issue: 4398221 Fix connection closure on SYN_RCVD

When closing a connection in SYN_RCVD state, we should transition
directly to CLOSED without sending FIN or RST packets, as specified
in the RFC. This patch properly handles this case by immediately
changing the state to CLOSED instead of going through the normal
shutdown procedure.

This prevents unnecessary control packets when a connection receives
a SYN and is immediately closed without exchanging data.

Signed-off-by: Tomer Cabouly <[email protected]>

Author: tomerdbz

src/core/lwip/tcp.c

@@ -148,6 +148,14 @@ static err_t tcp_close_shutdown(struct tcp_pcb *pcb, u8_t rst_on_unacked_data)
 {
     err_t err;
 
+    if (get_tcp_state(pcb) == SYN_RCVD) {
+        // according to the RFC, in case we get a SYN and no more data
+        // we should just close w/o FIN or RST
+        tcp_pcb_purge(pcb);
+        set_tcp_state(pcb, CLOSED);
+        return ERR_OK;
+    }
+
     if (rst_on_unacked_data &&
         ((get_tcp_state(pcb) == ESTABLISHED) || (get_tcp_state(pcb) == CLOSE_WAIT))) {
         if (pcb->rcv_wnd != pcb->rcv_wnd_max) {
@@ -194,12 +202,6 @@ static err_t tcp_close_shutdown(struct tcp_pcb *pcb, u8_t rst_on_unacked_data)
         tcp_pcb_remove(pcb);
         pcb = NULL;
         break;
-    case SYN_RCVD:
-        err = tcp_send_fin(pcb);
-        if (err == ERR_OK) {
-            set_tcp_state(pcb, FIN_WAIT_1);
-        }
-        break;
     case ESTABLISHED:
         err = tcp_send_fin(pcb);
         if (err == ERR_OK) {
@@ -258,7 +260,7 @@ err_t tcp_close(struct tcp_pcb *pcb)
         /* Set a flag not to receive any more data... */
         pcb->flags |= TF_RXCLOSED;
     }
-    /* ... and close */
+
     return tcp_close_shutdown(pcb, 1);
 }