Socket
[Core Netlink API]

Detailed Description

The socket is represented in a structure called the netlink handle, besides the socket, it stores various settings and values related to the socket. Every socket handle has a mandatory association with a set of callbacks which can be used to modify the behaviour when sending/receiving data from the socket.

Socket Attributes

Local Port: The local port is a netlink port identifying the local endpoint. It is used as source address for outgoing messages and will be addressed in replies. It must therefore be unique among all userspace applications. When the socket handle is allocated, a unique port number is generated automatically in the form of 22 bits Process Identifier + 10 bits Arbitary Number. Therefore the library is capable of generating 1024 unique local port numbers for every process. If more sockets are required, the application has to manage port numbers itself using nl_socket_set_local_port().
Group Subscriptions: A socket can subscribe to any number of multicast groups. It will then receive a copy of all messages sent to one of the groups. This method is mainly used for event notification. Prior to kernel 2.6.14, the group subscription was done via bitmask which limited to a total number of groups of 32. With 2.6.14 a new method was added based on continous identifiers which supports an arbitary number of groups. Both methods are supported, see nl_join_groups() respectively nl_socket_add_membership() and nl_socket_drop_membership().
Peer Port: The peer port is a netlink port identifying the peer's endpoint. If no peer port is specified, the kernel will try to autobind to a socket of the specified netlink family automatically. This is very common as typically only one listening socket exists on the kernel side. The peer port can be modified using nl_socket_set_peer_port().
Peer Groups:
File Descriptor: The file descriptor of the socket, it can be accessed via nl_socket_get_fd() to change socket options or monitor activity using poll()/select().
Protocol: Once connected, the socket is bound to stick to one netlink family. This field is invisible, it is maintained automatically. (See nl_connect())
Next Sequence Number: Next available sequence number to be used for the next message being sent out. (Initial value: UNIX time when the socket was allocated.) Sequence numbers can be used via nl_socket_use_seq().
Expected Sequence Number: Expected sequence number in the next message received from the socket. (Initial value: Equal to next sequence number.)
Callbacks Configuration:

1) Creating the netlink handle

 struct nl_handle *handle;

 // Allocate and initialize a new netlink handle
 handle = nl_handle_alloc();

 // Use nl_socket_get_fd() to fetch the file description, for example to
 // put a socket into non-blocking i/o mode.
 fcntl(nl_socket_get_fd(handle), F_SETFL, O_NONBLOCK);

2) Group Subscriptions

 // Event notifications are typically sent to multicast addresses which
 // represented by groups. Join a group to f.e. receive link notifications.
 nl_socket_add_membership(handle, RTNLGRP_LINK);

6) Cleaning up

 // Finally destroy the netlink handle
 nl_handle_destroy(handle);

Allocation

struct nl_handle * nl_handle_alloc (void)

Allocate new netlink socket handle.

struct nl_handle * nl_handle_alloc_cb (struct nl_cb *cb)

Allocate new socket handle with custom callbacks.

void nl_handle_destroy (struct nl_handle *handle)

Destroy netlink handle.

Sequence Numbers

void nl_disable_sequence_check (struct nl_handle *handle)

Disable sequence number checking.

unsigned int nl_socket_use_seq (struct nl_handle *handle)

Use next sequence number.

Source Idenficiation

uint32_t nl_socket_get_local_port (struct nl_handle *handle)

void nl_socket_set_local_port (struct nl_handle *handle, uint32_t port)

Set local port of socket.

Group Subscriptions

int nl_socket_add_membership (struct nl_handle *handle, int group)

Join a group.

int nl_socket_drop_membership (struct nl_handle *handle, int group)

Leave a group.

void nl_join_groups (struct nl_handle *handle, int groups)

Join multicast groups (deprecated).

Peer Identfication

uint32_t nl_socket_get_peer_port (struct nl_handle *handle)

void nl_socket_set_peer_port (struct nl_handle *handle, uint32_t port)

File Descriptor

int nl_socket_get_fd (struct nl_handle *handle)

int nl_socket_set_nonblocking (struct nl_handle *handle)

Set file descriptor of socket handle to non-blocking state.

void nl_socket_enable_msg_peek (struct nl_handle *handle)

Enable use of MSG_PEEK when reading from socket.

void nl_socket_disable_msg_peek (struct nl_handle *handle)

Disable use of MSG_PEEK when reading from socket.

