udp-reflector/udp-bcast-cidr-relay.go

// udp-bcast-cidr-relay.go
// Build (static, no cgo):
//
//	CGO_ENABLED=0 GOOS=linux GOARCH=mips64 go build -trimpath -ldflags='-s -w -extldflags "-static"' -o udp-bcast-cidr-relay ./udp-bcast-cidr-relay.go
package main

import (
	"encoding/binary"
	"errors"
	"flag"
	"fmt"
	"net"
	"os"
	"os/signal"
	"sort"
	"strconv"
	"strings"
	"syscall"
	"time"

	"github.com/rs/zerolog"
	"golang.org/x/sys/unix"
)

type subnetEntry struct {
	IfName string
	IfIdx  int
	Net    *net.IPNet // IPv4
	Bcast  net.IP     // 4 bytes
}

type recvMeta struct {
	IfIndex int
	TTL     int
}

func ip4ToU32(ip net.IP) (uint32, bool) {
	ip4 := ip.To4()
	if ip4 == nil || len(ip4) != 4 {
		return 0, false
	}
	return binary.BigEndian.Uint32(ip.To4()), true
}

func localIPv4SetFromSubnets(subnets []subnetEntry) map[uint32]bool {
	exists := map[uint32]bool{}
	for _, s := range subnets {
		if u, ok := ip4ToU32(s.Net.IP); ok {
			exists[u] = true
		}
	}
	return exists
}

func parseCIDRs(csv string) ([]*net.IPNet, error) {
	if strings.TrimSpace(csv) == "" {
		return nil, fmt.Errorf("cidrs required")
	}
	var out []*net.IPNet
	for _, s := range strings.Split(csv, ",") {
		s = strings.TrimSpace(s)
		if s == "" {
			continue
		}
		_, n, err := net.ParseCIDR(s)
		if err != nil {
			return nil, fmt.Errorf("bad cidr %q: %w", s, err)
		}
		// normalize to IPv4 nets only
		ip4 := n.IP.To4()
		if ip4 == nil || len(n.Mask) != 4 {
			return nil, fmt.Errorf("cidr must be IPv4: %q", s)
		}
		n.IP = ip4
		out = append(out, n)
	}
	return out, nil
}

func parsePorts(csv string) ([]int, error) {
	if strings.TrimSpace(csv) == "" {
		return nil, fmt.Errorf("ports required")
	}
	var out []int
	for _, s := range strings.Split(csv, ",") {
		s = strings.TrimSpace(s)
		if s == "" {
			continue
		}
		p, err := strconv.Atoi(s)
		if err != nil || p < 1 || p > 65535 {
			return nil, fmt.Errorf("bad port %q", s)
		}
		out = append(out, p)
	}
	return out, nil
}

func ipv4Bcast(ip net.IP, mask net.IPMask) (net.IP, bool) {
	ip4 := ip.To4()
	if ip4 == nil || len(mask) != 4 {
		return nil, false
	}
	b := make(net.IP, 4)
	for i := 0; i < 4; i++ {
		b[i] = ip4[i] | ^mask[i]
	}
	return b, true
}

func netWithinAny(n *net.IPNet, cidrs []*net.IPNet) bool {
	// treat "within" as: network address is inside CIDR and broadcast is inside CIDR
	b, ok := ipv4Bcast(n.IP, n.Mask)
	if !ok {
		return false
	}
	for _, c := range cidrs {
		if c.Contains(n.IP) && c.Contains(b) {
			return true
		}
	}
	return false
}

func discoverSubnets(cidrs []*net.IPNet, ifacesCSV string) ([]subnetEntry, error) {
	allow := map[string]bool{}
	if strings.TrimSpace(ifacesCSV) != "" {
		for _, s := range strings.Split(ifacesCSV, ",") {
			s = strings.TrimSpace(s)
			if s != "" {
				allow[s] = true
			}
		}
	}

