#include <asm/types.h>
#include <sys/socket.h>
#include <linux/netlink.h>
netlink_socket = socket(PF_NETLINK, socket_type, netlink_family);
Netlink is a datagram oriented service. Both SOCK_RAW and SOCK_DGRAM are valid values for socket_type; however the netlink protocol does not distinguish between datagram and raw sockets.
netlink_family selects the kernel module or netlink group to communicate with. The currently assigned netlink families are:
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 only be accessed with the standard NLMSG_* macros, see netlink(3).
Netlink is not a reliable protocol. 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 an NLMSG_ERROR packet with the error field set to 0. The application must generate acks for received messages itself. The kernel tries to send an NLMSG_ERROR message for every failed packet. A user process should follow this convention too.
Each netlink family has a set of 32 multicast groups. When bind(2) is called on the socket, the nl_groups field in the sockaddr_nl should be set to a bitmask of the groups which it wishes to listen to. The default value for this field is zero which means that no multicasts will be received. A socket may multicast messages to any of the multicast groups by setting nl_groups to a bitmask of the groups it wishes to send to when it calls sendmsg(2) or does a connect(2). Only users with an effective uid of 0 or the CAP_NET_ADMIN capability may send or listen to a netlink multicast group. Any replies to a message received for a multicast group should be sent back to the sending pid and the multicast group.
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; /* 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 specifies more message types, see the appropiate man pages for that, e.g. rtnetlink(7) 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. |
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. |
Note that NLM_F_ATOMIC requires CAP_NET_ADMIN or super user rights.
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.
Linux 2.0 supported a more primitive device based netlink interface (which is still available as a compatibility option). This obsolete interface is not described here.
ftp://ftp.inr.ac.ru/ip-routing/iproute2* for libnetlink