prober: per-network ICMP, real errqueue traceroute, sharper mDNS/ip-monitor

- IcmpProbe: attempt the echo on the default network AND each active
  network (Network.bindSocket + per-network DNS). Run 1's ping6 EAGAIN was
  topology (v6 only on cellular), not capability — now the report shows
  which networks carry which family instead of a bare ERROR.
- New traceroute.udp4 (TracerouteProbe): actual UDP traceroute reading
  ICMP time-exceeded via Os.recvmsg(MSG_ERRQUEUE) with cmsg parsing —
  reflection per repo convention (API surface exists ~34+). If this
  returns hops, the C-over-JNI shim is unnecessary on that device.
  sock_extended_err layout constants documented in OsAbi.
- MulticastProbe: 10 s window; meta-query PLUS concrete types
  (_http._tcp, _googlecast._tcp) — run 1 showed the meta-query alone
  returning 0 on a network with live services; capture names + failure
  codes as evidence.
- ShizukuProbe: ip monitor window 2s -> 5s, gateway ping in background to
  provoke a NEIGH transition instead of hoping for ambient churn.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

echolot-prober/app/src/main/java/app/echo_lot/prober/probe/IcmpProbe.kt

@@ -4,13 +4,15 @@
 package app.echo_lot.prober.probe
 
 import android.content.Context
+import android.net.ConnectivityManager
+import android.net.Network
+import android.net.NetworkCapabilities
 import android.system.Os
 import android.system.OsConstants
 import android.system.StructTimeval
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.withContext
 import java.io.FileDescriptor
-import java.net.Inet4Address
 import java.net.InetAddress
 import java.nio.ByteBuffer
 import java.util.Locale
@@ -19,6 +21,11 @@ import java.util.Locale
  * Probes the unprivileged ICMP echo path: socket(AF_INET, SOCK_DGRAM, IPPROTO_ICMP).
  * Android opens ping_group_range to all UIDs, so this should work without root or CAP_NET_RAW.
  * If it does, the production app never needs to shell out to /system/bin/ping.
+ *
+ * The echo is attempted once per active network (socket bound via Network.bindSocket), not just
+ * on the default network: the first device run showed why — v6 lived only on cellular while wifi
+ * was the v4-only default, so a default-network ping6 EAGAINed and looked like a capability
+ * failure. Per-network results turn that into topology evidence.
  */
 class IcmpProbe(
     private val v6: Boolean = false,
@@ -29,23 +36,65 @@ class IcmpProbe(
     else "ICMPv4 echo (unprivileged datagram socket)"
     override val tier = Tier.APP
 
