implement auto gateway Ip auto detection (#25)

Reviewed-on: #25
implemented #19

src/main.c

@@ -15,6 +15,7 @@
 #include <net/if.h>
 #include <sys/ioctl.h>
 #include <sys/socket.h>
+#include <net/route.h>
 #include <arpa/inet.h>
 #include <netinet/in.h>
 
@@ -64,6 +65,54 @@ static int get_ip(const char *iface, char *out, size_t sz)
     return ret;
 }
 
+/* Find the IPv4 next hop used by the kernel's default route. Prefer the
+ * configured LAN interface when more than one default route exists. */
+static int get_default_gateway(const char *preferred_iface,
+                               char *out, size_t sz)
+{
+    FILE *f = fopen("/proc/net/route", "r");
+    if (!f) return -1;
+
+    char line[256];
+    char iface[IFNAMSIZ];
+    unsigned long destination, gateway;
+    unsigned int flags, metric;
+    unsigned int best_metric = ~0U;
+    unsigned long best_gateway = 0;
+    int best_preferred = 0;
+
+    /* Skip the column headings. */
+    if (!fgets(line, sizeof(line), f)) {
+        fclose(f);
+        return -1;
+    }
+
+    while (fgets(line, sizeof(line), f)) {
+        if (sscanf(line, "%15s %lx %lx %x %*u %*u %u %*lx",
+                   iface, &destination, &gateway, &flags,
+                   &metric) != 5)
+            continue;
+        if (destination != 0 || gateway == 0 ||
+            !(flags & RTF_UP) || !(flags & RTF_GATEWAY))
+            continue;
+
+        int preferred = preferred_iface && preferred_iface[0] &&
+                        !strcmp(iface, preferred_iface);
+        if (!best_gateway || preferred > best_preferred ||
+            (preferred == best_preferred && metric < best_metric)) {
+            best_gateway = gateway;
+            best_metric = metric;
+            best_preferred = preferred;
+        }
+    }
+    fclose(f);
+
+    if (!best_gateway) return -1;
+
+    struct in_addr addr = { .s_addr = (in_addr_t)best_gateway };
+    return inet_ntop(AF_INET, &addr, out, sz) ? 0 : -1;
+}
+
 int main(int argc, char *argv[])
 {
     for (int i = 1; i < argc-1; i++)
@@ -105,9 +154,31 @@ int main(int argc, char *argv[])
     LOG("Initial device state: adopted=%d, authkey=%.8s...", state.adopted, 
         state.authkey[0] ? state.authkey : "DEFAULT");
 
-    if (!state.adopted || !state.inform_url[0])
+    /*
+     * A configured controller is an explicit override, including after
+     * adoption. With an empty setting, discover the initial controller from
+     * the default-route gateway and then preserve controller-issued state.
+     */
+    if (cfg.controller_ip[0]) {
         snprintf(state.inform_url, sizeof(state.inform_url),
                  "http://%s:%d%s", cfg.controller_ip, INFORM_PORT, INFORM_PATH);
+        LOG("Using configured controller override: %s", cfg.controller_ip);
+    } else if (!state.adopted || !state.inform_url[0]) {
+        char controller_ip[INET_ADDRSTRLEN] = {0};
+        if (get_default_gateway(cfg.lan_if, controller_ip,
+                                sizeof(controller_ip)) == 0) {
+            snprintf(state.inform_url, sizeof(state.inform_url),
+                     "http://%s:%d%s", controller_ip,
+                     INFORM_PORT, INFORM_PATH);
+            LOG("Autodetected controller from default gateway: %s",
+                controller_ip);
+        } else {
+            state.inform_url[0] = '\0';
+            fprintf(stderr,
+                    "[openuf] No controller configured and no IPv4 default "
+                    "gateway found\n");
+        }
+    }
     state_save(&state);
 
     printf("[openuf] Starting  model=%-8s  MAC=%s  IP=%s\n",
@@ -180,6 +251,20 @@ int main(int argc, char *argv[])
             last_inform = now;
             LOG("Sending inform");
 
+            /* The default route may appear after the service starts. */
+            if (!cfg.controller_ip[0] && !state.inform_url[0]) {
+                char controller_ip[INET_ADDRSTRLEN] = {0};
+                if (get_default_gateway(cfg.lan_if, controller_ip,
+                                        sizeof(controller_ip)) == 0) {
+                    snprintf(state.inform_url, sizeof(state.inform_url),
+                             "http://%s:%d%s", controller_ip,
+                             INFORM_PORT, INFORM_PATH);
+                    LOG("Autodetected controller after startup: %s",
+                        controller_ip);
+                    state_save(&state);
+                }
+            }
+
             /* Update IP each cycle */
             char new_ip[64] = {0};
             if (get_ip(cfg.lan_if, new_ip, sizeof(new_ip)) == 0 ||