Split MDNS.Run() into platform-specific implementations for Android

The standard go-libp2p mDNS service calls net.Interfaces() internally,
which triggers a bind() on netlink_route_socket — blocked by SELinux on
Android. Replace the single Run() with two build-tag variants:

- mdns_run.go (!android): original behaviour via mdns.NewMdnsService
- mdns_android.go (android): uses anet.Interfaces() to bypass netlinkrib,
  calls zeroconf.RegisterProxy/Browse directly with explicit interfaces,
  and runs the peer-handling loop in a goroutine so Run() returns
  immediately (matching the non-Android contract — blocking here prevents
  startNetwork() from returning and starves the ledger and VPN service)

Author: jimmykarily

pkg/discovery/mdns.go

@@ -21,7 +21,6 @@ import (
 
 	"github.com/libp2p/go-libp2p/core/host"
 	"github.com/libp2p/go-libp2p/core/peer"
-	"github.com/libp2p/go-libp2p/p2p/discovery/mdns"
 )
 
 type MDNS struct {
@@ -48,10 +47,3 @@ func (n *discoveryNotifee) HandlePeerFound(pi peer.AddrInfo) {
 func (d *MDNS) Option(ctx context.Context) func(c *libp2p.Config) error {
 	return func(*libp2p.Config) error { return nil }
 }
-
-func (d *MDNS) Run(l log.StandardLogger, ctx context.Context, host host.Host) error {
-	// setup mDNS discovery to find local peers
-
-	disc := mdns.NewMdnsService(host, d.DiscoveryServiceTag, &discoveryNotifee{h: host, c: l})
-	return disc.Start()
-}

