1

lib/platform_compat.c

@@ -3,16 +3,27 @@
 #include "platform_compat.h"
 #include <stddef.h>
 #include <stdint.h>
+#include <string.h>
+#include <stdio.h>
+#include <stdlib.h>
 
 #ifdef _WIN32
     #include <windows.h>
     #include <bcrypt.h>
+    #include <iphlpapi.h>
+    #pragma comment(lib, "iphlpapi.lib")
     #ifndef STATUS_SUCCESS
         #define STATUS_SUCCESS ((NTSTATUS)0x00000000L)
     #endif
 #else
     #include <fcntl.h>
     #include <unistd.h>
+    #include <sys/types.h>
+    #include <sys/socket.h>
+    #include <netinet/in.h>
+    #include <arpa/inet.h>
+    #include <ifaddrs.h>
+    #include <net/if.h>
 #endif
 
 /*
@@ -35,3 +46,93 @@ int random_bytes(uint8_t *buffer, size_t len) {
     return (ret == (ssize_t)len) ? 0 : -1;
 #endif
 }
+
+#ifndef _WIN32
+static uint32_t get_default_route_ip_linux(void) {
+    FILE *f = fopen("/proc/net/route", "r");
+    if (!f) return 0;
+
+    char line[256];
+    char *ifname = NULL;
+    unsigned int dest, gw;
+
+    if (!fgets(line, sizeof(line), f)) {
+        fclose(f);
+        return 0;
+    }
+
+    while (fgets(line, sizeof(line), f)) {
+        int items = sscanf(line, "%ms %x %x", &ifname, &dest, &gw);
+        if (items != 3) {
+            free(ifname);
+            continue;
+        }
+
+        if (dest == 0) {
+            free(ifname);
+            break;
+        }
+        free(ifname);
+        ifname = NULL;
+    }
+
+    fclose(f);
+
+    if (!ifname) return 0;
+
+    struct ifaddrs *ifaddr, *ifa;
+    if (getifaddrs(&ifaddr) == -1) {
+        free(ifname);
+        return 0;
+    }
+
+    uint32_t result = 0;
+    for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
+        if (ifa->ifa_addr == NULL) continue;
+        if (ifa->ifa_name == NULL) continue;
+        if (strcmp(ifa->ifa_name, ifname) != 0) continue;
+        if (ifa->ifa_addr->sa_family != AF_INET) continue;
+
+        struct sockaddr_in *addr = (struct sockaddr_in *)ifa->ifa_addr;
+        result = addr->sin_addr.s_addr;
+        break;
+    }
+
+    freeifaddrs(ifaddr);
+    free(ifname);
+    return result;
+}
+#else
+static uint32_t get_default_route_ip_windows(void) {
+    SOCKET s = socket(AF_INET, SOCK_DGRAM, 0);
+    if (s == INVALID_SOCKET) return 0;
+
+    struct sockaddr_in remote;
+    remote.sin_family = AF_INET;
+    remote.sin_port = htons(53);
+    remote.sin_addr.s_addr = inet_addr("8.8.8.8");
+
+    if (connect(s, (struct sockaddr *)&remote, sizeof(remote)) == SOCKET_ERROR) {
+        closesocket(s);
+        return 0;
+    }
+
+    struct sockaddr_in local;
+    int len = sizeof(local);
+    if (getsockname(s, (struct sockaddr *)&local, &len) == SOCKET_ERROR) {
+        closesocket(s);
+        return 0;
+    }
+
+    closesocket(s);
+    return local.sin_addr.s_addr;
+}
+#endif
+
+uint32_t get_default_route_ip(void) {
+#ifdef _WIN32
+    return get_default_route_ip_windows();
+#else
+    return get_default_route_ip_linux();
+#endif
+}

lib/platform_compat.h

@@ -136,8 +136,14 @@
     #include <poll.h>
 #endif
 
+#include <stdint.h>
+
 // Generate cryptographically secure random bytes
 // Returns 0 on success, -1 on error
 int random_bytes(uint8_t *buffer, size_t len);
 
+// Get IPv4 address of interface with default route (0.0.0.0 destination)
+// Returns IPv4 address in network byte order, 0 on error
+uint32_t get_default_route_ip(void);
+
 #endif // PLATFORM_COMPAT_H