+    private data class Attempt(val ok: Boolean, val detail: String)
+
     override suspend fun run(context: Context): ProbeResult = withContext(Dispatchers.IO) {
         val start = System.nanoTime()
         val ev = LinkedHashMap<String, String>()
+        val cm = context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
+
+        // Default-network attempt first — this is what a naive app gets.
+        val def = attempt(network = null)
+        ev["default"] = def.detail
+
+        var anyOk = def.ok
+        var okNet: String? = if (def.ok) "default" else null
+        @Suppress("DEPRECATION")
+        for (net in cm.allNetworks) {
+            val caps = cm.getNetworkCapabilities(net) ?: continue
+            val label = transportLabel(caps)
+            val res = attempt(net)
+            ev["net.$label"] = res.detail
+            if (res.ok && !anyOk) { anyOk = true; okNet = label }
+            if (res.ok && okNet == null) okNet = label
+        }
+
+        val durationMs = (System.nanoTime() - start) / 1_000_000
+        if (anyOk) {
+            val summary = if (def.ok) "Echo reply on the default network"
+            else "Echo reply on $okNet only — default network has no ${if (v6) "v6" else "v4"} path"
+            ProbeResult.of(this@IcmpProbe, Verdict.SUPPORTED, summary, ev, durationMs)
+        } else {
+            val permission = ev.values.any { "EACCES" in it || "EPERM" in it }
+            val verdict = if (permission) Verdict.UNSUPPORTED else Verdict.ERROR
+            ProbeResult.of(this@IcmpProbe, verdict,
+                "No echo reply on any of ${ev.size} attempt(s)", ev, durationMs)
+        }
+    }
+
+    private fun transportLabel(caps: NetworkCapabilities): String = when {
+        caps.hasTransport(NetworkCapabilities.TRANSPORT_WIFI) -> "wifi"
+        caps.hasTransport(NetworkCapabilities.TRANSPORT_CELLULAR) -> "cellular"
+        caps.hasTransport(NetworkCapabilities.TRANSPORT_ETHERNET) -> "ethernet"
+        caps.hasTransport(NetworkCapabilities.TRANSPORT_VPN) -> "vpn"
+        else -> "other"
+    }
+
+    /** One socket → optional bind to [network] → send → timed recv. Never throws. */
+    private fun attempt(network: Network?): Attempt {
         var fd: FileDescriptor? = null
-        try {
+        return try {
             val proto = if (v6) OsConstants.IPPROTO_ICMPV6 else OsConstants.IPPROTO_ICMP
             val family = if (v6) OsConstants.AF_INET6 else OsConstants.AF_INET
             fd = Os.socket(family, OsConstants.SOCK_DGRAM, proto)
-            ev["socket"] = "opened family=$family proto=$proto"
+            network?.bindSocket(fd)
-
             Os.setsockoptTimeval(
                 fd, OsConstants.SOL_SOCKET, OsConstants.SO_RCVTIMEO,
                 StructTimeval.fromMillis(3000),
             )
-
+            // Resolve via the bound network where there is one — the default resolver may
-            val addr = InetAddress.getByName(targetHost)
+            // not even have records for the other family.
-            ev["target"] = addr.hostAddress ?: targetHost
+            val addr = network?.getByName(targetHost) ?: InetAddress.getByName(targetHost)
 
             val ident = (Os.getpid() and 0xFFFF)
             val packet = buildEchoRequest(v6, ident.toShort(), seq = 1)
@@ -55,42 +104,21 @@ class IcmpProbe(
             val buf = ByteBuffer.allocate(1500)
             val received = Os.recvfrom(fd, buf, 0, null)
             val rttMs = (System.nanoTime() - t0) / 1_000_000.0
-            ev["bytes_received"] = received.toString()
-            // Locale.ROOT: the device's locale must not leak into the report ("38,1" broke
-            // the schema's number format on the first Austrian-locale device).
-            ev["rtt_ms"] = "%.1f".format(Locale.ROOT, rttMs)
-
             // ICMP datagram (ping) sockets deliver the ICMP message with NO IP header, so the
             // type byte is at offset 0 for both families. v4 echo reply = 0, v6 echo reply = 129.
             val replyType = if (received > 0) buf.get(0).toInt() and 0xFF else -1
-            ev["reply_type"] = replyType.toString()
+            val ok = replyType == (if (v6) 129 else 0)
-            ev["reply_type_meaning"] = when (replyType) {
+            // Locale.ROOT: the device's locale must not leak into the report ("38,1" broke
-                0 -> "echo reply (v4)"; 129 -> "echo reply (v6)"; else -> "other/$replyType"
+            // the schema's number format on the first Austrian-locale device).
-            }
+            Attempt(ok, "reply type=$replyType rtt_ms=${"%.1f".format(Locale.ROOT, rttMs)} " +
-
+                "bytes=$received target=${addr.hostAddress}")
-            ProbeResult.of(
-                this@IcmpProbe, Verdict.SUPPORTED,
-                "Echo reply from ${ev["target"]} in ${ev["rtt_ms"]} ms",
-                ev, (System.nanoTime() - start) / 1_000_000,
-            )
         } catch (e: Throwable) {
-            ev["error"] = e.message ?: e.javaClass.simpleName
+            Attempt(false, "error: ${e.message ?: e.javaClass.simpleName}")
-            val verdict = if (isPermissionLike(e)) Verdict.UNSUPPORTED else Verdict.ERROR
-            ProbeResult.of(
-                this@IcmpProbe, verdict,
-                "ICMP datagram socket failed: ${ev["error"]}",
-                ev, (System.nanoTime() - start) / 1_000_000,
-            )
         } finally {
             fd?.let { runCatching { Os.close(it) } }
         }
     }
 
