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 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 appropriate man pages for that, e.g. rtnetlink(7) for NETLINK_ROUTE.

Note that NLM_F_ATOMIC requires CAP_NET_ADMIN or super user rights.