Ubuntu Manpage: netlink - communication between kernel and user space (AF

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 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
Messages from 1-wire subsystem.

NETLINK_USERSOCK
Reserved for user-mode socket protocols.

NETLINK_FIREWALL
Transport IPv4 packets from netfilter to user space. Used by ip_queue kernel
module.

NETLINK_INET_DIAG
INET socket monitoring.

NETLINK_NFLOG
Netfilter/iptables ULOG.

NETLINK_XFRM
IPsec.

NETLINK_SELINUX
SELinux event notifications.

NETLINK_ISCSI
Open-iSCSI.

NETLINK_AUDIT
Auditing.

NETLINK_FIB_LOOKUP
Access to FIB lookup from user space.

NETLINK_CONNECTOR
Kernel connector. See Documentation/connector/* in the Linux kernel source tree
for further information.

NETLINK_NETFILTER
Netfilter subsystem.

NETLINK_IP6_FW
Transport IPv6 packets from netfilter to user space. Used by ip6_queue kernel
module.

NETLINK_DNRTMSG
DECnet routing messages.

NETLINK_KOBJECT_UEVENT
Kernel messages to user space.

NETLINK_GENERIC
Generic netlink family for simplified netlink usage.

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.

VERSIONS

       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.

       NETLINK_SELINUX appeared in Linux 2.6.4.

       NETLINK_AUDIT appeared in Linux 2.6.6.

       NETLINK_KOBJECT_UEVENT appeared in Linux 2.6.10.

       NETLINK_W1 and NETLINK_FIB_LOOKUP appeared in Linux 2.6.13.

       NETLINK_INET_DIAG, NETLINK_CONNECTOR and NETLINK_NETFILTER appeared in Linux 2.6.14.

       NETLINK_GENERIC and NETLINK_ISCSI appeared in Linux 2.6.15.

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  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[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. */
               ...
           }

COLOPHON

       This page is part of release 3.54 of the Linux man-pages project.  A  description  of  the
       project,     and    information    about    reporting    bugs,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.