-    private fun isPermissionLike(e: Throwable): Boolean {
-        val m = (e.message ?: "").uppercase()
-        return "EACCES" in m || "EPERM" in m || "EAFNOSUPPORT" in m || "EPROTONOSUPPORT" in m
-    }
-
     /** Minimal ICMP echo request; kernel fills checksum for ICMPv6, we compute it for ICMPv4. */
     private fun buildEchoRequest(v6: Boolean, ident: Short, seq: Short): ByteArray {
         val type = if (v6) 128 else 8 // echo request

echolot-prober/app/src/main/java/app/echo_lot/prober/probe/MulticastProbe.kt

@@ -10,18 +10,43 @@ import android.net.wifi.WifiManager
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.delay
 import kotlinx.coroutines.withContext
-import java.util.concurrent.atomic.AtomicInteger
+import java.util.Collections
 
 /**
  * Probes multicast reception (MulticastLock + NSD/mDNS discovery). Confirms the app can do the
  * mDNS/SSDP/LLMNR service inventory that doubles as the VLAN-leakage detector. Uses NsdManager
  * as the least-privileged path; a raw 224.0.0.251:5353 listener is the fuller implementation.
+ *
+ * Runs the `_services._dns-sd._udp.` meta-query AND concrete types in parallel: run 1 on a
+ * network with live mDNS services returned 0 via the meta-query alone — NsdManager's meta-query
+ * support is unreliable on many builds, while concrete-type discovery is what it is actually
+ * built for. A device where concrete types answer but the meta-query stays empty is itself a
+ * finding the production inventory needs to know about.
  */
 class MulticastProbe : Probe {
     override val id = "local.mdns_discover"
     override val title = "Multicast reception (MulticastLock + mDNS/NSD)"
     override val tier = Tier.APP
 
+    // Meta-query + common concrete types (HTTP covers HA/printers/NAS; googlecast is ubiquitous).
+    private val queries = listOf(
+        "meta" to "_services._dns-sd._udp.",
+        "http" to "_http._tcp.",
+        "googlecast" to "_googlecast._tcp.",
+    )
+
+    private class Recorder : NsdManager.DiscoveryListener {
+        val names: MutableList<String> = Collections.synchronizedList(mutableListOf())
+        @Volatile var started = false
+        @Volatile var startFailCode: Int? = null
+        override fun onStartDiscoveryFailed(t: String?, code: Int) { startFailCode = code }
+        override fun onStopDiscoveryFailed(t: String?, code: Int) {}
+        override fun onDiscoveryStarted(t: String?) { started = true }
+        override fun onDiscoveryStopped(t: String?) {}
+        override fun onServiceFound(s: NsdServiceInfo?) { s?.serviceName?.let { names.add(it) } }
+        override fun onServiceLost(s: NsdServiceInfo?) {}
+    }
+
     override suspend fun run(context: Context): ProbeResult = withContext(Dispatchers.IO) {
         val start = System.nanoTime()
         val ev = LinkedHashMap<String, String>()
@@ -33,35 +58,37 @@ class MulticastProbe : Probe {
         ev["multicast_lock"] = if (lock?.isHeld == true) "acquired" else "not held"
 