Callback Handler

struct nl_cb * nl_socket_get_cb (struct nl_handle *handle)

void nl_socket_set_cb (struct nl_handle *handle, struct nl_cb *cb)

int nl_socket_modify_cb (struct nl_handle *handle, enum nl_cb_type type, enum nl_cb_kind kind, nl_recvmsg_msg_cb_t func, void *arg)

Modify the callback handler associated to the socket.

Utilities

int nl_set_buffer_size (struct nl_handle *handle, int rxbuf, int txbuf)

Set socket buffer size of netlink handle.

int nl_set_passcred (struct nl_handle *handle, int state)

Enable/disable credential passing on netlink handle.

int nl_socket_recv_pktinfo (struct nl_handle *handle, int state)

Enable/disable receival of additional packet information.

Function Documentation

struct nl_handle* nl_handle_alloc ( void ) [read]

Returns:: Newly allocated netlink socket handle or NULL.

Definition at line 194 of file socket.c.

References nl_cb_alloc().

struct nl_handle* nl_handle_alloc_cb ( struct nl_cb * cb ) [read]

Parameters:

cb

Callback handler

The reference to the callback handler is taken into account automatically, it is released again upon calling nl_handle_destroy().

Returns:: Newly allocted socket handle or NULL.

Definition at line 216 of file socket.c.

void nl_handle_destroy ( struct nl_handle * handle )

Parameters:

handle

Netlink handle.

Definition at line 228 of file socket.c.

Referenced by nl_cache_mngr_free().

void nl_disable_sequence_check ( struct nl_handle * handle )

Parameters:

handle

Netlink handle.

Disables checking of sequence numbers on the netlink handle. This is required to allow messages to be processed which were not requested by a preceding request message, e.g. netlink events.

Note:: This function modifies the NL_CB_SEQ_CHECK configuration in the callback handle associated with the socket.

Definition at line 267 of file socket.c.

References NL_CB_CUSTOM, NL_CB_SEQ_CHECK, and nl_cb_set().

Referenced by nl_cache_mngr_alloc().

unsigned int nl_socket_use_seq ( struct nl_handle * handle )

Parameters:

handle

Netlink handle

Uses the next available sequence number and increases the counter by one for subsequent calls.

Returns:: Unique serial sequence number

Definition at line 282 of file socket.c.

void nl_socket_set_local_port	(	struct nl_handle *	handle,
		uint32_t	port
	)

Parameters:

	handle	Netlink handle
	port	Local port identifier

Assigns a local port identifier to the socket. If port is 0 a unique port identifier will be generated automatically.

Definition at line 307 of file socket.c.

int nl_socket_add_membership	(	struct nl_handle *	handle,
		int	group
	)

Parameters:

	handle	Netlink handle
	group	Group identifier

Joins the specified group using the modern socket option which is available since kernel version 2.6.14. It allows joining an almost arbitary number of groups without limitation.

Make sure to use the correct group definitions as the older bitmask definitions for nl_join_groups() are likely to still be present for backward compatibility reasons.

Returns:: 0 on sucess or a negative error code.

Definition at line 343 of file socket.c.

Referenced by nl_cache_mngr_add().

int nl_socket_drop_membership	(	struct nl_handle *	handle,
		int	group
	)

Parameters:

	handle	Netlink handle
	group	Group identifier

Leaves the specified group using the modern socket option which is available since kernel version 2.6.14.

See also:: nl_socket_add_membership

Returns:: 0 on success or a negative error code.

Definition at line 370 of file socket.c.

Referenced by nl_cache_mngr_add().

void nl_join_groups	(	struct nl_handle *	handle,
		int	groups
	)

Parameters:

	handle	Netlink handle.
	groups	Bitmask of groups to join.

This function defines the old way of joining multicast group which has to be done prior to calling nl_connect(). It works on any kernel version but is very limited as only 32 groups can be joined.

Definition at line 395 of file socket.c.

int nl_socket_set_nonblocking ( struct nl_handle * handle )

Parameters:

handle

Netlink socket

Returns:: 0 on success or a negative error code.

Definition at line 436 of file socket.c.

