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

(cherry picked from commit 3a45c01656)

nicfinder/localaddrs.go

@@ -0,0 +1,39 @@
+//go:build !android
+
+package nicfinder
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
+	"net"
+	"net/netip"
+)
+
+func localAddrs(ctx context.Context, l *slog.Logger, filter Filter) ([]netip.Addr, error) {
+	ifaces, err := net.Interfaces()
+	if err != nil {
+		return nil, fmt.Errorf("failed to enumerate local interfaces: %w", err)
+	}
+
+	var out []netip.Addr
+	var errs []error
+
+	for _, i := range ifaces {
+		if !allowName(ctx, l, filter, i.Name) {
+			continue
+		}
+		addrs, err := i.Addrs()
+		if err != nil {
+			errs = append(errs, fmt.Errorf("failed to get addresses for %s: %w", i.Name, err))
+			continue
+		}
+		for _, rawAddr := range addrs {
+			if addr, ok := allowedAddr(ctx, l, filter, rawAddr); ok {
+				out = append(out, addr)
+			}
+		}
+	}
+	return out, errors.Join(errs...)
+}