feat(cli): add get link.dns for resolver, gateway and lease DNS

Shows lwIP's live global DNS server list next to the DNS each medium's DHCP lease installed, so a node that is linked but failing to resolve can be diagnosed remotely.

src/helpers/NetworkLink.cpp

@@ -33,6 +33,38 @@ extern void tcpipInit();
 
 namespace {
 
+static void appendDnsAddr(char* out, size_t out_size, const ip_addr_t& addr) {
+  const size_t len = strlen(out);
+  if (len >= out_size) return;
+  if (len > 0 && len + 1 < out_size) {
+    out[len] = ',';
+    out[len + 1] = '\0';
+  }
+  const size_t at = strlen(out);
+  if (ip_addr_isany_val(addr)) {
+    snprintf(out + at, out_size - at, "-");
+  } else {
+    ipaddr_ntoa_r(&addr, out + at, (int)(out_size - at));
+  }
+}
+
+static esp_err_t readDnsServersOnTcpipThread(void* ctx) {
+  ip_addr_t* servers = static_cast<ip_addr_t*>(ctx);
+  for (int i = 0; i < DNS_MAX_SERVERS; i++) {
+    const ip_addr_t* server = dns_getserver(i);
+    servers[i] = (server != nullptr) ? *server : *IP_ADDR_ANY;
+  }
+  return ESP_OK;
+}
+
+// lwIP's server list is global, so this is what every socket resolves through.
+static void formatCurrentDnsServers(char* out, size_t out_size) {
+  ip_addr_t servers[DNS_MAX_SERVERS] = {};
+  esp_netif_tcpip_exec(readDnsServersOnTcpipThread, servers);
+  out[0] = '\0';
+  for (int i = 0; i < DNS_MAX_SERVERS; i++) appendDnsAddr(out, out_size, servers[i]);
+}
+
 class NetworkLinkBase : public NetworkLink {
  protected:
   std::atomic<uint64_t> _outage_bits{AlertFaultPolicy::packOutageSnapshot({false, 0, 0})};
@@ -144,6 +176,36 @@ class NetworkLinkBase : public NetworkLink {
  public:
   void snapshotDns() { esp_netif_tcpip_exec(snapshotDnsOnTcpipThread, this); }
 
+  // "none" until this medium has held a lease that carried a resolver.
+  void formatLeaseDns(char* out, size_t out_size) const {
+    struct Copy {
+      const NetworkLinkBase* self;
+      ip_addr_t servers[kDnsServers];
+      bool captured;
+    } copy = {this, {}, false};
+    esp_netif_tcpip_exec([](void* ctx) -> esp_err_t {
+      Copy* c = static_cast<Copy*>(ctx);
+      c->captured = c->self->_dns_captured;
+      for (int i = 0; i < kDnsServers; i++) c->servers[i] = c->self->_dns_snapshot[i];
+      return ESP_OK;
+    }, &copy);
+    out[0] = '\0';
+    if (!copy.captured) {
+      snprintf(out, out_size, "none");
+      return;
+    }
+    for (int i = 0; i < kDnsServers; i++) appendDnsAddr(out, out_size, copy.servers[i]);
+  }
+
+  void formatDnsFor(char* reply, size_t reply_size, IPAddress gateway) const {
+    char current[64];
+    char lease[40];
+    formatCurrentDnsServers(current, sizeof(current));
+    formatLeaseDns(lease, sizeof(lease));
+    snprintf(reply, reply_size, "> dns:%s gw:%s lease:%s",
+             current, gateway.toString().c_str(), lease);
+  }
+
   // A medium that has never held a lease with a resolver leaves the current one
   // alone rather than blanking it.
   void restoreDns() { esp_netif_tcpip_exec(restoreDnsOnTcpipThread, this); }
@@ -324,6 +386,10 @@ class WiFiNetworkLink final : public NetworkLinkBase {
              statusName(),
              localIP().toString().c_str());
   }
+  void formatDns(char* reply, size_t reply_size) const override {
+    formatDnsFor(reply, reply_size, WiFi.gatewayIP());
+  }
+  IPAddress gatewayIP() const { return WiFi.gatewayIP(); }
 };
 
 #if defined(NETWORK_PREFER_ETHERNET)
@@ -471,6 +537,10 @@ class EthernetNetworkLink final : public NetworkLinkBase {
     snprintf(reply, reply_size, "> ethernet:%s ip=%s",
              statusName(), localIP().toString().c_str());
   }
+  void formatDns(char* reply, size_t reply_size) const override {
+    formatDnsFor(reply, reply_size, CH390.gatewayIP());
+  }
+  IPAddress gatewayIP() const { return CH390.gatewayIP(); }
 
   EventState eventState() const {
     return static_cast<EventState>(
@@ -936,6 +1006,22 @@ class AutomaticNetworkLink final : public NetworkLink {
              wifiConfigured() ? "yes" : "no", wifi_started ? "yes" : "no",
              (wifi_started && _wifi.isConnected()) ? "up" : "down");
   }
+  void formatDns(char* reply, size_t reply_size) const override {
+    const NetworkMedium selected = _selected.load(std::memory_order_acquire);
+    char current[64];
+    char eth_lease[40];
+    char wifi_lease[40];
+    formatCurrentDnsServers(current, sizeof(current));
+    _ethernet.formatLeaseDns(eth_lease, sizeof(eth_lease));
+    _wifi.formatLeaseDns(wifi_lease, sizeof(wifi_lease));
+    const IPAddress gateway = selected == NetworkMedium::Ethernet ? _ethernet.gatewayIP()
+                            : selected == NetworkMedium::WiFi     ? _wifi.gatewayIP()
+                                                                  : IPAddress();
+    snprintf(reply, reply_size,
+             "> dns:%s selected:%s gw:%s\neth-lease:%s wifi-lease:%s",
+             current, NetworkPolicy::mediumName(selected),
+             gateway.toString().c_str(), eth_lease, wifi_lease);
+  }
   unsigned long connectedAtMillis() const override {
     const NetworkMedium selected = _selected.load(std::memory_order_acquire);
     return selected == NetworkMedium::None