         val nsd = context.getSystemService(NsdManager::class.java)
-        val found = AtomicInteger(0)
+        val recorders = queries.map { (label, type) ->
-        val started = java.util.concurrent.atomic.AtomicBoolean(false)
+            val r = Recorder()
-        val serviceType = "_services._dns-sd._udp." // meta-query: enumerates service types
+            runCatching { nsd.discoverServices(type, NsdManager.PROTOCOL_DNS_SD, r) }
-        val listener = object : NsdManager.DiscoveryListener {
+                .onFailure { r.startFailCode = -1 }
-            override fun onStartDiscoveryFailed(t: String?, code: Int) {}
+            Triple(label, type, r)
-            override fun onStopDiscoveryFailed(t: String?, code: Int) {}
-            override fun onDiscoveryStarted(t: String?) { started.set(true) }
-            override fun onDiscoveryStopped(t: String?) {}
-            override fun onServiceFound(s: NsdServiceInfo?) { found.incrementAndGet() }
-            override fun onServiceLost(s: NsdServiceInfo?) {}
         }
         try {
-            nsd.discoverServices(serviceType, NsdManager.PROTOCOL_DNS_SD, listener)
+            delay(10_000) // 4 s missed services that a longer listen catches (run-1 lesson)
-            delay(4000)
+            var total = 0
-            runCatching { nsd.stopServiceDiscovery(listener) }
+            var anyStarted = false
-            ev["discovery_started"] = started.get().toString()
+            for ((label, type, r) in recorders) {
-            ev["services_found"] = found.get().toString()
+                runCatching { nsd.stopServiceDiscovery(r) }
-            val verdict = when {
+                anyStarted = anyStarted || r.started
-                started.get() -> Verdict.SUPPORTED
+                total += r.names.size
-                else -> Verdict.INCONCLUSIVE
+                ev["$label.started"] = r.startFailCode?.let { "failed code=$it" } ?: r.started.toString()
+                ev["$label.found"] = r.names.size.toString()
+                if (r.names.isNotEmpty()) {
+                    ev["$label.names"] = r.names.distinct().joinToString(", ").take(300)
                 }
+                ev["$label.type"] = type
+            }
+            val verdict = if (anyStarted) Verdict.SUPPORTED else Verdict.INCONCLUSIVE
             ProbeResult.of(this@MulticastProbe, verdict,
-                "mDNS discovery ${if (started.get()) "ran" else "did not start"}; ${found.get()} service type(s) seen",
+                "mDNS discovery ${if (anyStarted) "ran" else "did not start"}; $total service(s)/type(s) seen across ${queries.size} queries",
                 ev, (System.nanoTime() - start) / 1_000_000)
         } catch (e: Throwable) {
             ev["error"] = e.message ?: e.javaClass.simpleName
             ProbeResult.of(this@MulticastProbe, Verdict.ERROR, "mDNS probe failed", ev,
                 (System.nanoTime() - start) / 1_000_000)
         } finally {
+            recorders.forEach { (_, _, r) -> runCatching { nsd.stopServiceDiscovery(r) } }
             runCatching { lock?.release() }
         }
     }

echolot-prober/app/src/main/java/app/echo_lot/prober/probe/OsAbi.kt

@@ -28,6 +28,18 @@ object OsAbi {
     const val IPV6_UNICAST_HOPS = 16
     const val IPV6_PMTUDISC_PROBE = 3
 
+    // recv flags — not in OsConstants on any current API level
+    const val MSG_ERRQUEUE = 0x2000
+    const val MSG_DONTWAIT = 0x40
+
+    // struct sock_extended_err (uapi/linux/errqueue.h), fixed layout on all Android ABIs:
+    // u32 ee_errno; u8 ee_origin; u8 ee_type; u8 ee_code; u8 ee_pad; u32 ee_info; u32 ee_data;
+    // followed directly by the offender sockaddr (SO_EE_OFFENDER).
+    const val SOCK_EE_SIZE = 16
+    const val SO_EE_ORIGIN_ICMP = 2
+    const val ICMP_TIME_EXCEEDED = 11
+    const val ICMP_DEST_UNREACH = 3
+
     /** Try setsockoptInt; return null on success, or the errno name on failure. */
     fun trySetIntOpt(fd: FileDescriptor, level: Int, opt: Int, value: Int): String? =
         try {

