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Michael Kerrisk
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man7/netlink.7
124
man7/netlink.7
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@ -14,38 +14,38 @@ netlink \- Communication between kernel and userspace (PF_NETLINK)
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.br
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.B #include <sys/socket.h>
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.br
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.B #include <linux/netlink.h>
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.B #include <linux/netlink.h>
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.br
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.PP
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.BI "netlink_socket = socket(PF_NETLINK, " socket_type ", " netlink_family );
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.BI "netlink_socket = socket(PF_NETLINK, " socket_type ", " netlink_family );
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.SH DESCRIPTION
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Netlink is used to transfer information between kernel and
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Netlink is used to transfer information between kernel and
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userspace processes.
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It consists of a standard sockets-based interface for userspace
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processes and an internal kernel API for kernel modules.
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The internal kernel interface is not documented in this manual page.
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It consists of a standard sockets-based interface for userspace
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processes and an internal kernel API for kernel modules.
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The internal kernel interface is not documented in this manual page.
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There is also an obsolete netlink interface
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via netlink character devices; this interface is not documented here
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via netlink character devices; this interface is not documented here
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and is only provided for backwards compatibility.
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Netlink is a datagram-oriented service. Both
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.B SOCK_RAW
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.B SOCK_RAW
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and
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.B SOCK_DGRAM
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are valid values for
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.IR socket_type .
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However, the netlink protocol does not distinguish between datagram
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However, the netlink protocol does not distinguish between datagram
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and raw sockets.
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.I netlink_family
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selects the kernel module or netlink group to communicate with.
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selects the kernel module or netlink group to communicate with.
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The currently assigned netlink families are:
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.TP
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.B NETLINK_ROUTE
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Receives routing and link updates and may be used to modify the routing
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tables (both IPv4 and IPv6), IP addresses, link parameters,
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neighbour setups, queueing disciplines, traffic classes and
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packet classifiers (see
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tables (both IPv4 and IPv6), IP addresses, link parameters,
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neighbour setups, queueing disciplines, traffic classes and
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packet classifiers (see
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.BR rtnetlink (7)).
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.TP
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.B NETLINK_W1
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@ -54,9 +54,9 @@ Messages from 1-wire subsystem.
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.B NETLINK_USERSOCK
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Reserved for user-mode socket protocols.
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.TP
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.B NETLINK_FIREWALL
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Transport IPv4 packets from netfilter to userspace. Used by
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.I ip_queue
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.B NETLINK_FIREWALL
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Transport IPv4 packets from netfilter to userspace. Used by
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.I ip_queue
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kernel module.
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.TP
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.B NETLINK_INET_DIAG
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@ -95,8 +95,8 @@ in the kernel source for further information.
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Netfilter subsystem.
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.TP
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.B NETLINK_IP6_FW
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Transport IPv6 packets from netfilter to userspace. Used by
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.I ip6_queue
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Transport IPv6 packets from netfilter to userspace. Used by
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.I ip6_queue
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kernel module.
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.TP
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.B NETLINK_DNRTMSG
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@ -109,9 +109,9 @@ Kernel messages to userspace.
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.B NETLINK_GENERIC
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Generic netlink family for simplified netlink usage.
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.PP
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Netlink messages consist of a byte stream with one or multiple
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Netlink messages consist of a byte stream with one or multiple
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.I nlmsghdr
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headers and associated payload.
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headers and associated payload.
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The byte stream should only be accessed with the standard
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.B NLMSG_*
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macros. See
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@ -168,16 +168,16 @@ appropriate manual pages for that, e.g.
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for
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.BR NETLINK_ROUTE .
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Standard flag bits in
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Standard flag bits in
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.I nlmsg_flags
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.br
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---------------------------------
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.TS
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tab(:);
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.TS
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tab(:);
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lB l.
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NLM_F_REQUEST:Must be set on all request messages.
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NLM_F_MULTI:T{
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The message is part of a multipart message terminated by
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The message is part of a multipart message terminated by
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.BR NLMSG_DONE .
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T}
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NLM_F_ACK:Request for an acknowledgment on success.
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@ -192,7 +192,7 @@ tab(:);
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lB l.
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NLM_F_ROOT:Return the complete table instead of a single entry.
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NLM_F_MATCH:T{
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Return all entries matching criteria passed in message content.
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Return all entries matching criteria passed in message content.
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Not implemented yet.
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T}
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.\" FIXME NLM_F_ATOMIC is not used any more?
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@ -203,7 +203,7 @@ NLM_F_DUMP:Convenience macro; equivalent to (NLM_F_ROOT|NLM_F_MATCH).
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Note that
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.B NLM_F_ATOMIC
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requires the
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.B CAP_NET_ADMIN
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.B CAP_NET_ADMIN
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capability or an effective UID of 0.
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Additional flag bits for NEW requests
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@ -223,10 +223,10 @@ and
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.I nlmsg_pid
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are used to track messages.
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.I nlmsg_pid
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shows the origin of the message.
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shows the origin of the message.
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Note that there isn't a 1:1 relationship between
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.I nlmsg_pid
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and the PID of the process if the message originated from a netlink
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.I nlmsg_pid
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and the PID of the process if the message originated from a netlink
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socket.
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See the
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.B ADDRESS FORMATS
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@ -239,26 +239,26 @@ and
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.\" FIXME Explain more about nlmsg_seq and nlmsg_pid.
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are opaque to netlink core.
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Netlink is not a reliable protocol.
