Generic Netlink

Collaboration diagram for Generic Netlink:

Modules
	Generic Netlink Family
	Management

Socket Creating
int	genl_connect (struct nl_sock *sk)

Sending
int	genl_send_simple (struct nl_sock *sk, int family, int cmd, int version, int flags)
	Send trivial generic netlink message. More...

Message Parsing
int	genlmsg_valid_hdr (struct nlmsghdr *nlh, int hdrlen)
int	genlmsg_validate (struct nlmsghdr *nlh, int hdrlen, int maxtype, struct nla_policy *policy)
int	genlmsg_parse (struct nlmsghdr *nlh, int hdrlen, struct nlattr *tb[], int maxtype, struct nla_policy *policy)
void *	genlmsg_data (const struct genlmsghdr *gnlh)
	Get head of message payload. More...
int	genlmsg_len (const struct genlmsghdr *gnlh)
	Get lenght of message payload. More...
struct nlattr *	genlmsg_attrdata (const struct genlmsghdr *gnlh, int hdrlen)
	Get head of attribute data. More...
int	genlmsg_attrlen (const struct genlmsghdr *gnlh, int hdrlen)
	Get length of attribute data. More...

Message Building
void *	genlmsg_put (struct nl_msg *msg, uint32_t pid, uint32_t seq, int family, int hdrlen, int flags, uint8_t cmd, uint8_t version)
	Add generic netlink header to netlink message. More...

Detailed Description

Message Format

 <------- NLMSG_ALIGN(hlen) ------> <---- NLMSG_ALIGN(len) --->
+----------------------------+- - -+- - - - - - - - - - -+- - -+
|           Header           | Pad |       Payload       | Pad |
|      struct nlmsghdr       |     |                     |     |
+----------------------------+- - -+- - - - - - - - - - -+- - -+

 <-------- GENL_HDRLEN -------> <--- hdrlen -->
                                <------- genlmsg_len(ghdr) ------>
+------------------------+- - -+---------------+- - -+------------+
| Generic Netlink Header | Pad | Family Header | Pad | Attributes |
|    struct genlmsghdr   |     |               |     |            |
+------------------------+- - -+---------------+- - -+------------+
genlmsg_data(ghdr)--------------^                     ^
genlmsg_attrdata(ghdr, hdrlen)-------------------------

Example

#include <netlink/netlink.h>
#include <netlink/genl/genl.h>
#include <netlink/genl/ctrl.h>

struct nl_sock *sock;
struct nl_msg *msg;
int family;

// Allocate a new netlink socket
sock = nl_socket_alloc();

// Connect to generic netlink socket on kernel side
genl_connect(sock);

// Ask kernel to resolve family name to family id
family = genl_ctrl_resolve(sock, "generic_netlink_family_name");

// Construct a generic netlink by allocating a new message, fill in
// the header and append a simple integer attribute.
msg = nlmsg_alloc();
genlmsg_put(msg, NL_AUTO_PID, NL_AUTO_SEQ, family, 0, NLM_F_ECHO,
            CMD_FOO_GET, FOO_VERSION);
nla_put_u32(msg, ATTR_FOO, 123);

// Send message over netlink socket
nl_send_auto_complete(sock, msg);

// Free message
nlmsg_free(msg);

// Prepare socket to receive the answer by specifying the callback
// function to be called for valid messages.
nl_socket_modify_cb(sock, NL_CB_VALID, NL_CB_CUSTOM, parse_cb, NULL);

// Wait for the answer and receive it
nl_recvmsgs_default(sock);

static int parse_cb(struct nl_msg *msg, void *arg)
{
    struct nlmsghdr *nlh = nlmsg_hdr(msg);
    struct nlattr *attrs[ATTR_MAX+1];

    // Validate message and parse attributes
    genlmsg_parse(nlh, 0, attrs, ATTR_MAX, policy);

    if (attrs[ATTR_FOO]) {
        uint32_t value = nla_get_u32(attrs[ATTR_FOO]);
        ...
    }

    return 0;
}

Function Documentation

int genl_connect

(

struct nl_sock *

sk

)

References NETLINK_GENERIC, and nl_connect().

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int genl_send_simple	(	struct nl_sock *	sk,
		int	family,
		int	cmd,
		int	version,
		int	flags
	)

Send trivial generic netlink message.

• sk Netlink socket.
• family Generic netlink family
• cmd Command
• version Version
• flags Additional netlink message flags.

Fills out a routing netlink request message and sends it out using nl_send_simple().

Returns

0 on success or a negative error code.

References genlmsghdr::cmd, nl_send_simple(), and version.

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struct nlattr* genlmsg_attrdata	(	const struct genlmsghdr *	gnlh,
		int	hdrlen
	)

Get head of attribute data.

• gnlh generic netlink message header
• hdrlen length of family specific header

References genlmsg_data(), and NLMSG_ALIGN.

Referenced by genlmsg_parse(), and genlmsg_validate().

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int genlmsg_attrlen	(	const struct genlmsghdr *	gnlh,
		int	hdrlen
	)

Get length of attribute data.

• gnlh generic netlink message header
• hdrlen length of family specific header

References genlmsg_len(), and NLMSG_ALIGN.

Referenced by genlmsg_parse(), and genlmsg_validate().

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void* genlmsg_data

(

const struct genlmsghdr *

gnlh

)

Get head of message payload.

• gnlh genetlink messsage header

References GENL_HDRLEN.

Referenced by genlmsg_attrdata().

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int genlmsg_len

(

const struct genlmsghdr *

gnlh

)

Get lenght of message payload.

• gnlh genetlink message header

References GENL_HDRLEN, nlh, NLMSG_HDRLEN, and nlmsghdr::nlmsg_len.

Referenced by genlmsg_attrlen(), and genlmsg_valid_hdr().

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int genlmsg_parse	(	struct nlmsghdr *	nlh,
		int	hdrlen,
		struct nlattr *	tb[],
		int	maxtype,
		struct nla_policy *	policy
	)

References genlmsg_attrdata(), genlmsg_attrlen(), genlmsg_valid_hdr(), nla_parse(), NLE_MSG_TOOSHORT, and nlmsg_data().

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void* genlmsg_put	(	struct nl_msg *	msg,
		uint32_t	pid,
		uint32_t	seq,
		int	family,
		int	hdrlen,
		int	flags,
		uint8_t	cmd,
		uint8_t	version
	)

Add generic netlink header to netlink message.

• msg netlink message
• pid netlink process id or NL_AUTO_PID
• seq sequence number of message or NL_AUTO_SEQ
• family generic netlink family
• hdrlen length of user specific header
• flags message flags
• cmd generic netlink command
• version protocol version

Returns pointer to user specific header.

References genlmsghdr::cmd, GENL_HDRLEN, NL_DBG, nlh, nlmsg_data(), nlmsg_put(), NULL, and version.

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int genlmsg_valid_hdr	(	struct nlmsghdr *	nlh,
		int	hdrlen
	)

References GENL_HDRLEN, genlmsg_len(), NLMSG_ALIGN, nlmsg_data(), and nlmsg_valid_hdr().

Referenced by genlmsg_parse(), and genlmsg_validate().

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int genlmsg_validate	(	struct nlmsghdr *	nlh,
		int	hdrlen,
		int	maxtype,
		struct nla_policy *	policy
	)

References genlmsg_attrdata(), genlmsg_attrlen(), genlmsg_valid_hdr(), nla_validate(), NLE_MSG_TOOSHORT, and nlmsg_data().

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