getting notification of new links and addresses

[Michael Richardson]

I am writing a routing advertisement / DHCPv6 daemon.
It has to handle thousands of (ppp) interfaces that come and go.

I scan the list of interfaces (and their addresseses) when I start
up using rtnl_link_alloc_cache()/rtnl_addr_alloc_cache(), and
nl_cache_foreach().  This works fine, and I see all the existing
interfaces, and I see the list of addresses, and I connect them to
the links in my data structure.

I subscribe to some multicast groups:
    /* subscribe to link info and address info */
    nl_socket_disable_seq_check(netlink_handle);
    nl_socket_add_memberships(netlink_handle, RTNLGRP_LINK, 0);
    nl_socket_add_memberships(netlink_handle, RTNLGRP_IPV6_IFADDR, 0);

I also mark the caches as being provided with nl_cache_mgmt_provide()
on the link and address cache I've created.

I enable libevent for the libnl3 socket, code which I append below, mostly
for google.   And with NLDBG=3 set, I can see messages coming up to
tell me about new interfaces.  ("ip monitor all" in another window also shows
things).

When I scan I see nice things like:
DBG<3>                nl.c:743  recvmsgs: Attempting to read from 0x9fec448
DBG<3>                nl.c:752  recvmsgs: recvmsgs(0x9fec448): Read 3320 bytes
DBG<3>                nl.c:756  recvmsgs: recvmsgs(0x9fec448): Processing valid message...
DBG<2>               msg.c:282  __nlmsg_alloc: msg 0x9fec490: Allocated new message, maxlen=548
DBG<3>             cache.c:452  __cache_add: Added object 0x9ff18a0 to cache
0x9fec018 <route/link>, nitems 1


when a new interface shows up, I see only:

[debug] event_process_active: event: 0x9ff17c0, EV_READ   call 0x804efdb
DBG<3>                nl.c:743  recvmsgs: Attempting to read from 0x9fec448
DBG<3>                nl.c:752  recvmsgs: recvmsgs(0x9fec448): Read 1052 bytes
DBG<3>                nl.c:756  recvmsgs: recvmsgs(0x9fec448): Processing valid message...
DBG<2>               msg.c:282  __nlmsg_alloc: msg 0x9fec490: Allocated new message, maxlen=1052
DBG<2>               msg.c:576  nlmsg_free: msg 0x9fec490: Freed


so the new interface is not going into any cache.
Do I have to write my own callback function?  If so, that would surprise me.

I'm about to "use the source", but I wanted to post this, perhaps I'll
manage to reply with an answer.


=========


(some of code. C++ for fun)

void network_interface::process_netlink_input(evutil_socket_t fd, short what,
void *arg)
{
    /* process messages from netlink */
    fprintf(stderr, "Processing messages due to event\n");
    nl_recvmsgs_default(netlink_handle);
}

void network_interface::setup_libevent(struct event_base *base)
{
    open_netlink();

    nl_socket_set_nonblocking(netlink_handle);
    int fd = nl_socket_get_fd(netlink_handle);

    netlink_read_event = event_new(base, fd,
                                   /* what:*/          EV_READ|EV_PERSIST,
                                   /* callback_fn: */  process_netlink_input,
                                   NULL);

    evutil_make_socket_nonblocking(fd);

    /* add the event with no time out */
    event_add(netlink_read_event, NULL);
}