Referenced by nl_cache_mngr_alloc().

void nl_socket_enable_msg_peek ( struct nl_handle * handle )

Parameters:

handle

Netlink socket

Definition at line 451 of file socket.c.

void nl_socket_disable_msg_peek ( struct nl_handle * handle )

Parameters:

handle

Netlink socket

Definition at line 460 of file socket.c.

int nl_socket_modify_cb	(	struct nl_handle *	handle,
		enum nl_cb_type	type,
		enum nl_cb_kind	kind,
		nl_recvmsg_msg_cb_t	func,
		void *	arg
	)

Parameters:

	handle	netlink handle
	type	which type callback to set
	kind	kind of callback
	func	callback function
	arg	argument to be passwd to callback function

See also:: nl_cb_set

Definition at line 493 of file socket.c.

References nl_cb_set().

Referenced by nl_cache_mngr_alloc().

int nl_set_buffer_size	(	struct nl_handle *	handle,
		int	rxbuf,
		int	txbuf
	)

Parameters:

	handle	Netlink handle.
	rxbuf	New receive socket buffer size in bytes.
	txbuf	New transmit socket buffer size in bytes.

Sets the socket buffer size of a netlink handle to the specified values rxbuf and txbuf. Providing a value of 0 assumes a good default value.

Note:: It is not required to call this function prior to nl_connect().

Returns:: 0 on sucess or a negative error code.

Definition at line 520 of file socket.c.

Referenced by nl_connect().

int nl_set_passcred	(	struct nl_handle *	handle,
		int	state
	)

Parameters:

	handle	Netlink handle
	state	New state (0 - disabled, 1 - enabled)

Returns:: 0 on success or a negative error code

Definition at line 555 of file socket.c.

int nl_socket_recv_pktinfo	(	struct nl_handle *	handle,
		int	state
	)

Parameters:

	handle	Netlink handle
	state	New state (0 - disabled, 1 - enabled)

Returns:: 0 on success or a negative error code

Definition at line 582 of file socket.c.


Allocation
struct nl_handle *	nl_handle_alloc (void)
	Allocate new netlink socket handle.
struct nl_handle *	nl_handle_alloc_cb (struct nl_cb *cb)
	Allocate new socket handle with custom callbacks.
void	nl_handle_destroy (struct nl_handle *handle)
	Destroy netlink handle.
Sequence Numbers
void	nl_disable_sequence_check (struct nl_handle *handle)
	Disable sequence number checking.
unsigned int	nl_socket_use_seq (struct nl_handle *handle)
	Use next sequence number.
Source Idenficiation
uint32_t	nl_socket_get_local_port (struct nl_handle *handle)
void	nl_socket_set_local_port (struct nl_handle *handle, uint32_t port)
	Set local port of socket.
Group Subscriptions
int	nl_socket_add_membership (struct nl_handle *handle, int group)
	Join a group.
int	nl_socket_drop_membership (struct nl_handle *handle, int group)
	Leave a group.
void	nl_join_groups (struct nl_handle *handle, int groups)
	Join multicast groups (deprecated).
Peer Identfication
uint32_t	nl_socket_get_peer_port (struct nl_handle *handle)
void	nl_socket_set_peer_port (struct nl_handle *handle, uint32_t port)
File Descriptor
int	nl_socket_get_fd (struct nl_handle *handle)
int	nl_socket_set_nonblocking (struct nl_handle *handle)
	Set file descriptor of socket handle to non-blocking state.
void	nl_socket_enable_msg_peek (struct nl_handle *handle)
	Enable use of MSG_PEEK when reading from socket.
void	nl_socket_disable_msg_peek (struct nl_handle *handle)
	Disable use of MSG_PEEK when reading from socket.
Callback Handler
struct nl_cb *	nl_socket_get_cb (struct nl_handle *handle)
void	nl_socket_set_cb (struct nl_handle handle, struct nl_cb cb)
int	nl_socket_modify_cb (struct nl_handle handle, enum nl_cb_type type, enum nl_cb_kind kind, nl_recvmsg_msg_cb_t func, void arg)
	Modify the callback handler associated to the socket.
Utilities
int	nl_set_buffer_size (struct nl_handle *handle, int rxbuf, int txbuf)
	Set socket buffer size of netlink handle.
int	nl_set_passcred (struct nl_handle *handle, int state)
	Enable/disable credential passing on netlink handle.
int	nl_socket_recv_pktinfo (struct nl_handle *handle, int state)
	Enable/disable receival of additional packet information.

Socket [Core Netlink API]

Detailed Description

Allocation

Sequence Numbers

Source Idenficiation

Group Subscriptions

Peer Identfication

File Descriptor

Callback Handler

Utilities

Function Documentation

Socket
[Core Netlink API]