tcp: Make initial sequence number random.

src/socket/tcp.rs

@@ -56,11 +56,6 @@ impl fmt::Display for State {
     }
 }
 
-/// Initial sequence number. This used to be 0, but some servers don't behave correctly
-/// with that, so we use a non-zero starting sequence number. TODO: randomize instead.
-/// https://github.com/smoltcp-rs/smoltcp/issues/489
-const INITIAL_SEQ_NO: TcpSeqNumber = TcpSeqNumber(42);
-
 // Conservative initial RTT estimate.
 const RTTE_INITIAL_RTT: u32 = 300;
 const RTTE_INITIAL_DEV: u32 = 100;
@@ -430,7 +425,7 @@ impl<'a> TcpSocket<'a> {
             listen_address: IpAddress::default(),
             local_endpoint: IpEndpoint::default(),
             remote_endpoint: IpEndpoint::default(),
-            local_seq_no: INITIAL_SEQ_NO,
+            local_seq_no: TcpSeqNumber::default(),
             remote_seq_no: TcpSeqNumber::default(),
             remote_last_seq: TcpSeqNumber::default(),
             remote_last_ack: None,
@@ -658,7 +653,7 @@ impl<'a> TcpSocket<'a> {
         self.listen_address = IpAddress::default();
         self.local_endpoint = IpEndpoint::default();
         self.remote_endpoint = IpEndpoint::default();
-        self.local_seq_no = INITIAL_SEQ_NO;
+        self.local_seq_no = TcpSeqNumber::default();
         self.remote_seq_no = TcpSeqNumber::default();
         self.remote_last_seq = TcpSeqNumber::default();
         self.remote_last_ack = None;
@@ -751,18 +746,24 @@ impl<'a> TcpSocket<'a> {
             ..local_endpoint
         };
 
-        // Carry over the local sequence number.
-        let local_seq_no = self.local_seq_no;
-
         self.reset();
         self.local_endpoint = local_endpoint;
         self.remote_endpoint = remote_endpoint;
-        self.local_seq_no = local_seq_no;
-        self.remote_last_seq = local_seq_no;
         self.set_state(State::SynSent);
+
+        let seq = Self::random_seq_no();
+        self.local_seq_no = seq;
+        self.remote_last_seq = seq;
         Ok(())
     }
 
+    fn random_seq_no() -> TcpSeqNumber {
+        #[cfg(test)]
+        return TcpSeqNumber(10000);
+        #[cfg(not(test))]
+        return TcpSeqNumber(crate::rand::rand_u32() as i32);
+    }
+
     /// Close the transmit half of the full-duplex connection.
     ///
     /// Note that there is no corresponding function for the receive half of the full-duplex
@@ -1575,8 +1576,7 @@ impl<'a> TcpSocket<'a> {
 
                 self.local_endpoint = IpEndpoint::new(ip_repr.dst_addr(), repr.dst_port);
                 self.remote_endpoint = IpEndpoint::new(ip_repr.src_addr(), repr.src_port);
-                // FIXME: use something more secure here
-                self.local_seq_no = TcpSeqNumber(!repr.seq_number.0);
+                self.local_seq_no = Self::random_seq_no();
                 self.remote_seq_no = repr.seq_number + 1;
                 self.remote_last_seq = self.local_seq_no;
                 self.remote_has_sack = repr.sack_permitted;