[cli] update br scan to allow network interface selection for mDNS …

…binding (#260)

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

This change introduces a new option for the CLI command `br
scan`. User can now specify a network interface using the syntax `br
scan --netif <network interface>`. The chosen interface will be used
for mDNS binding through socket options.

src/app/cli/README.md

@@ -743,7 +743,7 @@ Each advanced command always sends associative `MGMT_*_(GET|SET).req` requests.
 ```shell
 > help br
 usage:
-br scan [--nwk <network-alias-list> | --dom <domain-name>] [--export <json-file-path>] [--timeout <ms>]
+br scan [--nwk <network-alias-list> | --dom <domain-name>] [--export <json-file-path>] [--timeout <ms>] [--netif <network-interface>]
 br add <json-file-path>
 br list [--nwk <network-alias-list> | --dom <domain-name>]
 br delete (<br-record-id> | --nwk <network-alias-list> | --dom <domain-name>)

src/app/cli/interpreter.cpp

@@ -45,6 +45,7 @@
 #include <vector>
 
 #include <fcntl.h>
+#include <sys/socket.h>
 #include <sys/types.h>
 #include <unistd.h>
 
@@ -111,6 +112,8 @@
 #define WARN_NETWORK_SELECTION_DROPPED "Network selection was dropped by the command"
 #define WARN_NETWORK_SELECTION_CHANGED "Network selection was changed by the command"
 
+#define SO_BINDTODEVICE 25
+
 #define COLOR_ALIAS_FAILED Console::Color::kYellow
 
 namespace ot {
@@ -213,7 +216,8 @@ const std::map<std::string, std::string> &Interpreter::mUsageMap = *new std::map
     {"br", "br list [--nwk <network-alias-list> | --dom <domain-name>]\n"
            "br add <json-file-path>\n"
            "br delete (<br-record-id> | --nwk <network-alias-list> | --dom <domain-name>)\n"
-           "br scan [--nwk <network-alias-list> | --dom <domain-name>] [--export <json-file-path>] [--timeout <ms>]\n"},
+           "br scan [--nwk <network-alias-list> | --dom <domain-name>] [--export <json-file-path>] [--timeout <ms>] "
+           "[--netif <network-interface>]\n"},
     {"domain", "domain list [--dom <domain-name>]"},
     {"network", "network save <network-data-file>\n"
                 "network sync\n"
@@ -1421,6 +1425,7 @@ Interpreter::Value Interpreter::ProcessBr(const Expression &aExpr)
         const std::string                  kServiceName    = "_meshcop._udp.local";
 
         uint32_t                                  scanTimeout = 10000;
+        std::string                               netIf       = "";
         int                                       mdnsSocket  = -1;
         FDGuard                                   fdgMdnsSocket;
         std::thread                               selectThread;
@@ -1432,20 +1437,41 @@ Interpreter::Value Interpreter::ProcessBr(const Expression &aExpr)
         nlohmann::json                            baJson;
         char                                      mdnsSendBuffer[kMdnsBufferSize];
 
-        if (mContext.mCommandKeys.size() == 2 && mContext.mCommandKeys[0] == "--timeout")
+        auto it = std::find(mContext.mCommandKeys.begin(), mContext.mCommandKeys.end(), "--timeout");
+        if (it != mContext.mCommandKeys.end())
         {
-            try
+            if (++it != mContext.mCommandKeys.end())
             {
-                scanTimeout = stol(mContext.mCommandKeys[1]);
-            } catch (...)
+                SuccessOrExit(value = ParseInteger(scanTimeout, it[0]));
+            }
+            else
             {
-                ExitNow(value = ERROR_INVALID_ARGS("Imparsable timeout value '{}'", aExpr[3]));
+                ExitNow(value = ERROR_INVALID_ARGS("Missing --timeout value"));
+            }
+        }
+
+        it = std::find(mContext.mCommandKeys.begin(), mContext.mCommandKeys.end(), "--netif");
+        if (it != mContext.mCommandKeys.end())
+        {
+            if (++it != mContext.mCommandKeys.end())
+            {
+                netIf = it[0];
+            }
+            else
+            {
+                ExitNow(value = ERROR_INVALID_ARGS("Missing --netif value"));
             }
         }
 
         // Open IPv4 mDNS socket
         mdnsSocket = mdns_socket_open_ipv4();
         VerifyOrExit(mdnsSocket >= 0, value = ERROR_IO_ERROR("failed to open mDNS IPv4 socket"));
+
+        if (!netIf.empty() && setsockopt(mdnsSocket, SOL_SOCKET, SO_BINDTODEVICE, netIf.c_str(), netIf.size()) < 0)
+        {
+            ExitNow(value = ERROR_INVALID_ARGS("failed to bind network interface {}: {}", netIf, strerror(errno)));
+        }
+
         fdgMdnsSocket.mFD = mdnsSocket;
 
         //  Initialize event library