src/source/e1608/e1608_link.cpp

@@ -9,11 +9,14 @@
 #include <chrono>
 #include <cstdio>
 #include <cstring>
+#include <ifaddrs.h>
+#include <net/if.h>
 #include <netdb.h>
 #include <netinet/in.h>
 #include <netinet/tcp.h>
 #include <poll.h>
 #include <sys/socket.h>
+#include <thread>
 #include <unistd.h>
 
 namespace udpscope::e1608 {
@@ -120,6 +123,25 @@ Fd tcp_connect(const std::string& host, int port, int timeout_ms, int rcvbuf) {
   return fd;
 }
 
+/// A `D` reply (at least 29 bytes, first byte `D`, our product id). True when it added a board
+/// that `found` did not already hold (one entry per MAC).
+bool take_reply(const std::uint8_t* buf, ssize_t n, const sockaddr_in& from,
+                std::vector<DeviceInfo>& found) {
+  if (n < 29 || buf[0] != 'D') return false;
+  DeviceInfo info;
+  info.product_id = static_cast<std::uint16_t>(buf[7] | (buf[8] << 8));
+  if (info.product_id != kProductId) return false;
+  std::copy(buf + 1, buf + 7, info.mac.begin());
+  for (const DeviceInfo& d : found) {
+    if (d.mac == info.mac) return false;
+  }
+  char ip[INET_ADDRSTRLEN] = {};
+  ::inet_ntop(AF_INET, &from.sin_addr, ip, sizeof ip);
+  info.ip = ip;
+  found.push_back(info);
+  return true;
+}
+
 }  // namespace
 
 // --- addresses ------------------------------------------------------------------------------
@@ -222,22 +244,70 @@ std::vector<DeviceInfo> discover(const std::vector<std::string>& targets, int po
       if (errno == EAGAIN || errno == EWOULDBLOCK || errno == EINTR) continue;
       break;
     }
-    if (n < 29 || buf[0] != 'D') continue;
-    DeviceInfo info;
-    info.product_id = static_cast<std::uint16_t>(buf[7] | (buf[8] << 8));
-    if (info.product_id != kProductId) continue;
-    std::copy(buf + 1, buf + 7, info.mac.begin());
-    bool dup = false;
-    for (const DeviceInfo& d : found) dup = dup || d.mac == info.mac;
-    if (dup) continue;
-    char ip[INET_ADDRSTRLEN] = {};
-    ::inet_ntop(AF_INET, &from.sin_addr, ip, sizeof ip);
-    info.ip = ip;
-    found.push_back(info);
+    take_reply(buf, n, from, found);
   }
   return found;
 }
 
+Scan::Scan(int port) {
+  ifaddrs* list = nullptr;
+  if (::getifaddrs(&list) != 0) return;
+  sockaddr_in to{};
+  to.sin_family = AF_INET;
+  to.sin_port = htons(static_cast<std::uint16_t>(port));
+  to.sin_addr.s_addr = htonl(INADDR_BROADCAST);
+  const std::uint8_t probe = 'D';
+  std::vector<std::string> done;  // an interface with several addresses needs one probe, not several
+  for (const ifaddrs* a = list; a != nullptr; a = a->ifa_next) {
+    if (a->ifa_addr == nullptr || a->ifa_addr->sa_family != AF_INET) continue;
+    const unsigned fl = a->ifa_flags;
+    if ((fl & IFF_UP) == 0 || (fl & IFF_LOOPBACK) != 0 || (fl & IFF_BROADCAST) == 0) continue;
+    const std::string name = a->ifa_name;
+    if (std::find(done.begin(), done.end(), name) != done.end()) continue;
+    Fd s(::socket(AF_INET, SOCK_DGRAM | SOCK_NONBLOCK, 0));
+    if (!s.valid()) continue;
+    const int one = 1;
+    ::setsockopt(s.get(), SOL_SOCKET, SO_BROADCAST, &one, sizeof one);
+    if (::setsockopt(s.get(), SOL_SOCKET, SO_BINDTODEVICE, name.c_str(),
+                     static_cast<socklen_t>(name.size())) != 0) {
+      sockaddr_in me = *reinterpret_cast<const sockaddr_in*>(a->ifa_addr);
+      me.sin_port = 0;
+      if (::bind(s.get(), reinterpret_cast<const sockaddr*>(&me), sizeof me) != 0) continue;
+    }
+    if (::sendto(s.get(), &probe, 1, MSG_NOSIGNAL, reinterpret_cast<const sockaddr*>(&to),
+                 sizeof to) != 1)
+      continue;
+    done.push_back(name);
+    socks_.push_back(std::move(s));
+  }
+  ::freeifaddrs(list);
+}
+
+const std::vector<DeviceInfo>& Scan::poll() {
+  for (const Fd& s : socks_) {
+    for (;;) {
+      std::uint8_t buf[128];
+      sockaddr_in from{};
+      socklen_t flen = sizeof from;
+      const ssize_t n = ::recvfrom(s.get(), buf, sizeof buf, MSG_DONTWAIT,
+                                   reinterpret_cast<sockaddr*>(&from), &flen);
+      if (n < 0) break;  // nothing more on this socket for now
+      take_reply(buf, n, from, found_);
+    }
+  }
+  return found_;
+}
+
+std::vector<DeviceInfo> discover_broadcast(int port, int wait_ms) {
+  Scan scan(port);
+  const auto deadline = Clock::now() + std::chrono::milliseconds(wait_ms);
+  while (ms_left(deadline) > 0) {
+    scan.poll();
+    std::this_thread::sleep_for(std::chrono::milliseconds(10));
+  }
+  return scan.poll();
+}
+
 // --- Link -----------------------------------------------------------------------------------
 
