View Source: netlink(7) - Waikato Linux Users Group

Edit PageHistory Diff Info LikePages
NETLINK
!!!NETLINK
NAME
SYNOPSIS
DESCRIPTION
ADDRESS FORMATS
BUGS
NOTES
VERSIONS
SEE ALSO
----
!!NAME


netlink, PF_NETLINK - Communication between kernel and user.
!!SYNOPSIS


__#include
__ ''socket_type''__,__ ''netlink_family''__);
__
!!DESCRIPTION


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:


__NETLINK_ROUTE__


Receives routing updates and may be used to modify the IPv4
routing table (see rtnetlink(7)).


__NETLINK_FIREWALL__


Receives packets sent by the IPv4 firewall
code.


__NETLINK_ARPD__


For managing the arp table in user space.


__NETLINK_ROUTE6__


Receives and sends 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 an ethernet
driver to be simulated from user space.


__NETLINK_SKIP__


Reserved for ENskip.


__NETLINK_USERSOCK__


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''.


Note that NLM_F_ATOMIC requires CAP_NET_ADMIN or super user rights.
!!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 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.
!!BUGS


This man page is not complete.
!!NOTES


It is often better to use netlink via __libnetlink__ than
via the low level kernel interface.
!!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.
!!SEE ALSO


cmsg(3), rtnetlink(7),
netlink(3)


ftp://ftp.inr.ac.ru/ip-routing/iproute2* for
libnetlink
----
10 pages link to netlink(7):
This page is a man page (or other imported legacy content). We are unable to automatically determine the license status of this page.
Last edited on Tuesday, June 4, 2002 12:30:59 am by "perry"
Edit PageHistory Diff Info LikePages