echolot-prober/app/src/main/java/app/echo_lot/prober/probe/ProbeRegistry.kt

@@ -11,6 +11,7 @@ object ProbeRegistry {
         IcmpProbe(v6 = true),
         SockOptProbe(),
         ErrqueueProbe(),
+        TracerouteProbe(),
         MultiNetworkProbe(),
         MulticastProbe(),
         BleAdvertiseProbe(),

echolot-prober/app/src/main/java/app/echo_lot/prober/probe/ShizukuProbe.kt

@@ -26,7 +26,9 @@ class ShizukuProbe : Probe {
         "ip_neigh" to "ip neigh show",
         "ip6_route" to "ip -6 route show table all",
         "ip_addr" to "ip addr show",
-        "ip_monitor" to "timeout 2 ip monitor all || true",
+        // 5 s window (2 s caught nothing on a quiet net); ping the gateway concurrently so at
+        // least one NEIGH transition is provoked rather than hoping for ambient churn.
+        "ip_monitor" to "(ping -c 2 -W 1 \$(ip route show default | head -1 | cut -d' ' -f3) >/dev/null 2>&1 &); timeout 5 ip monitor all || true",
         "dhcp_log" to "dumpsys network_stack 2>/dev/null | grep -iA2 -m 20 -e dhcp -e 'IpClient' || true",
         "wifi_dump" to "dumpsys wifi 2>/dev/null | grep -iA1 -m 20 -e 'mDhcpResults' -e 'Gateway' -e 'DNS' || true",
     )

