netlink(7) -- Linux man page



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);


Netlink is used to transfer information between kernel modules and user space processes. 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. Also there is an obsolete netlink interface via netlink character devices, this interface is not documented here and is only provided for backwards 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:

Receives routing updates and may be used to modify the IPv4 routing table (see rtnetlink(7)).
Receives packets sent by the IPv4 firewall code.
For managing the arp table in user space.
Receives and sends IPv6 routing table updates.
to receive packets that failed the IPv6 firewall checks (currently not implemented).
are the instances of the ethertap device. Ethertap is a pseudo network tunnel device that allows an ethernet driver to be simulated from user space.
Reserved for ENskip.
is reserved 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 only be accessed with the standard NLMSG_* macros, see netlink(3).

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 acks 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.

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 bitmask 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 bitmask of the groups it wishes to send to when it calls sendmsg(2) or does a connect(2). Only users 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.

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;  /* PID of the process that opened the socket */

struct nlmsgerr
    int      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 specifies more message types, see the appropriate man pages for that, e.g. rtnetlink(7) 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.
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.

Note that NLM_F_ATOMIC requires CAP_NET_ADMIN or super user rights.



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 process owning the destination socket, or 0 if the destination is in the kernel. nl_groups is a bitmask with every bit representing a netlink group number.



This man page is not complete.



It is often better to use netlink via libnetlink than via the low level kernel interface.



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 obsolete interface is not described here.



cmsg(3), rtnetlink(7), netlink(3)* for libnetlink