Add network block LD_PRELOAD script

Author: MrLGamer

network_block.c

@@ -0,0 +1,48 @@
+#define _GNU_SOURCE
+#include <dlfcn.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <errno.h>
+#include <stdbool.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <netinet/in.h>
+#include <arpa/inet.h>
+
+static int (*real_connect)(int, const struct sockaddr *, socklen_t) = NULL;
+static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
+static int connect_count = 0;
+
+static void init_real_connect(void) {
+    if (!real_connect) {
+        real_connect = dlsym(RTLD_NEXT, "connect");
+    }
+}
+
+int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen) {
+    init_real_connect();
+
+    pthread_mutex_lock(&lock);
+    connect_count++;
+    int n = connect_count;
+    pthread_mutex_unlock(&lock);
+
+    // Optional: log destination for debugging
+    if (addr && addrlen >= sizeof(struct sockaddr_in) && addr->sa_family == AF_INET) {
+        const struct sockaddr_in *sa = (const struct sockaddr_in *)addr;
+        char ip[INET_ADDRSTRLEN];
+        inet_ntop(AF_INET, &sa->sin_addr, ip, sizeof(ip));
+        fprintf(stderr, "[blockfirst] #%d connect to %s:%d\n",
+                n, ip, ntohs(sa->sin_port));
+    }
+
+    // Block *only* the first connect() call for this process
+    if (n == 1) {
+        errno = ECONNREFUSED;
+        fprintf(stderr, "[blockfirst] blocking first connect()\n");
+        return -1;
+    }
+
+    // All later calls: behave normally
+    return real_connect(sockfd, addr, addrlen);
+}