echolot-prober/app/src/main/java/app/echo_lot/prober/probe/TracerouteProbe.kt

@@ -0,0 +1,204 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.prober.probe
+
+import android.content.Context
+import android.system.Os
+import android.system.OsConstants
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.withContext
+import java.io.FileDescriptor
+import java.net.InetAddress
+import java.net.InetSocketAddress
+import java.nio.ByteBuffer
+import java.nio.ByteOrder
+
+/**
+ * The real thing that trace.errqueue_reachable only smoke-tests: a UDP traceroute reading ICMP
+ * time-exceeded off the socket error queue via Os.recvmsg(MSG_ERRQUEUE) — no root, no raw socket,
+ * no native code. If this returns actual hop addresses, the planned C-over-JNI shim is dead on
+ * this device; if the Os surface falls short, the evidence says exactly where.
+ *
+ * StructMsghdr/StructCmsghdr/recvmsg are reached via reflection (repo convention for uncertain
+ * OS paths): present since roughly API 34, absent before, and the probe must run — and report —
+ * on both.
+ */
+class TracerouteProbe(
+    private val targetHost: String = "1.1.1.1",
+    private val maxHops: Int = 6,
+) : Probe {
+    override val id = "traceroute.udp4"
+    override val title = "UDP traceroute via MSG_ERRQUEUE (no root, no JNI)"
+    override val tier = Tier.APP
+
+    override suspend fun run(context: Context): ProbeResult = withContext(Dispatchers.IO) {
+        val start = System.nanoTime()
+        val ev = LinkedHashMap<String, String>()
+        var fd: FileDescriptor? = null
+
+        val api = ErrqueueApi.resolve()
+        ev["recvmsg_api"] = api?.describe() ?: "unavailable (StructMsghdr/recvmsg not on this API level)"
+        if (api == null) {
+            return@withContext ProbeResult.of(this@TracerouteProbe, Verdict.PARTIAL,
+                "Os.recvmsg surface missing — errqueue traceroute needs the native shim here",
+                ev, (System.nanoTime() - start) / 1_000_000)
+        }
+
+        try {
+            fd = Os.socket(OsConstants.AF_INET, OsConstants.SOCK_DGRAM, OsConstants.IPPROTO_UDP)
+            OsAbi.trySetIntOpt(fd, OsConstants.IPPROTO_IP, OsAbi.IP_RECVERR, 1)?.let {
+                ev["IP_RECVERR"] = "reject: $it"
+                return@withContext ProbeResult.of(this@TracerouteProbe, Verdict.UNSUPPORTED,
+                    "IP_RECVERR rejected", ev, (System.nanoTime() - start) / 1_000_000)
+            }
+            val target = InetAddress.getByName(targetHost)
+            ev["target"] = target.hostAddress ?: targetHost
+
+            var hopsSeen = 0
+            var reachedTarget = false
+            for (ttl in 1..maxHops) {
+                OsAbi.trySetIntOpt(fd, OsConstants.IPPROTO_IP, OsAbi.IP_TTL, ttl)
+                val t0 = System.nanoTime()
+                val sent = runCatching {
+                    Os.sendto(fd, ByteArray(32), 0, 32, 0, target, 33434 + ttl)
+                }
+                if (sent.isFailure) {
+                    ev["hop.$ttl"] = "sendto failed: ${sent.exceptionOrNull()?.message}"
+                    continue
+                }
+
+                // The ICMP error takes one RTT to come back; poll the errqueue briefly.
+                var hop: ErrqueueApi.ErrEvent? = null
+                val deadline = System.nanoTime() + 900_000_000L
+                while (hop == null && System.nanoTime() < deadline) {
+                    hop = api.pollErrqueue(fd)
+                    if (hop == null) delay(40)
+                }
+                val rttMs = (System.nanoTime() - t0) / 1_000_000
+                when {
+                    hop == null -> ev["hop.$ttl"] = "* (no errqueue event in 900 ms)"
+                    hop.parseError != null ->
+                        ev["hop.$ttl"] = "errqueue event, cmsg unparsed: ${hop.parseError}"
+                    else -> {
+                        hopsSeen++
+                        ev["hop.$ttl"] = "${hop.offender ?: "?"} icmp type=${hop.icmpType} " +
+                            "origin=${hop.origin} ~${rttMs} ms"
+                        if (hop.icmpType == OsAbi.ICMP_DEST_UNREACH) reachedTarget = true
+                    }
+                }
+                if (reachedTarget) break
+            }
+
+            val verdict = when {
+                hopsSeen > 0 -> Verdict.SUPPORTED
+                else -> Verdict.PARTIAL // API present, sent fine, but no parseable events
+            }
+            val summary = when {
+                hopsSeen > 0 && reachedTarget ->
+                    "$hopsSeen hop(s) via errqueue, target reached — no native shim needed"
+                hopsSeen > 0 -> "$hopsSeen hop(s) read via errqueue — no native shim needed"
+                else -> "recvmsg present but no errqueue events surfaced — shim still the fallback"
