From b0032c534c8ac6e2c82cfd5725a73653bb7c1971 Mon Sep 17 00:00:00 2001
From: Christian Ulrich <christian@ulrich.earth>
Date: Thu, 8 Oct 2020 18:12:59 +0200
Subject: [PATCH] use random sequence number for outgoing low-TTL SYN

---
 tcp_syni.nim | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/tcp_syni.nim b/tcp_syni.nim
index e74e6b3..82cca21 100644
--- a/tcp_syni.nim
+++ b/tcp_syni.nim
@@ -1,6 +1,7 @@
 import asyncfutures, asyncdispatch, asyncnet, strformat
 from net import IpAddress, Port, `$`, `==`, getPrimaryIPAddr, toSockAddr, parseIpAddress
 from nativesockets import SockAddr, Sockaddr_storage, SockLen, setSockOptInt
+from random import randomize, rand
 from sequtils import any
 import asyncutils
 import ip_packet
@@ -137,7 +138,9 @@ proc captureAndResendAck(attempt: ConnectAttempt, captureFd: AsyncFD,
   closeSocket(captureFd)
   closeSocket(injectFd)
 
-proc initPuncher*(): TcpSyniPuncher = TcpSyniPuncher()
+proc initPuncher*(): TcpSyniPuncher =
+  randomize()
+  TcpSyniPuncher()
 
 proc findConnectAttempt(puncher: TcpSyniPuncher, srcIp: IpAddress,
                         srcPort: Port, dstIp: IpAddress,
@@ -277,8 +280,8 @@ proc accept*(puncher: TcpSyniPuncher, srcPort: Port, dstIp: IpAddress,
       let synOut = IpPacket(protocol: tcp, ipAddrSrc: attempt.srcIp,
                             ipAddrDst: attempt.dstIp, ipTTL: 2,
                             tcpPortSrc: attempt.srcPort, tcpPortDst: dstPort,
-                            tcpSeqNumber: 0, tcpAckNumber: 0, tcpFlags: {SYN},
-                            tcpWindowSize: 1452 * 10)
+                            tcpSeqNumber: rand(uint32), tcpAckNumber: 0,
+                            tcpFlags: {SYN}, tcpWindowSize: 1452 * 10)
       await rawFd.injectTcpPacket(synOut)
       for seqNum in attempt.seqNums:
         let synIn = IpPacket(protocol: tcp, ipAddrSrc: attempt.dstIp,