pkg/discovery/mdns_android.go

@@ -0,0 +1,287 @@
+//go:build android
+
+package discovery
+
+// Android-specific mDNS implementation that avoids net.Interfaces() (blocked by
+// SELinux on Android) by using anet.Interfaces() instead.
+//
+// go-libp2p's mdns.NewMdnsService always passes nil to zeroconf.RegisterProxy and
+// zeroconf.Browse, which causes both to call listMulticastInterfaces() → net.Interfaces()
+// → syscall.NetlinkRIB → bind() on netlink_route_socket → EACCES on Android.
+//
+// We bypass this by calling zeroconf.RegisterProxy and zeroconf.Browse ourselves
+// with explicit interfaces obtained from anet, which uses sendto() instead of bind()
+// on the netlink socket.
+//
+// We also avoid h.Addrs() at startup because the address manager's background goroutine
+// may not have populated currentAddrs.localAddrs yet. Instead we derive IPs directly from
+// anet.InterfaceAddrs() and ports from h.Network().ListenAddresses().
+
+import (
+	"context"
+	"math/rand"
+	"net"
+	"strings"
+
+	"github.com/ipfs/go-log"
+	"github.com/libp2p/go-libp2p/core/host"
+	"github.com/libp2p/go-libp2p/core/peer"
+	"github.com/libp2p/zeroconf/v2"
+	ma "github.com/multiformats/go-multiaddr"
+	manet "github.com/multiformats/go-multiaddr/net"
+	"github.com/wlynxg/anet"
+)
+
+const (
+	mdnsServiceName = "_p2p._udp"
+	mdnsDomain      = "local"
+	dnsaddrPrefix   = "dnsaddr="
+)
+
+func (d *MDNS) Run(l log.StandardLogger, ctx context.Context, h host.Host) error {
+	serviceName := d.DiscoveryServiceTag
+	if serviceName == "" {
+		serviceName = mdnsServiceName
+	}
+	l.Infof("mdns(android): starting, service=%s", serviceName)
+
+	// Get network interfaces without calling net.Interfaces() (blocked on Android).
+	ifaces, err := anet.Interfaces()
+	if err != nil {
+		return err
+	}
+
+	// Keep only interfaces that are up and support multicast.
+	var mcIfaces []net.Interface
+	for _, iface := range ifaces {
+		if iface.Flags&net.FlagUp == 0 || iface.Flags&net.FlagMulticast == 0 {
+			continue
+		}
+		mcIfaces = append(mcIfaces, iface)
+	}
+	if len(mcIfaces) == 0 {
+		l.Warnf("mdns(android): no multicast-capable interfaces found (WiFi off?), mDNS skipped")
+		return nil
+	}
+	l.Infof("mdns(android): using interfaces: %v", ifaceNames(mcIfaces))
+
+	// Get real interface IPs via anet (avoids netlinkrib bind() call).
+	// These are used both for the mDNS A/AAAA records and for expanding
+	// wildcard/loopback listen addresses into routable addresses.
+	ifAddrs, err := anet.InterfaceAddrs()
+	if err != nil {
+		return err
+	}
+	var routableIPs []net.IP
+	for _, a := range ifAddrs {
+		ipNet, ok := a.(*net.IPNet)
+		if !ok {
+			continue
+		}
+		ip := ipNet.IP
+		if ip.IsLoopback() || ip.IsLinkLocalUnicast() {
+			continue
+		}
+		routableIPs = append(routableIPs, ip)
+	}
+	if len(routableIPs) == 0 {
+		l.Warnf("mdns(android): no routable interface addresses found, mDNS skipped")
+		return nil
+	}
+
+	// Build multiaddrs for each routable IP (without port — used as base for expansion).
+	var routableMaddrs []ma.Multiaddr
+	for _, ip := range routableIPs {
+		maddr, err := manet.FromIP(ip)
+		if err != nil {
+			continue
+		}
+		routableMaddrs = append(routableMaddrs, maddr)
+	}
+
+	// Get the raw listen addresses (e.g. /ip4/0.0.0.0/tcp/PORT or /ip4/127.0.0.1/tcp/PORT).
+	// These may be wildcards or loopback; we expand them to real IPs below.
+	listenAddrs := h.Network().ListenAddresses()
+
+	// Expand wildcard/loopback listen addresses to routable IPs, keeping transport/port.
+	var expandedListenAddrs []ma.Multiaddr
+	for _, la := range listenAddrs {
+		ip, err := manet.ToIP(la)
+		if err != nil {
+			// Non-IP address (e.g. /p2p-circuit); skip for mDNS.
+			continue
+		}
+		if !ip.IsUnspecified() && !ip.IsLoopback() {
+			// Already a routable address.
+			expandedListenAddrs = append(expandedListenAddrs, la)
+			continue
+		}
+		// Wildcard or loopback → expand to real IPs with same transport/port.
+		_, transport := ma.SplitFirst(la)
+		for _, rAddr := range routableMaddrs {
+			expanded := rAddr
+			if transport != nil {
+				expanded = rAddr.Encapsulate(transport)
+			}
+			expandedListenAddrs = append(expandedListenAddrs, expanded)
+		}
+	}
+	if len(expandedListenAddrs) == 0 {
+		l.Warnf("mdns(android): no listen addresses to advertise, mDNS skipped")
+		return nil
+	}
+
+	// Build p2p multiaddrs (with /p2p/PeerID suffix) for TXT records.
+	p2pAddrs, err := peer.AddrInfoToP2pAddrs(&peer.AddrInfo{
+		ID:    h.ID(),