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It tries its best to deliver a message to its destination(s),
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Netlink is not a reliable protocol.
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It tries its best to deliver a message to its destination(s),
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but may drop messages when an out-of-memory condition or
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other error occurs. For reliable transfer the sender can request an
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acknowledgement from the receiver by setting the
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.B NLM_F_ACK
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flag. An acknowledgment is an
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.B NLMSG_ERROR
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packet with the error field set to 0.
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packet with the error field set to 0.
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The application must generate acknowledgements for
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received messages itself. The kernel tries to send an
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.B NLMSG_ERROR
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message for every failed packet.
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message for every failed packet.
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A user process should follow this convention too.
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However, reliable transmissions from kernel to user are impossible
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However, reliable transmissions from kernel to user are impossible
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in any case.
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The kernel can't send a netlink message if the socket buffer is full:
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the message will be dropped and the kernel and the userspace process will
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no longer have the same view of kernel state.
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the message will be dropped and the kernel and the userspace process will
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no longer have the same view of kernel state.
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It is up to the application to detect when this happens (via the
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.B ENOBUFS
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error returned by
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@ -269,12 +269,12 @@ and resynchronise.
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The
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.I sockaddr_nl
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structure describes a netlink client in user space or in the kernel.
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A
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.I sockaddr_nl
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A
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.I sockaddr_nl
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can be either unicast (only sent to one peer) or sent to
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netlink multicast groups
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.RI ( nl_groups
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not equal 0).
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netlink multicast groups
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.RI ( nl_groups
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not equal 0).
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.in +0.25i
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.nf
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@ -289,36 +289,36 @@ struct sockaddr_nl {
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.I nl_pid
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is the unicast address of netlink socket.
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It's always 0 if the destination is in the kernel.
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It's always 0 if the destination is in the kernel.
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For a userspace process,
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.I nl_pid
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is usually the PID of the process owning the destination socket.
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is usually the PID of the process owning the destination socket.
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However,
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.I nl_pid
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identifies a netlink socket, not a process.
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identifies a netlink socket, not a process.
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If a process owns several netlink
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sockets, then
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.I nl_pid
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can only be equal to the process ID for at most one socket.
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.I nl_pid
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can only be equal to the process ID for at most one socket.
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There are two ways to assign
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.I nl_pid
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to a netlink socket.
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to a netlink socket.
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If the application sets
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.I nl_pid
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before calling
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.BR bind (2),
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then it is up to the application to make sure that
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.I nl_pid
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is unique.
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is unique.
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If the application sets it to 0, the kernel takes care of assigning it.
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The kernel assigns the process ID to the first netlink socket the process
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opens and assigns a unique
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.I nl_pid
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to every netlink socket that the process subsequently creates.
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.I nl_groups
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.I nl_groups
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is a bitmask with every bit representing a netlink group number.
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Each netlink family has a set of 32 multicast groups.
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Each netlink family has a set of 32 multicast groups.
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When
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.BR bind (2)
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is called on the socket, the
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@ -337,16 +337,16 @@ or does a
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Only processes with an effective UID of 0 or the
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.B CAP_NET_ADMIN
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capability may send or listen to a netlink multicast group.
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Any replies to a message received for a multicast group should be
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Any replies to a message received for a multicast group should be
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sent back to the sending PID and the multicast group.
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.SH EXAMPLES
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The following example creates a
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.B NETLINK_ROUTE
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.B NETLINK_ROUTE
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netlink socket which will listen to the
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.B RTMGRP_LINK
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.B RTMGRP_LINK
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(network interface create/delete/up/down events) and
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.B RTMGRP_IPV4_IFADDR
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.B RTMGRP_IPV4_IFADDR
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(IPv4 addresses add/delete events) multicast groups.
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.in +0.25i
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@ -362,9 +362,9 @@ bind(fd, (struct sockaddr*)&sa, sizeof(sa));
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.fi
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.in -0.25i
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The next example demonstrates how to send a netlink message to the
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kernel (pid 0).
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Note that application must take care of message sequence numbers
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The next example demonstrates how to send a netlink message to the
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kernel (pid 0).
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Note that application must take care of message sequence numbers
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in order to reliably track acknowledgements.
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.in +0.25i
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@ -417,10 +417,10 @@ for (nh = (struct nlmsghdr *) buf; NLMSG_OK (nh, len);
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.in -0.25i
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.SH BUGS
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This manual page is not complete.
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This manual page is not complete.
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.SH NOTES
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It is often better to use netlink via
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It is often better to use netlink via
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.B libnetlink
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or
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.B libnl
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@ -429,8 +429,8 @@ than via the low level kernel interface.
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.SH VERSIONS
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The socket interface to netlink is a new feature of Linux 2.2.
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Linux 2.0 supported a more primitive device based netlink interface
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(which is still available as a compatibility option).
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Linux 2.0 supported a more primitive device based netlink interface
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(which is still available as a compatibility option).
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This obsolete interface is not described here.
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NETLINK_SELINUX appeared in Linux 2.6.4.
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@ -452,7 +452,7 @@ NETLINK_GENERIC and NETLINK_ISCSI appeared in Linux 2.6.15.
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.BR capabilities (7),
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.BR rtnetlink (7)
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.PP
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ftp://ftp.inr.ac.ru/ip-routing/iproute2*
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ftp://ftp.inr.ac.ru/ip-routing/iproute2*
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for information about libnetlink.
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http://people.suug.ch/~tgr/libnl/
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