internal/clientinfo: watch NDP table changes on Linux

So with clients which only use SLAAC, ctrld could see client's new ip as
soon as its state changes to REACHABLE.

Moreover, the NDP listener is also changed to listen on all possible
ipv6 link local interfaces. That would allow ctrld to get all NDP events
happening in local network.

SLAAC RFC: https://datatracker.ietf.org/doc/html/rfc4862

internal/clientinfo/ndp_linux.go

@@ -1,7 +1,10 @@
 package clientinfo
 
 import (
+	"context"
+
 	"github.com/vishvananda/netlink"
+	"golang.org/x/sys/unix"
 
 	"github.com/Control-D-Inc/ctrld"
 )
@@ -21,6 +24,41 @@ func (nd *ndpDiscover) scan() {
 		}
 		ip := n.IP.String()
 		mac := n.HardwareAddr.String()
-		nd.saveInfo(ip, mac)
+		nd.saveInfo(ip, mac, false)
+	}
+}
+
+// subscribe watches NDP table changes and update new information to local table.
+func (nd *ndpDiscover) subscribe(ctx context.Context) {
+	ch := make(chan netlink.NeighUpdate)
+	done := make(chan struct{})
+	defer close(done)
+	if err := netlink.NeighSubscribe(ch, done); err != nil {
+		ctrld.ProxyLogger.Load().Err(err).Msg("could not perform neighbor subscribing")
+		return
+	}
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case nu := <-ch:
+			if nu.Family != netlink.FAMILY_V6 {
+				continue
+			}
+			ip := normalizeIP(nu.IP.String())
+			if nu.Type == unix.RTM_DELNEIGH {
+				ctrld.ProxyLogger.Load().Debug().Msgf("removing NDP neighbor: %s", ip)
+				nd.mac.Delete(ip)
+				continue
+			}
+			mac := nu.HardwareAddr.String()
+			switch nu.State {
+			case netlink.NUD_REACHABLE:
+				nd.saveInfo(ip, mac, false)
+			case netlink.NUD_FAILED:
+				ctrld.ProxyLogger.Load().Debug().Msgf("removing NDP neighbor with failed state: %s", ip)
+				nd.mac.Delete(ip)
+			}
+		}
 	}
 }