core: always convert hostname to IP for UDP remotes

Otherwise we end up with two connections, one for the hostname that we
add and one for the IP where we receive messages from.

src/mavsdk/core/CMakeLists.txt

@@ -24,6 +24,7 @@ target_sources(mavsdk
     file_cache.cpp
     flight_mode.cpp
     fs_utils.cpp
+    hostname_to_ip.cpp
     inflate_lzma.cpp
     math_conversions.cpp
     mavsdk.cpp
@@ -85,8 +86,9 @@ if (NOT BUILD_WITHOUT_CURL)
     )
 
     list(APPEND UNIT_TEST_SOURCES
-        # TODO: add this again
         ${PROJECT_SOURCE_DIR}/mavsdk/core/curl_test.cpp
+        ${PROJECT_SOURCE_DIR}/mavsdk/core/hostname_to_ip_test.cpp
+        # TODO: add this again
         #${PROJECT_SOURCE_DIR}/mavsdk/core/http_loader_test.cpp
     )
 else()

src/mavsdk/core/hostname_to_ip.cpp

@@ -0,0 +1,65 @@
+#include "hostname_to_ip.h"
+#include "log.h"
+
+#include <cstring>
+#include <netdb.h>
+#include <arpa/inet.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <ifaddrs.h>
+#include <netinet/in.h>
+
+#if defined(WINDOWS)
+#include <winsock2.h>
+#include <ws2tcpip.h>
+#pragma comment(lib, "ws2_32.lib")
+#endif
+
+namespace mavsdk {
+
+std::optional<std::string> resolve_hostname_to_ip(const std::string& hostname)
+{
+#if defined(WINDOWS)
+    WSADATA wsaData;
+    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
+        std::cerr << "WSAStartup failed" << std::endl;
+        return "";
+    }
+#endif
+
+    addrinfo hints{};
+    hints.ai_family = AF_INET; // IPv4
+    hints.ai_socktype = SOCK_STREAM;
+    hints.ai_flags = AI_PASSIVE;
+
+    addrinfo* result = nullptr;
+    int res = getaddrinfo(hostname.c_str(), nullptr, &hints, &result);
+    if (res != 0) {
+#if defined(WINDOWS)
+        LogErr() << "getaddrinfo failed: " << WSAGetLastError();
+#else
+        LogErr() << "getaddrinfo failed: " << gai_strerror(res);
+#endif
+#if defined(WINDOWS)
+        WSACleanup();
+#endif
+        return {};
+    }
+
+    std::string ipAddress;
+    for (addrinfo* ptr = result; ptr != nullptr; ptr = ptr->ai_next) {
+        sockaddr_in* sockaddrIpv4 = reinterpret_cast<sockaddr_in*>(ptr->ai_addr);
+        char ipStr[INET_ADDRSTRLEN];
+        inet_ntop(AF_INET, &(sockaddrIpv4->sin_addr), ipStr, INET_ADDRSTRLEN);
+        ipAddress = ipStr;
+        break; // Take the first result
+    }
+
+    freeaddrinfo(result);
+#if defined(WINDOWS)
+    WSACleanup();
+#endif
+    return {ipAddress};
+}
+
+} // namespace mavsdk

src/mavsdk/core/hostname_to_ip.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <string>
+#include <optional>
+
+namespace mavsdk {
+
+std::optional<std::string> resolve_hostname_to_ip(const std::string& hostname);
+
+} // namespace mavsdk

src/mavsdk/core/hostname_to_ip_test.cpp

@@ -0,0 +1,34 @@
+#include "hostname_to_ip.h"
+#include <gtest/gtest.h>
+
+using namespace mavsdk;
+
+TEST(HostnameToIp, Localhost)
+{
+    auto host = "localhost";
+    auto ip = resolve_hostname_to_ip(host);
+    ASSERT_TRUE(ip);
+    EXPECT_EQ(ip.value(), "127.0.0.1");
+}
+
+TEST(HostnameToIp, Ip)
+{
+    auto host = "127.0.0.1";
+    auto ip = resolve_hostname_to_ip(host);
+    ASSERT_TRUE(ip);
+    EXPECT_EQ(ip.value(), host);
+}
+
+TEST(HostnameToIp, Empty)
+{
+    auto host = "";
+    auto ip = resolve_hostname_to_ip(host);
+    EXPECT_FALSE(ip);
+}
+
+TEST(HostnameToIp, Something)
+{
+    auto host = "something";
+    auto ip = resolve_hostname_to_ip(host);
+    EXPECT_FALSE(ip);
+}

src/mavsdk/core/mavsdk_impl.cpp

@@ -16,6 +16,7 @@
 #include "plugin_base.h"
 #include "mavlink_channels.h"
 #include "callback_list.tpp"
+#include "hostname_to_ip.h"
 
 namespace mavsdk {
 
@@ -538,7 +539,15 @@ MavsdkImpl::add_udp_connection(const CliArg::Udp& udp, ForwardingOption forwardi
     auto handle = add_connection(new_conn);
 
     if (udp.mode == CliArg::Udp::Mode::Out) {
-        new_conn->add_remote(udp.host, udp.port);
+        // We need to add the IP rather than a hostname, otherwise we end up with two remotes:
+        // one for the IP, and one for a hostname.
+        auto remote_ip = resolve_hostname_to_ip(udp.host);
+
+        if (!remote_ip) {
+            return {ConnectionResult::DestinationIpUnknown, Mavsdk::ConnectionHandle{}};
+        }
+
+        new_conn->add_remote(remote_ip.value(), udp.port);
         std::lock_guard<std::recursive_mutex> lock(_systems_mutex);
 
         // With a UDP remote, we need to initiate the connection by sending heartbeats.