feat: SYN flag only

Author: Chion82

src/client.c

@@ -145,7 +145,7 @@ int main(int argc, char* argv[]) {
   vector_init(&open_connections_vector);
 
   if (argc < 5) {
-    printf("Usage: kcpraw_client SERVER_IP SERVER_PORT LISTEN_IP LISTEN_PORT [--mode MODE] [--key KEY] [--noseq] [--nobpf]\n");
+    printf("Usage: kcpraw_client SERVER_IP SERVER_PORT LISTEN_IP LISTEN_PORT [--mode MODE] [--key KEY] [--noseq] [--syn-only] [--nobpf]\n");
     exit(1);
   }
 
@@ -178,12 +178,17 @@ int main(int argc, char* argv[]) {
   packetinfo.on_packet_recv = on_packet_recv;
   packetinfo.is_server = 0;
   packetinfo.disable_seq_update = 0;
+  packetinfo.syn_only = 0;
 
   for (int i=0; i<argc; i++) {
     if (!strcmp(argv[i], "--noseq")) {
       LOG("Disable TCP sequense counter.");
       packetinfo.disable_seq_update = 1;
     }
+    if (!strcmp(argv[i], "--syn-only")) {
+      LOG("Use SYN-only mode");
+      packetinfo.syn_only = 1;
+    }
   }
 
   strcpy(bind_ip, argv[3]);

src/common.c

@@ -468,7 +468,9 @@ void reinit_fake_tcp() {
   update_src_addr();
 
   init_bpf();
-  send_packet(&packetinfo, "", 0, FIRST_SYN);
+
+  if (!packetinfo.syn_only)
+    send_packet(&packetinfo, "", 0, FIRST_SYN);
 }
 #endif
 

src/server.c

@@ -69,7 +69,7 @@ int main(int argc, char* argv[]) {
   vector_init(&open_connections_vector);
 
   if (argc < 5) {
-    printf("Usage: kcpraw_server TCP_CONNECT_TO_IP TCP_CONNECT_TO_PORT SERVER_IP SERVER_PORT [--mode MODE] [--key KEY] [--noseq] [--nobpf]\n");
+    printf("Usage: kcpraw_server TCP_CONNECT_TO_IP TCP_CONNECT_TO_PORT SERVER_IP SERVER_PORT [--mode MODE] [--key KEY] [--noseq] [--syn-only] [--nobpf]\n");
     exit(1);
   }
 
@@ -105,12 +105,17 @@ int main(int argc, char* argv[]) {
   packetinfo.on_packet_recv = on_packet_recv;
   packetinfo.is_server = 1;
   packetinfo.disable_seq_update = 0;
+  packetinfo.syn_only = 0;
 
   for (int i=0; i<argc; i++) {
     if (!strcmp(argv[i], "--noseq")) {
       LOG("Disable TCP sequense counter.");
       packetinfo.disable_seq_update = 1;
     }
+    if (!strcmp(argv[i], "--syn-only")) {
+      LOG("Use SYN-only mode");
+      packetinfo.syn_only = 1;
+    }
   }
 
   loop = ev_default_loop(0);

src/trans_packet.c

@@ -164,7 +164,7 @@ void check_packet_recv(struct packet_info* packetinfo) {
     }
     strcpy(packetinfo->source_ip, inet_ntoa(to_addr));
 
-    if (tcph->syn == 1 && tcph->ack == 0 && tcph->psh == 0) {
+    if ((!packetinfo->syn_only) && tcph->syn == 1 && tcph->ack == 0 && tcph->psh == 0) {
         // Server replies SYN + ACK
         (packetinfo->state).seq = 1;
         (packetinfo->state).ack = 1;
@@ -176,7 +176,7 @@ void check_packet_recv(struct packet_info* packetinfo) {
 #endif
 
 #ifndef SERVER
-    if (tcph->syn == 1 && tcph->ack == 1 && tcph->psh == 0) {
+    if ((!packetinfo->syn_only) && tcph->syn == 1 && tcph->ack == 1 && tcph->psh == 0) {
         //Client replies first ACK
         (packetinfo->state).seq = 1;
         (packetinfo->state).ack = 1;
@@ -311,12 +311,21 @@ int send_packet(struct packet_info* packetinfo, char* source_payload, int source
     tcph->seq = htonl((packetinfo->state).seq);
     tcph->ack_seq = htonl((packetinfo->state).ack);
     tcph->doff = 5;  //tcp header size
-    tcph->fin=0;
-    tcph->syn=0;
-    tcph->rst=0;
-    tcph->psh=1;
-    tcph->ack=1;
-    tcph->urg=0;
+    if (packetinfo->syn_only) {
+        tcph->fin=0;
+        tcph->syn=1;
+        tcph->rst=0;
+        tcph->psh=0;
+        tcph->ack=0;
+        tcph->urg=0;
+    } else {
+        tcph->fin=0;
+        tcph->syn=0;
+        tcph->rst=0;
+        tcph->psh=1;
+        tcph->ack=1;
+        tcph->urg=0;
+    }
     tcph->window = htons(129600);
     tcph->check = 0; //leave checksum 0 now, filled later by pseudo header
     tcph->urg_ptr = 0;

src/trans_packet.h

@@ -20,6 +20,7 @@ struct packet_info {
   int is_server;
   struct trans_packet_state state;
   int disable_seq_update;
+  int syn_only;
 };
 
 int packet_send_sd;