	ifaces, err := net.Interfaces()
	if err != nil {
		return nil, err
	}
	var subs []subnetEntry
	for _, ifc := range ifaces {
		if ifc.Flags&net.FlagUp == 0 || ifc.Flags&net.FlagLoopback != 0 {
			continue
		}
		if len(allow) > 0 && !allow[ifc.Name] {
			continue
		}
		addrs, err := ifc.Addrs()
		if err != nil {
			return nil, fmt.Errorf("addrs(%s): %w", ifc.Name, err)
		}
		for _, a := range addrs {
			ipnet, ok := a.(*net.IPNet)
			if !ok || ipnet.IP == nil {
				continue
			}
			ip4 := ipnet.IP.To4()
			if ip4 == nil || len(ipnet.Mask) != 4 {
				continue
			}
			n := &net.IPNet{IP: ip4, Mask: ipnet.Mask}
			if !netWithinAny(n, cidrs) {
				continue
			}
			b, ok := ipv4Bcast(n.IP, n.Mask)
			if !ok {
				continue
			}
			subs = append(subs, subnetEntry{
				IfName: ifc.Name,
				IfIdx:  ifc.Index,
				Net:    n,
				Bcast:  b,
			})
		}
	}

	// de-dupe (same subnet repeated)
	seen := map[string]bool{}
	var out []subnetEntry
	for _, s := range subs {
		k := fmt.Sprintf("%d-%s/%d", s.IfIdx, s.Net.IP.String(), maskBits(s.Net.Mask))
		if !seen[k] {
			seen[k] = true
			out = append(out, s)
		}
	}
	sort.Slice(out, func(i, j int) bool {
		if out[i].IfName == out[j].IfName {
			return binary.BigEndian.Uint32(out[i].Bcast) < binary.BigEndian.Uint32(out[j].Bcast)
		}
		return out[i].IfName < out[j].IfName
	})
	return out, nil
}

func maskBits(m net.IPMask) int {
	ones, _ := m.Size()
	return ones
}

func parseOOB(oob []byte) recvMeta {
	meta := recvMeta{IfIndex: 0, TTL: -1}
	msgs, err := unix.ParseSocketControlMessage(oob)
	if err != nil {
		return meta
	}
	for _, m := range msgs {
		switch {
		case m.Header.Level == unix.IPPROTO_IP && m.Header.Type == unix.IP_PKTINFO:
			// in_pktinfo: ifindex (u32 LE) + spec_dst + addr
			if len(m.Data) >= 12 {
				meta.IfIndex = int(binary.LittleEndian.Uint32(m.Data[0:4]))
			}
		case m.Header.Level == unix.IPPROTO_IP && m.Header.Type == unix.IP_TTL:
			switch len(m.Data) {
			case 1:
				meta.TTL = int(m.Data[0])
			case 4:
				ttlLE := int(binary.LittleEndian.Uint32(m.Data))
				ttlBE := int(binary.BigEndian.Uint32(m.Data))
				if ttlLE >= 0 && ttlLE <= 255 {
					meta.TTL = ttlLE
				} else {
					meta.TTL = ttlBE
				}
			}
		}
	}
	return meta
}

type sender struct {
	ifname string
	conn   net.PacketConn
}

func newSender(ifname string, ttl int) (*sender, error) {
	fd, err := unix.Socket(unix.AF_INET, unix.SOCK_DGRAM, unix.IPPROTO_UDP)
	if err != nil {
		return nil, err
	}
	if err = unix.SetsockoptInt(fd, unix.SOL_SOCKET, unix.SO_BROADCAST, 1); err != nil {
		_ = unix.Close(fd)
		return nil, err
	}
	if err = unix.SetsockoptString(fd, unix.SOL_SOCKET, unix.SO_BINDTODEVICE, ifname); err != nil {
		_ = unix.Close(fd)
		return nil, err
	}
	if err = unix.SetsockoptInt(fd, unix.IPPROTO_IP, unix.IP_TTL, ttl); err != nil {
		_ = unix.Close(fd)
		return nil, err
	}

