replace fake ICMP echo reply with test_loopback flag, honest error when no raw socket

src/icmp_proxy.c

@@ -124,8 +124,7 @@ static void exit_handle_request(struct ETCP_CONN* conn, struct ll_entry* entry)
 
     if (g_icmp_ctx->raw_sock != SOCKET_INVALID) {
         exit_send_echo(inst, client_node_id, dst_ip, echo_id, echo_seq, payload, payload_len);
-    } else {
-        // No raw socket (e.g. no root): send simulated echo reply back to client
+    } else if (g_icmp_ctx->test_loopback) {
         struct ll_entry* e = queue_entry_new(0);
         if (e) {
             e->dgram = u_malloc(ICMP_PROXY_HDR_SIZE + payload_len);
@@ -141,6 +140,8 @@ static void exit_handle_request(struct ETCP_CONN* conn, struct ll_entry* entry)
                 etcp_route_send(inst, client_node_id, e);
             } else queue_entry_free(e);
         }
+    } else {
+        DEBUG_ERROR(DEBUG_CATEGORY_SOCKET, "icmp_proxy: no raw socket, dropping echo request to 0x%08x", dst_ip);
     }
 
 drop:
@@ -240,6 +241,11 @@ int icmp_proxy_deliver_reply(struct UTUN_INSTANCE* inst,
     return 0;
 }
 
+void icmp_proxy_set_test_loopback(struct UTUN_INSTANCE* inst, int enabled) {
+    (void)inst;
+    if (g_icmp_ctx) g_icmp_ctx->test_loopback = enabled;
+}
+
 static void req_expire(struct icmp_proxy_ctx* ctx) {
     uint64_t now = get_time_tb(); struct icmp_request** prev = &ctx->pending;
     while (*prev) {
@@ -259,6 +265,7 @@ int icmp_proxy_init(struct UTUN_INSTANCE* inst, struct UASYNC* ua) {
     ctx->inst = inst; ctx->ua = ua; ctx->raw_sock = SOCKET_INVALID;
     ctx->request_timeout_tb = ICMP_TIMEOUT_TB;
     ctx->is_exit = inst->remote_proxy.enabled;
+    ctx->test_loopback = 0;
     g_icmp_ctx = ctx;
 
     if (ctx->is_exit) {

src/icmp_proxy.h

@@ -40,6 +40,7 @@ struct icmp_proxy_ctx {
     void*        raw_read_id;
     struct icmp_request* pending;
     uint64_t     request_timeout_tb;
+    int          test_loopback;    // 1 = virtual loopback when no raw socket (test only)
 };
 
 int icmp_proxy_init(struct UTUN_INSTANCE* inst, struct UASYNC* ua);
@@ -57,4 +58,6 @@ int icmp_proxy_deliver_reply(struct UTUN_INSTANCE* inst,
                              uint32_t src_ip, uint16_t echo_id, uint16_t echo_seq,
                              const uint8_t* payload, size_t payload_len);
 
+void icmp_proxy_set_test_loopback(struct UTUN_INSTANCE* inst, int enabled);
+
 #endif // ICMP_PROXY_H

tests/test_icmp_proxy.c

@@ -154,6 +154,8 @@ int main(void) {
     if (utun_instance_init(cli) < 0) { printf("[FAIL] client init\n"); goto done; }
     if (utun_instance_init(exit_node) < 0) { printf("[FAIL] exit init\n"); goto done; }
 
+    icmp_proxy_set_test_loopback(exit_node, 1);
+
     g_to_id = uasync_set_timeout(ua, 100, NULL, monitor, "mon");
     void* to_id = uasync_set_timeout(ua, TEST_TIMEOUT_MS * 10, NULL, test_timeout, "to");