+            }
+            ProbeResult.of(this@TracerouteProbe, verdict, summary, ev,
+                (System.nanoTime() - start) / 1_000_000)
+        } catch (e: Throwable) {
+            ev["error"] = e.message ?: e.javaClass.simpleName
+            ProbeResult.of(this@TracerouteProbe, Verdict.ERROR, "traceroute probe failed", ev,
+                (System.nanoTime() - start) / 1_000_000)
+        } finally {
+            fd?.let { runCatching { Os.close(it) } }
+        }
+    }
+}
+
+/**
+ * Reflection facade over android.system.{StructMsghdr, StructCmsghdr, Os.recvmsg}.
+ * Resolved once; null if any piece is missing on this API level.
+ */
+private class ErrqueueApi private constructor(
+    private val msghdrCtor: java.lang.reflect.Constructor<*>,
+    private val recvmsg: java.lang.reflect.Method,
+    private val cmsgLevel: java.lang.reflect.Field,
+    private val cmsgType: java.lang.reflect.Field,
+    private val cmsgData: java.lang.reflect.Field,
+    private val msgControl: java.lang.reflect.Field,
+) {
+    class ErrEvent(
+        val offender: String?,
+        val icmpType: Int,
+        val origin: Int,
+        val parseError: String? = null,
+    )
+
+    fun describe() = "StructMsghdr + Os.recvmsg via reflection"
+
+    /** One non-blocking MSG_ERRQUEUE read; null when the queue is empty. */
+    fun pollErrqueue(fd: FileDescriptor): ErrEvent? {
+        return try {
+            val iov = arrayOf(ByteBuffer.allocate(512))
+            // (SocketAddress msg_name, ByteBuffer[] msg_iov, StructCmsghdr[] msg_control, flags)
+            val msghdr = msghdrCtor.newInstance(
+                InetSocketAddress(0), iov, null, 0,
+            )
+            recvmsg.invoke(null, fd, msghdr, OsAbi.MSG_ERRQUEUE or OsAbi.MSG_DONTWAIT)
+            val control = msgControl.get(msghdr) as? Array<*>
+                ?: return ErrEvent(null, -1, -1, "msg_control empty after recvmsg")
+            for (cmsg in control.filterNotNull()) {
+                val level = cmsgLevel.getInt(cmsg)
+                val type = cmsgType.getInt(cmsg)
+                if (level == OsConstants.IPPROTO_IP && type == OsAbi.IP_RECVERR) {
+                    return parseSockExtendedErr(cmsgData.get(cmsg))
+                }
+            }
+            ErrEvent(null, -1, -1, "no IP_RECVERR cmsg among ${control.size}")
+        } catch (e: Throwable) {
+            val cause = (e as? java.lang.reflect.InvocationTargetException)?.cause ?: e
+            val msg = cause.message ?: cause.javaClass.simpleName
+            if ("EAGAIN" in msg || "EWOULDBLOCK" in msg) null // queue empty — not an error
+            else ErrEvent(null, -1, -1, msg)
+        }
+    }
+
+    /** cmsg_data = struct sock_extended_err + offender sockaddr_in (see OsAbi). */
+    private fun parseSockExtendedErr(data: Any?): ErrEvent {
+        val bytes: ByteArray = when (data) {
+            is ByteArray -> data
+            is ByteBuffer -> ByteArray(data.remaining()).also { data.duplicate().get(it) }
+            else -> return ErrEvent(null, -1, -1, "cmsg_data is ${data?.javaClass?.name}")
+        }
+        if (bytes.size < OsAbi.SOCK_EE_SIZE) {
+            return ErrEvent(null, -1, -1, "cmsg_data too short: ${bytes.size}")
+        }
+        val origin = bytes[4].toInt() and 0xFF
+        val icmpType = bytes[5].toInt() and 0xFF
+        // SO_EE_OFFENDER: sockaddr_in directly after the fixed struct; sin_addr at offset +4.
+        val offender = if (bytes.size >= OsAbi.SOCK_EE_SIZE + 8) {
+            val family = ByteBuffer.wrap(bytes, OsAbi.SOCK_EE_SIZE, 2)
+                .order(ByteOrder.nativeOrder()).short.toInt()
+            if (family == OsConstants.AF_INET) {
+                val a = bytes.copyOfRange(OsAbi.SOCK_EE_SIZE + 4, OsAbi.SOCK_EE_SIZE + 8)
+                InetAddress.getByAddress(a).hostAddress
+            } else null
+        } else null
+        return ErrEvent(offender, icmpType, origin)
+    }
+
+    companion object {
+        fun resolve(): ErrqueueApi? = runCatching {
+            val msghdrCls = Class.forName("android.system.StructMsghdr")
+            val cmsghdrCls = Class.forName("android.system.StructCmsghdr")
+            ErrqueueApi(
+                msghdrCtor = msghdrCls.constructors.first { it.parameterCount == 4 },
+                recvmsg = Os::class.java.getMethod(
+                    "recvmsg", FileDescriptor::class.java, msghdrCls, Int::class.javaPrimitiveType,
+                ),
+                cmsgLevel = cmsghdrCls.getField("cmsg_level"),
+                cmsgType = cmsghdrCls.getField("cmsg_type"),
+                cmsgData = cmsghdrCls.getField("cmsg_data"),
+                msgControl = msghdrCls.getField("msg_control"),
+            )
+        }.getOrNull()
+    }
+}