Initial Commit

.gitignore

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+**/go.sum

README.md

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+# Simple Discovery Service
+
+Client utilizes packet capture to listen to response to bypass host firewalls.
+

client/client.go

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+package main
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"net"
+	"time"
+
+	"github.com/google/gopacket"
+	"github.com/google/gopacket/pcap"
+)
+
+func errCheck(err error) {
+	if err != nil {
+		panic(err)
+	}
+	return
+}
+
+func listenDiscoverResponse() {
+	handle, err := pcap.OpenLive("any", int32(1600), true, pcap.BlockForever)
+	errCheck(err)
+
+	handle.SetBPFFilter("udp and port 8850")
+
+	packets := gopacket.NewPacketSource(handle, handle.LinkType()).Packets()
+
+	for pkt := range packets {
+		fmt.Printf("%s\n", pkt.NetworkLayer().NetworkFlow().Src())
+	}
+
+	return
+}
+
+func toMaxHost(n *net.IPNet) (net.IP, error) {
+	if n.IP.To4() == nil {
+		return net.IP{}, errors.New("IP address is not IPv4")
+	}
+
+	ip := make(net.IP, len(n.IP.To4()))
+	binary.BigEndian.PutUint32(ip,
+		binary.BigEndian.Uint32(n.IP.To4())|^binary.BigEndian.Uint32(net.IP(n.Mask).To4()))
+
+	return ip, nil
+}
+
+func bcastAddresses() ([]string) {
+	var addrs []string = nil
+	interfaceInfo, _ := net.Interfaces()
+	for i := 0; i < len(interfaceInfo); i++ {
+		addr, err := interfaceInfo[i].Addrs()
+		errCheck(err)
+		for j := 0; j < len(addr); j++ {
+			_, ipNet, _ := net.ParseCIDR(addr[j].String())
+			bcast, err := toMaxHost(ipNet)
+
+			if err != nil {
+			} else {
+				addrs = append(addrs, bcast.String())
+			}
+		}
+	}
+
+	return addrs
+}
+
+func main() {
+	sendAddrs := bcastAddresses()
+
+	go listenDiscoverResponse()
+
+	time.Sleep(1 * time.Second)
+
+	for i := 0; i < len(sendAddrs); i++ {
+		remoteAddr := sendAddrs[i] + ":8888"
+		remote, err := net.ResolveUDPAddr("udp4", remoteAddr)
+		errCheck(err)
+		conn, err := net.DialUDP("udp4", nil, remote)
+		errCheck(err)
+		defer conn.Close()
+
+		_, err = conn.Write([]byte(conn.LocalAddr().String()))
+		errCheck(err)
+	}
+	time.Sleep(30 * time.Second)
+}
+

client/go.mod

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+module client
+
+go 1.18
+
+require github.com/google/gopacket v1.1.19
+
+require golang.org/x/sys v0.0.0-20190412213103-97732733099d // indirect

server/server.go

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+package main
+
+import (
+	"fmt"
+	"net"
+	"strings"
+)
+
+func main() {
+	serv, err := net.ListenPacket("udp4", ":8888")
+
+	if err != nil {
+		panic(err)
+	}
+
+	defer serv.Close()
+
+	buff := make([]byte, 2048)
+
+	for {
+
+		n, addr, err := serv.ReadFrom(buff)
+		if err != nil {
+			panic(err)
+		}
+
+		remoteAddr := strings.Split(addr.String(), ":")
+
+		remote, err := net.ResolveUDPAddr("udp4", remoteAddr[0] + ":8850")
+
+		if err != nil {
+			panic(err)
+		}
+
+		conn, _ := net.DialUDP("udp4", nil, remote)
+		defer conn.Close()
+
+		conn.Write([]byte("Hello"))
+		conn.Close()
+
+		fmt.Printf("%s sent this: %s\n", addr, buff[:n])
+
+	}
+}