Add CGNAT STUN port discovery with TCP proxy keepalive

- Independent CGNATEnabled toggle (default off, decoupled from UPnP)
- UDP STUN discovers external port for uTP
- Standalone TCP proxy maintains NAT mapping + forwards inbound connections
- Fix STUN XOR-MAPPED-ADDRESS byte-order bug
- WebUI settings: enable toggle, STUN server, probe interval
- Default STUN server empty (user-configured)

Verified: 364MB upload through CGNAT proxy port on pure IPv4 Full Cone NAT.

Author: qzgeek

src/base/CMakeLists.txt

@@ -10,6 +10,7 @@ add_library(qbt_base STATIC
     bittorrent/bandwidthscheduler.h
     bittorrent/bencoderesumedatastorage.h
     bittorrent/cachestatus.h
+    bittorrent/cgnatmanager.h
     bittorrent/categoryoptions.h
     bittorrent/common.h
     bittorrent/customstorage.h
@@ -138,6 +139,7 @@ add_library(qbt_base STATIC
     bittorrent/bandwidthscheduler.cpp
     bittorrent/bencoderesumedatastorage.cpp
     bittorrent/categoryoptions.cpp
+    bittorrent/cgnatmanager.cpp
     bittorrent/customstorage.cpp
     bittorrent/dbresumedatastorage.cpp
     bittorrent/downloadpathoption.cpp
@@ -304,3 +306,8 @@ if (PLUGINS)
             plugins/pluginversion.h
     )
 endif()
+
+# CGNAT TCP proxy helper (pure C, standalone)
+add_executable(cgnat_proxy bittorrent/cgnat_proxy.cpp)
+
+set_target_properties(cgnat_proxy PROPERTIES AUTOMOC OFF)

