Advertise underlay addresses on Android (#1881) (#1887)

(cherry picked from commit 3a45c01656)

nicfinder/nicfinder.go

@@ -0,0 +1,59 @@
+package nicfinder
+
+import (
+	"context"
+	"log/slog"
+	"net"
+	"net/netip"
+
+	"github.com/slackhq/nebula/logging"
+)
+
+// Filter decides which interfaces, by name, and which of their addresses Find returns.
+type Filter interface {
+	AllowName(string) bool
+	Allow(netip.Addr) bool
+}
+
+// Find returns the addresses of this host's network interfaces that allowList accepts, with
+// loopback and link-local addresses left out. When only some interfaces could be read it returns
+// their addresses alongside an error for the rest. allowList must not be nil; it is called as
+// given, so a nil pointer of a type whose methods accept a nil receiver is fine.
+func Find(ctx context.Context, l *slog.Logger, allowList Filter) ([]netip.Addr, error) {
+	return localAddrs(ctx, l, allowList)
+}
+
+// allowName asks filter about an interface name and traces the answer.
+func allowName(ctx context.Context, l *slog.Logger, filter Filter, name string) bool {
+	allow := filter.AllowName(name)
+	if l.Enabled(ctx, logging.LevelTrace) {
+		l.Log(ctx, logging.LevelTrace, "localAllowList.AllowName", "interfaceName", name, "allow", allow)
+	}
+	return allow
+}
+
+// allowedAddr converts one address a platform reports and says whether to advertise it: it must
+// be a *net.IPNet or *net.IPAddr carrying an IP, not be loopback or link-local, and be accepted by
+// filter, whose answer is traced. IPv4 comes back unmapped.
+func allowedAddr(ctx context.Context, l *slog.Logger, filter Filter, rawAddr net.Addr) (netip.Addr, bool) {
+	var ip net.IP
+	switch v := rawAddr.(type) {
+	case *net.IPNet:
+		ip = v.IP
+	case *net.IPAddr:
+		ip = v.IP
+	}
+	addr, ok := netip.AddrFromSlice(ip)
+	if !ok {
+		return netip.Addr{}, false
+	}
+	addr = addr.Unmap()
+	if addr.IsLoopback() || addr.IsLinkLocalUnicast() {
+		return netip.Addr{}, false
+	}
+	isAllowed := filter.Allow(addr)
+	if l.Enabled(ctx, logging.LevelTrace) {
+		l.Log(ctx, logging.LevelTrace, "localAllowList.Allow", "localAddr", addr, "allowed", isAllowed)
+	}
+	return addr, isAllowed
+}