must use sendTo to inject ip packet

tcp_syni.nim

@@ -1,6 +1,6 @@
 import asyncdispatch, asyncnet, strformat
-from net import IpAddress, Port, `$`, `==`, getPrimaryIPAddr
-from nativesockets import setSockOptInt
+from net import IpAddress, Port, `$`, `==`, getPrimaryIPAddr, toSockAddr
+from nativesockets import SockAddr, Sockaddr_storage, SockLen, setSockOptInt
 import asyncutils
 import ip_packet
 import network_interface
@@ -90,7 +90,17 @@ proc injectSyns(rawFd: AsyncFD, srcIp: IpAddress, srcPort: Port,
                             tcpPortDst: srcPort,
                             tcpSeqNumber: seqNum,
                             tcpFlags: {SYN})
-    asyncCheck rawFd.send(serialize(ipPacket))
+    echo &"injecting {srcIP}:{srcPort.int} -> {dstIp}:{dstPort.int} (seq {seqNum})"
+    try:
+      let packet = serialize(ipPacket)
+      var sockaddr: Sockaddr_storage
+      var sockaddrLen: SockLen
+      toSockAddr(dstIp, dstPort, sockaddr, sockaddrLen)
+      await rawFd.sendTo(packet.cstring, packet.len,
+                         cast[ptr SockAddr](addr sockaddr), sockaddrLen)
+      echo &"injected {srcIP}:{srcPort.int} -> {dstIp}:{dstPort.int} (seq {seqNum})"
+    except OSError as e:
+      echo "cannot inject: ", e.msg
 
 proc initPuncher*(srcPort: Port, dstIp: IpAddress, dstPorts: array[3, Port],
                   seqNums: seq[uint32] = @[]): Future[TcpSyniPuncher] {.async.} =