[cli] Update command "borderagent discovery" to allow network interfa…

…ce selection for mDNS binding.

Within the OTBR practice application, the BR host will have several
interfaces. When users want to discover the border agent using mDNS,
they'll need to choose the specific interface the border agent is bound
to.

This change introduces a new option for the CLI command `borderagent
dicover`. Users can now specify a network interface using the syntax
`borderagent discovery <timeout> <network interface>`. The chosen
interface will be used for mDNS binding through socket options.

src/app/br_discover.cpp

@@ -30,6 +30,7 @@
 
 #include <chrono>
 #include <thread>
+#include <sys/socket.h>
 
 #include "common/error_macros.hpp"
 #include "common/utils.hpp"
@@ -38,7 +39,7 @@ namespace ot {
 
 namespace commissioner {
 
-Error DiscoverBorderAgent(BorderAgentHandler aBorderAgentHandler, size_t aTimeout)
+Error DiscoverBorderAgent(BorderAgentHandler aBorderAgentHandler, size_t aTimeout, const std::string &aNetIf)
 {
     static constexpr size_t             kDefaultBufferSize = 1024 * 16;
     static constexpr mdns_record_type_t kMdnsQueryType     = MDNS_RECORDTYPE_PTR;
@@ -52,6 +53,10 @@ Error DiscoverBorderAgent(BorderAgentHandler aBorderAgentHandler, size_t aTimeou
     int socket = mdns_socket_open_ipv4();
     VerifyOrExit(socket >= 0, error = ERROR_IO_ERROR("failed to open mDNS IPv4 socket"));
 
+    if (aNetIf != "" && setsockopt(socket, SOL_SOCKET, SO_BINDTODEVICE, &aNetIf[0], sizeof(aNetIf)) < 0) {
+         ExitNow(error = ERROR_IO_ERROR("failed to bind network interface: {}", aNetIf));
+    }
+
     if (mdns_query_send(socket, kMdnsQueryType, kServiceName, strlen(kServiceName), buf, sizeof(buf)) != 0)
     {
         ExitNow(error = ERROR_IO_ERROR("failed to send mDNS query"));

src/app/br_discover.hpp

@@ -56,9 +56,11 @@ using BorderAgentHandler = std::function<void(const BorderAgent *aBorderAgent, c
  * @param[in] aBorderAgentHandler  The handler of a single Border Agent response.
  * @param[in] aTimeout             The time to wait for mDNS responses. Any
  *                                 response not within the interval is ignored.
+ * @param[in] aNetIf               The specified network interface for mDNS binding.
+ *
  *
  */
-Error DiscoverBorderAgent(BorderAgentHandler aBorderAgentHandler, size_t aTimeout);
+Error DiscoverBorderAgent(BorderAgentHandler aBorderAgentHandler, size_t aTimeout, const std::string &aNetIf);
 
 } // namespace commissioner
 

src/app/cli/README.md

@@ -377,7 +377,7 @@ The command `borderagent` provides access to Border Agent information and scans
 ```shell
 > help borderagent
 usage:
-borderagent discover [<timeout-in-milliseconds>]
+borderagent discover [<timeout-in-milliseconds>] [<specified-network-interface>]
 borderagent get locator
 [done]
 >

src/app/cli/interpreter.cpp

@@ -1793,8 +1793,14 @@ Interpreter::Value Interpreter::ProcessBorderAgent(const Expression &aExpr)
         {
             SuccessOrExit(value = ParseInteger(timeout, aExpr[2]));
         }
+        std::string netIf = "";
 
-        SuccessOrExit(value = DiscoverBorderAgent(BorderAgentHandler, static_cast<size_t>(timeout)));
+        if (aExpr.size() == 4)
+        {
+            netIf = aExpr[3];
+        }
+
+        SuccessOrExit(value = DiscoverBorderAgent(BorderAgentHandler, static_cast<size_t>(timeout), netIf));
     }
     else if (CaseInsensitiveEqual(aExpr[1], "get"))
     {