associated payload. For multipart messages the first and all following headers have the NLM_F_MULTI flag set, except for the last header which has the type NLMSG_DONE. The byte stream should be only accessed with the standard NLMSG_* macros.

    Netlink is pseudo reliable. This means 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 a NLMSG_ERROR packet with the error field set to 0. The user has to handle generating acks for received messages himself. The kernel tries to send an NLMSG_ERROR message for every failed packet.

    struct nlmsghdr
            __u32           nlmsg_len;      /* Length of message including header */
            __u16           nlmsg_type;     /* Message content */
            __u16           nlmsg_flags;    /* Additional flags */
            __u32           nlmsg_seq;      /* Sequence number */
            __u32           nlmsg_pid;      /* Sending process PID */
    struct nlmsgerr
            int     error;          /* error: negative errno or 0 for acks. */ 
            struct nlmsghdr msg;    /* message header that caused the error */ 

    After each nlmsghdr the payload follows. nlmsg_type can be one of the standard message types: NLMSG_NOOP message is to be ignored, NLMSG_ERROR the message signals an error and the payload contains a nlmsgerr structure, NLMSG_DONE message terminates a multipart message,

    A netlink family usually has more specified message types, see the appropiate man pages for that, e.g. rtnetlink(4) for NETLINK_ROUTE.

    Standard Flag bits in nlmsg_flags
    NLM_F_REQUEST:set on all request messages

    the message is part of a multipart message terminated by



    NLM_F_ACK:reply with an acknowledgment on success

    NLM_F_ECHO:echo this request

    Additional flag bits for GET requests
    NLM_F_ROOTReturn the complete table instead of a single entry.
    NLM_F_MATCHNot implemented yet.
    NLM_F_ATOMICReturn an atomic snapshot of the table. Requires CAP_NET_ADMIN or super user rights.
    NLM_F_DUMPnot documented yet.

    Additional flag bits for NEW requests
    NLM_F_REPLACEOverride existing object.
    NLM_F_EXCLDon't replace if the object already exists.
    NLM_F_CREATECreate object if it doesn't already exist.
    NLM_F_APPENDAdd to the end of the object list.



    The sockaddr_nl structure describes a netlink client in user space or in the kernel. A sockaddr_nl can be either unicast (only send to one peer) or send to netlink 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 pid */
        __u32       nl_groups;    /* multicast groups mask */

    nl_pid is the pid of the user space netlink, or 0 if the destination is in the kernel. nl_groups is a bitmask with every bit representing a netlink group number.



    Netlink defines several standard macros to access or create a netlink datagram. They are similar in spirit to the macros defined in cmsg(3) for auxilliary data. The buffer passed to and from a netlink socket should be only accessed using these macros.
    Round the length of a netlink message up to align it properly.
    Gets the payload length as argument and returns the aligned length to store in the nlmsg_len field of the nlmsghdr.

    Return the number of bytes a netlink message with payload of the passed length would occupy.
    Return a pointer to the payload associated with the passed nlmsghdr
    NLMSG_NEXT Get the next nlmsghdr in a multipart message. The caller must check if the current nlmsghdr didn't have the NLMSG_DONE set - this function doesn't return NULL on end. The length parameter is an lvalue containing the remaining length of the message buffer. This macro decrements it by the length of the message header.
    Return true if the netlink message is not truncated and ok to parse.
    Return the length of the payload associated with the nlmsghdr



    This man page is not complete.



    cmsg(3), rtnetlink(4)



    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 compatibility option). This old interface is not described here.




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