p2p/nat: use default route for nat-pmp gateway

p2p/nat/natpmp.go

@@ -110,14 +110,29 @@ func discoverPMP() Interface {
 	return nil
 }
 
-// TODO: improve this. We currently assume that (on most networks)
-// the router is X.X.X.1 in a local LAN range.
+// TODO: improve this further. We prefer discovered default gateways when
+// available and otherwise fall back to assuming the router is X.X.X.1 in a
+// local LAN range.
 func potentialGateways() (gws []net.IP) {
+	seen := make(map[string]struct{})
+	for _, ip := range defaultGatewayIPs() {
+		if ip = ip.To4(); ip != nil {
+			key := ip.String()
+			if _, ok := seen[key]; !ok {
+				seen[key] = struct{}{}
+				gws = append(gws, ip)
+			}
+		}
+	}
+
 	ifaces, err := net.Interfaces()
 	if err != nil {
-		return nil
+		return gws
 	}
 	for _, iface := range ifaces {
+		if iface.Flags&net.FlagUp == 0 || iface.Flags&net.FlagLoopback != 0 {
+			continue
+		}
 		ifaddrs, err := iface.Addrs()
 		if err != nil {
 			return gws
@@ -128,7 +143,11 @@ func potentialGateways() (gws []net.IP) {
 					ip := x.IP.Mask(x.Mask).To4()
 					if ip != nil {
 						ip[3] = ip[3] | 0x01
-						gws = append(gws, ip)
+						key := ip.String()
+						if _, ok := seen[key]; !ok {
+							seen[key] = struct{}{}
+							gws = append(gws, ip)
+						}
 					}
 				}
 			}

p2p/nat/natpmp_gateway_linux.go

@@ -0,0 +1,72 @@
+// Copyright 2026 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+//go:build linux
+
+package nat
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/binary"
+	"net"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func defaultGatewayIPs() []net.IP {
+	data, err := os.ReadFile("/proc/net/route")
+	if err != nil {
+		return nil
+	}
+	return parseLinuxRouteTable(data)
+}
+
+func parseLinuxRouteTable(data []byte) []net.IP {
+	var gws []net.IP
+
+	scanner := bufio.NewScanner(bytes.NewReader(data))
+	if !scanner.Scan() {
+		return nil
+	}
+	for scanner.Scan() {
+		fields := strings.Fields(scanner.Text())
+		if len(fields) < 4 || fields[1] != "00000000" {
+			continue
+		}
+		flags, err := strconv.ParseUint(fields[3], 16, 16)
+		if err != nil || flags&0x2 == 0 {
+			continue
+		}
+		gw, err := parseLinuxRouteHexIPv4(fields[2])
+		if err != nil || gw == nil {
+			continue
+		}
+		gws = append(gws, gw)
+	}
+	return gws
+}
+
+func parseLinuxRouteHexIPv4(s string) (net.IP, error) {
+	n, err := strconv.ParseUint(s, 16, 32)
+	if err != nil {
+		return nil, err
+	}
+	var buf [4]byte
+	binary.LittleEndian.PutUint32(buf[:], uint32(n))
+	return net.IPv4(buf[0], buf[1], buf[2], buf[3]).To4(), nil
+}

p2p/nat/natpmp_gateway_linux_test.go

@@ -0,0 +1,56 @@
+// Copyright 2026 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+//go:build linux
+
+package nat
+
+import (
+	"net"
+	"testing"
+)
+
+func TestParseLinuxRouteTable(t *testing.T) {
+	data := []byte(`Iface	Destination Gateway 	Flags RefCnt Use Metric Mask MTU Window IRTT
+eth0	00000000	0101A8C0	0003	0	0	100	00000000	0	0	0
+eth0	0001A8C0	00000000	0001	0	0	100	00FFFFFF	0	0	0
+wlan0	00000000	FE01A8C0	0003	0	0	600	00000000	0	0	0
+`)
+	got := parseLinuxRouteTable(data)
+	want := []net.IP{
+		net.IPv4(192, 168, 1, 1).To4(),
+		net.IPv4(192, 168, 1, 254).To4(),
+	}
+	if len(got) != len(want) {
+		t.Fatalf("got %d gateways, want %d", len(got), len(want))
+	}
+	for i := range want {
+		if !got[i].Equal(want[i]) {
+			t.Fatalf("gateway %d: got %v, want %v", i, got[i], want[i])
+		}
+	}
+}
+
+func TestParseLinuxRouteHexIPv4(t *testing.T) {
+	got, err := parseLinuxRouteHexIPv4("0101A8C0")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	want := net.IPv4(192, 168, 1, 1).To4()
+	if !got.Equal(want) {
+		t.Fatalf("got %v, want %v", got, want)
+	}
+}

p2p/nat/natpmp_gateway_other.go

@@ -0,0 +1,25 @@
+// Copyright 2026 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+//go:build !linux
+
+package nat
+
+import "net"
+
+func defaultGatewayIPs() []net.IP {
+	return nil
+}