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// Copyright (c) Tailscale Inc & contributors
// SPDX-License-Identifier: BSD-3-Clause
// Common code for FreeBSD and Darwin. This might also work on other
// BSD systems (e.g. OpenBSD) but has not been tested.
//go:build darwin || freebsd
package netmon
import (
"errors"
"fmt"
"log"
"net/netip"
"syscall"
"golang.org/x/net/route"
"golang.org/x/sys/unix"
"tailscale.com/net/netaddr"
)
// ErrNoGatewayIndexFound is returned by DefaultRouteInterfaceIndex when no
// default route is found.
var ErrNoGatewayIndexFound = errors.New("no gateway index found")
// DefaultRouteInterfaceIndex returns the index of the network interface that
// owns the default route. It returns the first IPv4 or IPv6 default route it
// finds (it does not prefer one or the other).
func DefaultRouteInterfaceIndex() (int, error) {
// $ netstat -nr
// Routing tables
// Internet:
// Destination Gateway Flags Netif Expire
// default 10.0.0.1 UGSc en0 <-- want this one
// default 10.0.0.1 UGScI en1
// From man netstat:
// U RTF_UP Route usable
// G RTF_GATEWAY Destination requires forwarding by intermediary
// S RTF_STATIC Manually added
// c RTF_PRCLONING Protocol-specified generate new routes on use
// I RTF_IFSCOPE Route is associated with an interface scope
rib, err := fetchRoutingTable()
if err != nil {
return 0, fmt.Errorf("route.FetchRIB: %w", err)
}
msgs, err := parseRoutingTable(rib)
if err != nil {
return 0, fmt.Errorf("route.ParseRIB: %w", err)
}
for _, m := range msgs {
rm, ok := m.(*route.RouteMessage)
if !ok {
continue
}
if isDefaultGateway(rm) {
if delegatedIndex, err := getDelegatedInterface(rm.Index); err == nil && delegatedIndex != 0 {
return delegatedIndex, nil
} else if err != nil {
log.Printf("interfaces_bsd: could not get delegated interface: %v", err)
}
return rm.Index, nil
}
}
return 0, ErrNoGatewayIndexFound
}
func init() {
likelyHomeRouterIP = likelyHomeRouterIPBSDFetchRIB
}
func likelyHomeRouterIPBSDFetchRIB() (ret, myIP netip.Addr, ok bool) {
rib, err := fetchRoutingTable()
if err != nil {
log.Printf("routerIP/FetchRIB: %v", err)
return ret, myIP, false
}
msgs, err := parseRoutingTable(rib)
if err != nil {
log.Printf("routerIP/ParseRIB: %v", err)
return ret, myIP, false
}
for _, m := range msgs {
rm, ok := m.(*route.RouteMessage)
if !ok {
continue
}
if !isDefaultGateway(rm) {
continue
}
gw, ok := rm.Addrs[unix.RTAX_GATEWAY].(*route.Inet4Addr)
if !ok {
continue
}
// If the route entry has an interface address associated with
// it, then parse and return that. This is optional.
if len(rm.Addrs) >= unix.RTAX_IFA {
if addr, ok := rm.Addrs[unix.RTAX_IFA].(*route.Inet4Addr); ok {
myIP = netaddr.IPv4(addr.IP[0], addr.IP[1], addr.IP[2], addr.IP[3])
}
}
return netaddr.IPv4(gw.IP[0], gw.IP[1], gw.IP[2], gw.IP[3]), myIP, true
}
return ret, myIP, false
}
var v4default = [4]byte{0, 0, 0, 0}
var v6default = [16]byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
func isDefaultGateway(rm *route.RouteMessage) bool {
if rm.Flags&unix.RTF_GATEWAY == 0 {
return false
}
// Defined locally because FreeBSD does not have unix.RTF_IFSCOPE.
const RTF_IFSCOPE = 0x1000000
if rm.Flags&RTF_IFSCOPE != 0 {
return false
}
// Addrs is [RTAX_DST, RTAX_GATEWAY, RTAX_NETMASK, ...]
if len(rm.Addrs) <= unix.RTAX_NETMASK {
return false
}
dst := rm.Addrs[unix.RTAX_DST]
netmask := rm.Addrs[unix.RTAX_NETMASK]
if dst == nil || netmask == nil {
return false
}
if dst.Family() == syscall.AF_INET && netmask.Family() == syscall.AF_INET {
dstAddr, dstOk := dst.(*route.Inet4Addr)
nmAddr, nmOk := netmask.(*route.Inet4Addr)
if dstOk && nmOk && dstAddr.IP == v4default && nmAddr.IP == v4default {
return true
}
}
if dst.Family() == syscall.AF_INET6 && netmask.Family() == syscall.AF_INET6 {
dstAddr, dstOk := dst.(*route.Inet6Addr)
nmAddr, nmOk := netmask.(*route.Inet6Addr)
if dstOk && nmOk && dstAddr.IP == v6default && nmAddr.IP == v6default {
return true
}
}
return false
}
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