+		Addrs: expandedListenAddrs,
+	})
+	if err != nil {
+		return err
+	}
+
+	var txts []string
+	for _, addr := range p2pAddrs {
+		if isSuitableForMDNS(addr) {
+			txts = append(txts, dnsaddrPrefix+addr.String())
+		}
+	}
+
+	// Build the IP strings for the mDNS A/AAAA records.
+	ips := ipsToStrings(routableIPs)
+
+	peerName := randomString(32 + rand.Intn(32))
+
+	l.Infof("mdns(android): advertising IPs=%v txts=%v", ips, txts)
+	server, err := zeroconf.RegisterProxy(
+		peerName,
+		serviceName,
+		mdnsDomain,
+		4001, // port is carried in TXT records; this value is required but ignored by libp2p peers
+		peerName,
+		ips,
+		txts,
+		mcIfaces,
+	)
+	if err != nil {
+		return err
+	}
+	l.Infof("mdns(android): registered proxy, browsing for peers...")
+
+	// Browse for peers on the same service. SelectIfaces bypasses listMulticastInterfaces().
+	entryChan := make(chan *zeroconf.ServiceEntry, 1000)
+
+	errCh := make(chan error, 1)
+	go func() {
+		errCh <- zeroconf.Browse(ctx, serviceName, mdnsDomain, entryChan, zeroconf.SelectIfaces(mcIfaces))
+	}()
+
+	notifee := &discoveryNotifee{h: h, c: l}
+
+	// Run the peer-handling loop in the background so Run() returns immediately,
+	// matching the non-android implementation (mdns_run.go) which is also non-blocking.
+	// startNetwork() must return for the ledger and VPN network services to start.
+	go func() {
+		defer server.Shutdown()
+		for {
+			select {
+			case entry, ok := <-entryChan:
+				if !ok {
+					return
+				}
+				var addrs []ma.Multiaddr
+				for _, txt := range entry.Text {
+					if !strings.HasPrefix(txt, dnsaddrPrefix) {
+						continue
+					}
+					addr, err := ma.NewMultiaddr(txt[len(dnsaddrPrefix):])
+					if err != nil {
+						continue
+					}
+					addrs = append(addrs, addr)
+				}
+				infos, err := peer.AddrInfosFromP2pAddrs(addrs...)
+				if err != nil {
+					continue
+				}
+				for _, info := range infos {
+					if info.ID == h.ID() {
+						continue
+					}
+					l.Infof("mdns(android): found peer %s addrs=%v", info.ID, info.Addrs)
+					go notifee.HandlePeerFound(info)
+				}
+			case err := <-errCh:
+				if err != nil {
+					l.Warnf("mdns(android): browse error: %v", err)
+				}
+				return
+			case <-ctx.Done():
+				return
+			}
+		}
+	}()
+
+	return nil
+}
+
+// ifaceNames returns the names of the given interfaces for logging.
+func ifaceNames(ifaces []net.Interface) []string {
+	names := make([]string, 0, len(ifaces))
+	for _, iface := range ifaces {
+		names = append(names, iface.Name)
+	}
+	return names
+}
+
+// ipsToStrings converts net.IP slice to string slice (IPv4 as dotted decimal, IPv6 as standard).
+func ipsToStrings(ips []net.IP) []string {
+	ss := make([]string, 0, len(ips))
+	for _, ip := range ips {
+		ss = append(ss, ip.String())
+	}
+	return ss
+}
+
+// isSuitableForMDNS mirrors the same function from go-libp2p's mdns package.
+// It filters multiaddrs to those suitable for LAN mDNS advertisement.
+func isSuitableForMDNS(addr ma.Multiaddr) bool {
+	if addr == nil {
+		return false
+	}
+	first, _ := ma.SplitFirst(addr)
+	if first == nil {
+		return false
+	}
+	switch first.Protocol().Code {
+	case ma.P_IP4, ma.P_IP6:
+		// ok
+	case ma.P_DNS, ma.P_DNS4, ma.P_DNS6, ma.P_DNSADDR:
+		if !strings.HasSuffix(strings.ToLower(first.Value()), ".local") {
+			return false
+		}
+	default:
+		return false
+	}
+	// Reject circuit relay and browser-only transports.
+	unsuitable := false
+	ma.ForEach(addr, func(c ma.Component) bool {
+		switch c.Protocol().Code {
+		case ma.P_CIRCUIT, ma.P_WEBTRANSPORT, ma.P_WEBRTC, ma.P_WEBRTC_DIRECT, ma.P_P2P_WEBRTC_DIRECT, ma.P_WS, ma.P_WSS:
+			unsuitable = true
+			return false
+		}
+		return true
+	})
+	return !unsuitable
+}
+
+// randomString generates a random lowercase alphanumeric string of length l.
+func randomString(l int) string {
+	const alphabet = "abcdefghijklmnopqrstuvwxyz0123456789"
+	s := make([]byte, 0, l)
+	for i := 0; i < l; i++ {
+		s = append(s, alphabet[rand.Intn(len(alphabet))])
+	}
+	return string(s)
+}

pkg/discovery/mdns_run.go

@@ -0,0 +1,16 @@
+//go:build !android
+
+package discovery
+
+import (
+	"context"
+
+	"github.com/ipfs/go-log"
+	"github.com/libp2p/go-libp2p/core/host"
+	"github.com/libp2p/go-libp2p/p2p/discovery/mdns"
+)
+
+func (d *MDNS) Run(l log.StandardLogger, ctx context.Context, host host.Host) error {
+	disc := mdns.NewMdnsService(host, d.DiscoveryServiceTag, &discoveryNotifee{h: host, c: l})
+	return disc.Start()
+}