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'\" t
.\" Don't change the first line, it tells man that tbl is needed.
.\" This man page copyright 1998 by Andi Kleen. Subject to the GPL.
.\" This manpage copyright 1998 by Andi Kleen. Subject to the GPL.
.\" Based on the original comments from Alexey Kuznetsov
.\" $Id: netlink.7,v 1.8 2000/06/22 13:23:00 ak Exp $
.TH NETLINK 7 1999-04-27 "Linux Man Page" "Linux Programmer's Manual"
.SH NAME
netlink, PF_NETLINK \- Communication between kernel and user
.SH SYNOPSIS
.nf
.\" XXX
.B #include <asm/types.h>
.br
.B #include <sys/socket.h>
.br
.B #include <linux/netlink.h>
.br
.PP
.BI "netlink_socket = socket(PF_NETLINK, " socket_type ", " netlink_family );
.SH 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
.B SOCK_RAW
and
.B SOCK_DGRAM
are valid values for
.IR socket_type ;
however the netlink protocol does not distinguish between
datagram and raw sockets.
.I netlink_family
selects the kernel module or netlink group to communicate with.
The currently assigned netlink families are:
.TP
.B NETLINK_ROUTE
Receives routing updates and may be used to modify the IPv4 routing
table (see
.BR rtnetlink (7)).
.TP
.B NETLINK_FIREWALL
Receives packets sent by the IPv4 firewall code.
.TP
.B NETLINK_ARPD
For managing the arp table in user space.
.TP
.B NETLINK_ROUTE6
Receives and sends IPv6 routing table updates.
.TP
.B NETLINK_IP6_FW
to receive packets that failed the IPv6 firewall checks (currently not
implemented).
.TP
.BR NETLINK_TAPBASE ... NETLINK_TAPBASE+15
are the instances of the
.B ethertap
device. Ethertap is a pseudo network tunnel device that allows an
ethernet driver to be simulated from user space.
.TP
.B NETLINK_SKIP
Reserved for ENskip.
.TP
.B NETLINK_USERSOCK
is reserved for future user space protocols.
.PP
Netlink messages consist of a byte stream with one or multiple
.B nlmsghdr
headers and associated payload.
For multipart messages the first and all following headers have the
.B NLM_F_MULTI
flag set, except for the last header
which has the type
.BR NLMSG_DONE .
The byte stream should only be accessed with the standard
.B NLMSG_*
macros, see
.BR 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
.B NLM_F_ACK
flag. An acknowledgment is an
.B 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
.B 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
.BR bind (2)
is called on the socket, the
.B nl_groups
field in the
.B 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
.B nl_groups
to a bitmask of the groups it wishes to send to when it calls
.BR sendmsg (2)
or does a
.BR connect (2).
Only users with an effective uid of 0 or the
.B 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.
.RS
.nf
.ta 4 13 25
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 */
};
.ta
.fi
.RE
After each
.B nlmsghdr
the payload follows.
.B nlmsg_type
can be one of the standard message types:
.B NLMSG_NOOP
message is to be ignored,
.B NLMSG_ERROR
the message signals an error and the payload contains a
.I nlmsgerr
structure,
.B NLMSG_DONE
message terminates a multipart message,
.\" 2.1.130 does not seem to use it.
.\" .B NLMSG_OVERRUN
.\" data was lost.
A netlink family usually specifies more message types, see the
appropriate man pages for that, e.g.
.BR rtnetlink (7)
for
.IR NETLINK_ROUTE .
.TS
tab(:);
l s
l l.
Standard Flag bits in nlmsg_flags
NLM_F_REQUEST:set on all request messages
NLM_F_MULTI:T{
the message is part of a multipart message terminated by
.B
NLMSG_DONE
.\" XXX describe that
T}
NLM_F_ACK:reply with an acknowledgment on success
NLM_F_ECHO:echo this request
.TE
.TS
tab(:);
l s
l l.
Additional flag bits for GET requests
NLM_F_ROOT:Return the complete table instead of a single entry.
NLM_F_MATCH:Not implemented yet.
NLM_F_ATOMIC:Return an atomic snapshot of the table.
NLM_F_DUMP:not documented yet.
.TE
.TS
tab(:);
l s
l l.
Additional flag bits for NEW requests
NLM_F_REPLACE:Override existing 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.
.TE
Note that NLM_F_ATOMIC requires CAP_NET_ADMIN or super user rights.
.SH "ADDRESS FORMATS"
The
.B 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).
.RS
.nf
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 */
};
.fi
.RE
.B nl_pid
is the pid of the process owning the destination socket, or 0 if the
destination is in the kernel.
.B nl_groups
is a bitmask with every bit representing a netlink group number.
.\" XXX describe what that is.
.SH BUGS
This man page is not complete.
.SH NOTES
It is often better to use netlink via
.B libnetlink
than via the low level kernel interface.
.SH 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.
.SH "SEE ALSO"
.BR cmsg (3),
.BR netlink (3),
.BR capabilities (7),
.BR rtnetlink (7)
.PP
ftp://ftp.inr.ac.ru/ip-routing/iproute2*
for libnetlink
|