src/base/bittorrent/cgnat_proxy.cpp

@@ -0,0 +1,178 @@
+// cgnat_proxy.cpp — standalone TCP proxy + keepalive for CGNAT
+// Usage: cgnat_proxy <listen_port> <backend_port> <port_check_url>
+// Outputs "PORT=<ext_port>\n" on startup, then proxies.
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <errno.h>
+#include <signal.h>
+#include <time.h>
+#include <sys/socket.h>
+#include <sys/select.h>
+#include <netinet/in.h>
+#include <netdb.h>
+#include <arpa/inet.h>
+
+#ifndef SO_REUSEPORT
+#define SO_REUSEPORT 15
+#endif
+
+static volatile int running = 1;
+static void sig_handler(int) { running = 0; }
+
+static int discover_port(int bind_port, const char *url)
+{
+    char host[256] = {0}, path[256] = "/";
+    int port = 80;
+    const char *p = url;
+    if (strncmp(p, "http://", 7) == 0) p += 7;
+    const char *slash = strchr(p, '/'), *colon = strchr(p, ':');
+    if (colon && (!slash || colon < slash)) {
+        memcpy(host, p, colon - p); port = atoi(colon + 1);
+    } else if (slash) memcpy(host, p, slash - p);
+    else strncpy(host, p, sizeof(host)-1);
+    if (slash) strncpy(path, slash, sizeof(path)-1);
+
+    struct hostent *he = gethostbyname(host);
+    if (!he) return -1;
+
+    int fd = socket(AF_INET, SOCK_STREAM, 0);
+    if (fd < 0) return -1;
+    int opt = 1;
+    setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
+    setsockopt(fd, SOL_SOCKET, SO_REUSEPORT, &opt, sizeof(opt));
+
+    struct sockaddr_in local = {0};
+    local.sin_family = AF_INET; local.sin_port = htons(bind_port); local.sin_addr.s_addr = INADDR_ANY;
+    if (bind(fd, (struct sockaddr *)&local, sizeof(local)) < 0) { close(fd); return -1; }
+
+    struct sockaddr_in remote = {0};
+    remote.sin_family = AF_INET; remote.sin_port = htons(port);
+    memcpy(&remote.sin_addr, he->h_addr_list[0], he->h_length);
+    if (connect(fd, (struct sockaddr *)&remote, sizeof(remote)) < 0) { close(fd); return -1; }
+
+    char req[512];
+    snprintf(req, sizeof(req), "GET %s HTTP/1.0\r\nHost: %s\r\nConnection: close\r\n\r\n", path, host);
+    send(fd, req, strlen(req), 0);
+
+    char buf[4096]; int n = 0;
+    fd_set fds; struct timeval tv = {3, 0};
+    FD_ZERO(&fds); FD_SET(fd, &fds);
+    if (select(fd+1, &fds, NULL, NULL, &tv) > 0) n = recv(fd, buf, sizeof(buf)-1, 0);
+    close(fd);
+    if (n <= 0) return -1;
+    buf[n] = 0;
+
+    char *body = strstr(buf, "\r\n\r\n");
+    if (!body) return -1;
+    body += 4;
+    while (*body == ' ' || *body == '\r' || *body == '\n') body++;
+    return atoi(body);
+}
+
+int main(int argc, char *argv[])
+{
+    if (argc < 4) { fprintf(stderr, "Usage: %s <listen_port> <backend_port> <url>\n", argv[0]); return 1; }
+    int lp = atoi(argv[1]), bp = atoi(argv[2]);
+    const char *url = argv[3];
+    signal(SIGTERM, sig_handler); signal(SIGINT, sig_handler); signal(SIGPIPE, SIG_IGN);
+
+    // Discover
+    int ext = discover_port(lp, url);
+    if (ext <= 0) { fprintf(stderr, "port discovery failed\n"); return 1; }
+    printf("PORT=%d\n", ext); fflush(stdout);
+
+    // Listen socket
+    int ls = socket(AF_INET, SOCK_STREAM, 0);
+    int opt = 1;
+    setsockopt(ls, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
+    setsockopt(ls, SOL_SOCKET, SO_REUSEPORT, &opt, sizeof(opt));
+    struct sockaddr_in la = {0};
+    la.sin_family = AF_INET; la.sin_port = htons(lp); la.sin_addr.s_addr = INADDR_ANY;
+    if (bind(ls, (struct sockaddr *)&la, sizeof(la)) < 0 || listen(ls, 32) < 0)
+    { perror("bind/listen"); return 1; }
+
+    // Keepalive
+    int kf = -1; time_t last_ka = 0;
+    char ka_host[256] = {0};
+    { const char *p = url; if (strncmp(p, "http://", 7) == 0) p += 7;
+      const char *s = strchr(p, '/'); size_t n = s ? (size_t)(s-p) : strlen(p);
+      memcpy(ka_host, p, n < 255 ? n : 255); }
+
+    auto open_ka = [&]() -> int {
+        struct hostent *he = gethostbyname(ka_host);
+        if (!he) return -1;
+        int fd = socket(AF_INET, SOCK_STREAM, 0);
+        if (fd < 0) return -1;
+        setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
+        setsockopt(fd, SOL_SOCKET, SO_REUSEPORT, &opt, sizeof(opt));
+        struct sockaddr_in l = {0};
+        l.sin_family = AF_INET; l.sin_port = htons(lp); l.sin_addr.s_addr = INADDR_ANY;
+        if (bind(fd, (struct sockaddr *)&l, sizeof(l)) < 0) { close(fd); return -1; }
+        struct sockaddr_in r = {0};
+        r.sin_family = AF_INET; r.sin_port = htons(80);
+        memcpy(&r.sin_addr, he->h_addr_list[0], he->h_length);
+        if (connect(fd, (struct sockaddr *)&r, sizeof(r)) < 0) { close(fd); return -1; }
+        return fd;
+    };
+
+    // Client slots
+    struct { int cf, bf; } clients[64];
+    memset(clients, -1, sizeof(clients));
+
+    while (running)
+    {
+        fd_set rfds; FD_ZERO(&rfds);
+        FD_SET(ls, &rfds); int maxfd = ls;
+        if (kf >= 0) { FD_SET(kf, &rfds); if (kf > maxfd) maxfd = kf; }
+        for (int i = 0; i < 64; i++) {
+            if (clients[i].cf >= 0) { FD_SET(clients[i].cf, &rfds); if (clients[i].cf > maxfd) maxfd = clients[i].cf; }
+            if (clients[i].bf >= 0) { FD_SET(clients[i].bf, &rfds); if (clients[i].bf > maxfd) maxfd = clients[i].bf; }
+        }
+        struct timeval tv = {1, 0};
+        if (select(maxfd+1, &rfds, NULL, NULL, &tv) < 0) break;
+
+        if (FD_ISSET(ls, &rfds)) {
+            int cf = accept(ls, NULL, NULL);
+            if (cf >= 0) {
+                int bf = socket(AF_INET, SOCK_STREAM, 0);
+                struct sockaddr_in be = {0};
+                be.sin_family = AF_INET; be.sin_port = htons(bp); be.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
+                if (bf < 0 || connect(bf, (struct sockaddr *)&be, sizeof(be)) < 0) {
+                    if (bf >= 0) close(bf); close(cf);
+                } else {
+                    for (int i = 0; i < 64; i++) if (clients[i].cf < 0) { clients[i].cf = cf; clients[i].bf = bf; break; }
+                }
+            }
+        }
+
+        for (int i = 0; i < 64; i++) {
+            if (clients[i].cf >= 0 && FD_ISSET(clients[i].cf, &rfds)) {
+                char buf[8192]; ssize_t n = recv(clients[i].cf, buf, sizeof(buf), 0);
+                if (n <= 0) { close(clients[i].cf); close(clients[i].bf); clients[i].cf = -1; }
+                else if (clients[i].bf >= 0) send(clients[i].bf, buf, n, MSG_NOSIGNAL);
+            }
+            if (clients[i].bf >= 0 && FD_ISSET(clients[i].bf, &rfds)) {
+                char buf[8192]; ssize_t n = recv(clients[i].bf, buf, sizeof(buf), 0);
+                if (n <= 0) { close(clients[i].cf); close(clients[i].bf); clients[i].cf = -1; }
+                else if (clients[i].cf >= 0) send(clients[i].cf, buf, n, MSG_NOSIGNAL);
+            }
+        }
+
+        time_t now = time(NULL);
+        if (now - last_ka > 25) {
+            if (kf >= 0) {
+                char head[256]; snprintf(head, sizeof(head), "HEAD / HTTP/1.0\r\nHost: %s\r\nConnection: keep-alive\r\n\r\n", ka_host);
+                if (send(kf, head, strlen(head), MSG_NOSIGNAL) < 0) { close(kf); kf = -1; }
+            }
+            if (kf < 0) kf = open_ka();
+            last_ka = now;
+        }
+    }
+
+    for (int i = 0; i < 64; i++) { if (clients[i].cf >= 0) { close(clients[i].cf); close(clients[i].bf); } }
+    if (kf >= 0) close(kf); close(ls);
+    return 0;
+}