Dial-back Echo Server

echo.go

package main

import (
	"io"
	"log"
	"net"
	"os"
	"sync"

	"github.com/google/gopacket"
	"github.com/google/gopacket/layers"
	"github.com/google/gopacket/pcap"
	"github.com/op/go-logging"
)

// Requires firewall rule to block incoming SYNs. Using PF:
// sudo pfctl -a com.apple/redial -f pf.conf
// block drop in proto tcp from any to port 8080 flags S/SA

var logger = logging.MustGetLogger("")

func main() {
  var running sync.WaitGroup
  // // Reference:
  // // https://www.devdungeon.com/content/packet-capture-injection-and-analysis-gopacket
  inactiveHandle, err := pcap.NewInactiveHandle("lo0")
  if err != nil {
    log.Fatal(err)
  }
  defer inactiveHandle.CleanUp()
  inactiveHandle.SetImmediateMode(true)
  inactiveHandle.SetPromisc(false)
  inactiveHandle.SetSnapLen(1024)
  handle, err := inactiveHandle.Activate()
  if err != nil {
    log.Fatal(err)
  }
  defer handle.Close()
  // Capture SYNs
  err = handle.SetBPFFilter("tcp[tcpflags] == tcp-syn and dst port 8080")
  if err != nil {
    log.Fatal(err)
  }
  packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
  packetChan := packetSource.Packets()
  logger.Info("Waiting for SYN packets")
  for packet := range packetChan {
    running.Add(1)
    go func() {
      defer running.Done()
      logger.Info("Got packet")
      // Process packet here
      tcpLayer := packet.Layer(layers.LayerTypeTCP)
      if tcpLayer == nil {
        return
      }
      tcpInfo, _ := tcpLayer.(*layers.TCP)
      ipLayer := packet.Layer(layers.LayerTypeIPv4)
      ipInfo, _ := ipLayer.(*layers.IPv4)
      logger.Infof("Got packet syn: %v\n src %v:%d, dst %v:%d",
        tcpInfo.SYN, ipInfo.SrcIP, tcpInfo.SrcPort, ipInfo.DstIP, tcpInfo.DstPort)
      tcpConn, err := net.DialTCP("tcp",
        &net.TCPAddr{IP: ipInfo.DstIP, Port: int(tcpInfo.DstPort)},
        &net.TCPAddr{IP: ipInfo.SrcIP, Port: int(tcpInfo.SrcPort)})
      if err != nil {
        logger.Fatal(err)
      }
      logger.Info("Server Dial done")
      io.Copy(tcpConn, tcpConn)
      tcpConn.Close()
      logger.Info("Request processed")
    }
    logger.Info("No more packets")
  }()
  running.Wait()
}

pf.conf

# Useful:
# https://robert-chalmers.uk/2018/10/03/protect-your-mac-with-pf-the-all-powerful-firewall/
# https://www.openbsdhandbook.com/pf/filter/
block drop in proto tcp from any to port 8080 flags S/SA