 void Link::connect(const std::string& host, int port, std::uint32_t code, int timeout_ms) {

src/source/e1608/e1608_link.hpp

@@ -76,6 +76,30 @@ std::vector<std::string> sweep_targets(const std::string& subnet);
 /// `kProductId`) and one entry per MAC.
 std::vector<DeviceInfo> discover(const std::vector<std::string>& targets, int port, int wait_ms);
 
+/// A broadcast discovery that never blocks. The limited broadcast alone leaves through the default
+/// route only, so on a host with several interfaces it can miss a board on another one; this opens
+/// one socket per up, non-loopback IPv4 interface with a broadcast address (bound to the device, or
+/// to the interface address when binding to the device is refused), sends `D` to
+/// 255.255.255.255 on each, and collects the replies as `poll()` is called. The caller owns the
+/// clock. An interface that cannot be opened or sent on is skipped; no interface is an empty scan.
+class Scan {
+ public:
+  explicit Scan(int port);
+  Scan(const Scan&) = delete;
+  Scan& operator=(const Scan&) = delete;
+  /// Drains every socket without blocking. The devices found so far: E-1608s only, one per MAC.
+  const std::vector<DeviceInfo>& poll();
+  /// How many interfaces a probe was sent on.
+  std::size_t interfaces() const noexcept { return socks_.size(); }
+
+ private:
+  std::vector<Fd> socks_;
+  std::vector<DeviceInfo> found_;
+};
+
+/// `Scan` plus a blocking wait of `wait_ms`: what a lookup by MAC with no subnet uses.
+std::vector<DeviceInfo> discover_broadcast(int port, int wait_ms);
+
 /// One board: the UDP handshake, the TCP command channel, and the scan socket (`port + 1`).
 /// Not thread safe; the source's producer thread owns it.
 class Link {

src/source/e1608/e1608_source.cpp

@@ -274,8 +274,13 @@ void E1608Source::connect_and_start() {
   if (host.empty() || have_mac) {
     const std::vector<std::string> targets =
         host.empty() ? sweep_targets(cfg_.subnet) : std::vector<std::string>{host};
+    const int wait_ms = std::min(cfg_.timeout_ms, 1500);
+    // No address and no subnet: the limited broadcast, sent on every interface rather than only
+    // the default route's.
+    const bool broadcast =
+        host.empty() && targets.size() == 1 && targets[0] == "255.255.255.255";
     const std::vector<DeviceInfo> found =
-        discover(targets, cfg_.port, std::min(cfg_.timeout_ms, 1500));
+        broadcast ? discover_broadcast(cfg_.port, wait_ms) : discover(targets, cfg_.port, wait_ms);
     if (host.empty()) {
       for (const DeviceInfo& d : found) {
         if (d.mac == want) host = d.ip;