	f := os.NewFile(uintptr(fd), "udp-send-"+ifname)
	pc, err := net.FilePacketConn(f)
	_ = f.Close()
	if err != nil {
		_ = unix.Close(fd)
		return nil, err
	}
	return &sender{ifname: ifname, conn: pc}, nil
}

func main() {
	cidrsFlag := flag.String("cidrs", "", "comma-separated IPv4 CIDRs to include (for example 192.168.0.0/16,10.0.0.0/8)")
	ifacesFlag := flag.String("ifaces", "", "optional comma-separated interface allow-list (empty = all UP non-loopback)")
	portsFlag := flag.String("ports", "32410,32412,32413,32414,50222", "comma-separated UDP ports to relay")
	ttlFlag := flag.Int("ttl", 1, "outbound TTL for forwarded packets (use 1)")
	dropTTLFlag := flag.Int("drop-ttl", 1, "drop inbound packets with this TTL (use 1 to drop your forwarded copies)")
	timeoutMS := flag.Int("timeout-ms", 250, "read timeout in ms")
	logLevelFlag := flag.String("log-level", "info", "log level: trace|debug|info|warn|error|fatal|panic")
	flag.Parse()

	lvl, err := zerolog.ParseLevel(strings.ToLower(strings.TrimSpace(*logLevelFlag)))
	if err != nil {
		fmt.Fprintf(os.Stderr, "WARN: bad --log-level %q; using info\n", *logLevelFlag)
		lvl = zerolog.InfoLevel
	}
	lg := zerolog.New(os.Stderr).With().Timestamp().Logger().Level(lvl)

	cidrs, err := parseCIDRs(*cidrsFlag)
	if err != nil {
		lg.Fatal().Msgf("cidrs: %v", err)
	}
	ports, err := parsePorts(*portsFlag)
	if err != nil {
		lg.Fatal().Msgf("ports: %v", err)
	}

	subnets, err := discoverSubnets(cidrs, *ifacesFlag)
	if err != nil {
		lg.Fatal().Msgf("discover: %v", err)
	}
	if len(subnets) < 2 {
		lg.Fatal().Msgf("need at least 2 discovered subnets inside -cidrs; got %d", len(subnets))
	}

	localIPs := localIPv4SetFromSubnets(subnets)

	// One sender per interface name
	senders := map[string]*sender{}
	for _, s := range subnets {
		if _, ok := senders[s.IfName]; ok {
			continue
		}
		se, err := newSender(s.IfName, *ttlFlag)
		if err != nil {
			lg.Fatal().Msgf("sender(%s): %v", s.IfName, err)
		}
		senders[s.IfName] = se
	}
	defer func() {
		for _, s := range senders {
			_ = s.conn.Close()
		}
	}()

	lg.Info().Msgf("Discovered subnets in %s:", *cidrsFlag)
	for _, s := range subnets {
		lg.Info().Msgf("  if=%s idx=%d net=%s bcast=%s", s.IfName, s.IfIdx, s.Net.String(), s.Bcast.String())
	}
	lg.Info().Msgf("Relaying UDP ports=%v ttl=%d dropTTL=%d", ports, *ttlFlag, *dropTTLFlag)

	stop := make(chan os.Signal, 1)
	signal.Notify(stop, os.Interrupt, syscall.SIGTERM)

	for _, p := range ports {
		port := p
		go func() {
			fd, err := unix.Socket(unix.AF_INET, unix.SOCK_DGRAM, unix.IPPROTO_UDP)
			if err != nil {
				lg.Fatal().Msgf("socket :%d: %v", port, err)
			}
			defer unix.Close(fd)

			if err := unix.SetsockoptInt(fd, unix.SOL_SOCKET, unix.SO_REUSEADDR, 1); err != nil {
				lg.Fatal().Msgf("reuseaddr :%d: %v", port, err)
			}
			if err := unix.SetsockoptInt(fd, unix.SOL_SOCKET, unix.SO_BROADCAST, 1); err != nil {
				lg.Fatal().Msgf("broadcast :%d: %v", port, err)
			}
			if err := unix.SetsockoptInt(fd, unix.IPPROTO_IP, unix.IP_PKTINFO, 1); err != nil {
				lg.Fatal().Msgf("pktinfo :%d: %v", port, err)
			}
			if err := unix.SetsockoptInt(fd, unix.IPPROTO_IP, unix.IP_RECVTTL, 1); err != nil {
				lg.Fatal().Msgf("recvttl :%d: %v", port, err)
			}

