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NAME

netlink, PF_NETLINK - Communication between kernel and user.
 

SYNOPSIS

int NLMSG_ALIGN(size_t len);
int NLMSG_LENGTH(size_t len);
int NLMSG_SPACE(size_t len);
void *NLMSG_DATA(struct nlmsghdr *);
struct nlmsghdr *NLMSG_NEXT(struct nlmsghdr *, int length);
int NLMSG_OK(struct nlmsghdr *, int length);
int NLMSG_PAYLOAD(struct nlmsghdr *, int length);  

DESCRIPTION

Netlink is a datagram oriented service. Valid socket types are SOCK_RAW and SOCK_DGRAM.

netlink_family selects the kernel module or netlink group to communicate with. Currently assigned netlink families are: NETLINK_ROUTE to receive routing updates and to modify the IP routing table. NETLINK_FIREWALL to receive packets sent by the IPv4 firewall code. NETLINK_ARPD to manage the arp table in user space. NETLINK_ROUTE6 to receive and send IPv6 routing table updates. NETLINK_IP6_FW to receive packets that failed the IPv6 firewall checks (currently not implemented). NETLINK_TAPBASE ... NETLINK_TAPBASE+15 are the instances of the ethertap device. Ethertap is a pseudo network tunnel device that allows to simulate an ethernet driver from user space. Reserved are NETLINK_SKIP for ENskip and NETLINK_USERSOCK for future user space protocols.

Netlink messages consist of a byte stream with one or multiple nlmsghdr headers and associated payload. For multipart messages the first and all following headers have the NLM_F_MULTI flag set, except for the last header which has the type NLMSG_DONE. The byte stream should be only accessed with the standard NLMSG_* macros.

Netlink is pseudo reliable. This means 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 NLM_F_ACK flag. An acknowledgment is a NLMSG_ERROR packet with the error field set to 0. The user has to handle generating acks for received messages himself. The kernel tries to send an NLMSG_ERROR message for every failed packet.

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;      /* Sending process PID */
};


struct nlmsgerr
{
        int     error;          /* error: negative errno or 0 for acks. */ 
        struct nlmsghdr msg;    /* message header that caused the error */ 
};

After each nlmsghdr the payload follows. nlmsg_type can be one of the standard message types: NLMSG_NOOP message is to be ignored, NLMSG_ERROR the message signals an error and the payload contains a nlmsgerr structure, NLMSG_DONE message terminates a multipart message,

A netlink family usually has more specified message types, see the appropiate man pages for that, e.g. rtnetlink(4) for NETLINK_ROUTE.

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
NLMSG_DONE
T}
NLM_F_ACK:reply with an acknowledgment on success
NLM_F_ECHO:echo this request

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. Requires CAP_NET_ADMIN or super user rights.
NLM_F_DUMP	not documented yet.

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.

   

ADDRESS FORMATS

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 */
};

nl_pid is the pid of the user space netlink, or 0 if the destination is in the kernel. nl_groups is a bitmask with every bit representing a netlink group number.

NETLINK MACROS

NLMSG_ALIGN

Round the length of a netlink message up to align it properly.

NLMSG_LENGTH

Gets the payload length as argument and returns the aligned length to store in the nlmsg_len field of the nlmsghdr.

NLMSG_SPACE

Return the number of bytes a netlink message with payload of the passed length would occupy.

NLMSG_DATA

Return a pointer to the payload associated with the passed nlmsghdr

NLMSG_NEXT Get the next nlmsghdr in a multipart message. The caller must check if the current nlmsghdr didn't have the NLMSG_DONE set - this function doesn't return NULL on end. The length parameter is an lvalue containing the remaining length of the message buffer. This macro decrements it by the length of the message header.

NLMSG_OK

Return true if the netlink message is not truncated and ok to parse.

NLMSG_PAYLOAD

Return the length of the payload associated with the nlmsghdr

BUGS

SEE ALSO

VERSIONS

Linux 2.0 supported a more primitive device based netlink interface (which is still available as a compatibility option). This old interface is not described here.

Index

NAME

SYNOPSIS

DESCRIPTION

ADDRESS FORMATS

NETLINK MACROS

BUGS

SEE ALSO

VERSIONS

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