Generic Netlink

Modules
	Generic Netlink Family
	Management

Socket Creating
int	genl_connect (struct nl_handle *handle)

Sending
int	genl_send_simple (struct nl_handle *handle, 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_handle *sock;
struct nl_msg *msg;
int family;
 
// Allocate a new netlink socket
sock = nl_handle_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

genl_send_simple()

int genl_send_simple	(	struct nl_handle *	handle,
		int	family,
		int	cmd,
		int	version,
		int	flags
	)

Parameters

handle	Netlink handle.
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.

Definition at line 128 of file genl.c.

{
        struct genlmsghdr hdr = {
                .cmd = cmd,
                .version = version,
        };
 
        return nl_send_simple(handle, family, flags, &hdr, sizeof(hdr));
}

References nl_send_simple().

genlmsg_data()

void* genlmsg_data

(

const struct genlmsghdr *

gnlh

)

Parameters

gnlh	genetlink messsage header

Definition at line 191 of file genl.c.

{
        return ((unsigned char *) gnlh + GENL_HDRLEN);
}

Referenced by genlmsg_attrdata().

genlmsg_len()

int genlmsg_len

(

const struct genlmsghdr *

gnlh

)

Parameters

gnlh	genetlink message header

Definition at line 200 of file genl.c.

{
        struct nlmsghdr *nlh = (struct nlmsghdr *)((unsigned char *)gnlh -
                                                        NLMSG_HDRLEN);
        return (nlh->nlmsg_len - GENL_HDRLEN - NLMSG_HDRLEN);
}

References nlmsghdr::nlmsg_len.

Referenced by genlmsg_attrlen().

genlmsg_attrdata()

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

Parameters

gnlh	generic netlink message header
hdrlen	length of family specific header

Definition at line 212 of file genl.c.

{
        return genlmsg_data(gnlh) + NLMSG_ALIGN(hdrlen);
}

References genlmsg_data().

genlmsg_attrlen()

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

Parameters

gnlh	generic netlink message header
hdrlen	length of family specific header

Definition at line 222 of file genl.c.

{
        return genlmsg_len(gnlh) - NLMSG_ALIGN(hdrlen);
}

References genlmsg_len().

genlmsg_put()

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
	)

Parameters

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.

Definition at line 247 of file genl.c.

{
        struct nlmsghdr *nlh;
        struct genlmsghdr hdr = {
                .cmd = cmd,
                .version = version,
        };
 
        nlh = nlmsg_put(msg, pid, seq, family, GENL_HDRLEN + hdrlen, flags);
        if (nlh == NULL)
                return NULL;
 
        memcpy(nlmsg_data(nlh), &hdr, sizeof(hdr));
        NL_DBG(2, "msg %p: Added generic netlink header cmd=%d version=%d\n",
               msg, cmd, version);
 
        return nlmsg_data(nlh) + GENL_HDRLEN;
}

References nlmsg_data(), and nlmsg_put().