			sa := &unix.SockaddrInet4{Port: port}
			copy(sa.Addr[:], net.IPv4zero.To4())
			if err := unix.Bind(fd, sa); err != nil {
				lg.Fatal().Msgf("bind :%d: %v", port, err)
			}

			buf := make([]byte, 65535)
			oob := make([]byte, 256)
			for {
				_ = unix.SetsockoptTimeval(fd, unix.SOL_SOCKET, unix.SO_RCVTIMEO, &unix.Timeval{
					Sec:  0,
					Usec: int64(*timeoutMS) * 1000,
				})

				n, oobn, _, from, err := unix.Recvmsg(fd, buf, oob, 0)
				if err != nil {
					// timeout
					if errors.Is(err, unix.EAGAIN) ||
						errors.Is(err, unix.EWOULDBLOCK) ||
						errors.Is(err, unix.EINTR) {
						continue
					}
					lg.Warn().Msgf("recvmsg :%d: %v (stopping listener goroutine)", port, err)
					return
				}
				meta := parseOOB(oob[:oobn])
				if meta.TTL == *dropTTLFlag || meta.IfIndex == 0 {
					lg.Debug().Msgf("drop :%d: meta ttl=%d ifindex=%d (dropTTL=%d)", port, meta.TTL, meta.IfIndex, *dropTTLFlag)
					continue
				}

				var srcIP net.IP
				switch fa := from.(type) {
				case *unix.SockaddrInet4:
					srcIP = net.IPv4(fa.Addr[0], fa.Addr[1], fa.Addr[2], fa.Addr[3]).To4()
				default:
					lg.Debug().Msgf("drop :%d: unexpected from addr type %T", port, from)
					continue
				}
				if srcIP == nil {
					continue
				}

				// if TTL is our forwarded TTL, drop.
				if meta.TTL == *dropTTLFlag && *dropTTLFlag >= 0 {
					lg.Debug().Msgf("drop :%d: ttl matches dropTTL src=%s ttl=%d", port, srcIP.String(), meta.TTL)
					continue
				}

				// If the packet's source IP is one of ours, it's a rebroadcast
				// we generated. Drop it to avoid looping.
				if u, ok := ip4ToU32(srcIP); ok && localIPs[u] {
					lg.Debug().Msgf("drop :%d: looped packet src=%s", port, srcIP.String())
					continue
				}

				// Determine which discovered subnet this packet originated from (on the ingress ifindex).
				origin := map[string]bool{} // key: ifidx+net string
				for _, sn := range subnets {
					if sn.IfIdx != meta.IfIndex {
						continue
					}
					if sn.Net.Contains(srcIP) {
						origin[fmt.Sprintf("%d-%s", sn.IfIdx, sn.Net.String())] = true
					}
				}

				payload := buf[:n]

				lg.Debug().Msgf("recv :%d: %d bytes from=%s ifindex=%d ttl=%d", port, n, srcIP.String(), meta.IfIndex, meta.TTL)

				// Fan out to every broadcast target except the origin subnet(s).
				for _, sn := range subnets {
					k := fmt.Sprintf("%d-%s", sn.IfIdx, sn.Net.String())
					if origin[k] {
						continue
					}
					s := senders[sn.IfName]
					if s == nil {
						continue
					}
					dst := &net.UDPAddr{IP: sn.Bcast, Port: port}
					_, _ = s.conn.WriteTo(payload, dst)
					lg.Debug().Msgf("sent :%d: %d bytes to=%s via=%s", port, len(payload), dst.String(), sn.IfName)
				}
			}
		}()
	}

	<-stop
	lg.Info().Msg("Shutting down")
	time.Sleep(100 * time.Millisecond)
}