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fa932fefe7
With #6566 we added an external mechanism for getting the default interface, and used it on macOS and iOS (see tailscale/corp#8201). The goal was to be able to get the default physical interface even when using an exit node (in which case the routing table would say that the Tailscale utun* interface is the default). However, the external mechanism turns out to be unreliable in some cases, e.g. when multiple cellular interfaces are present/toggled (I have occasionally gotten my phone into a state where it reports the pdp_ip1 interface as the default, even though it can't actually route traffic). It was observed that `ifconfig -v` on macOS reports an "effective interface" for the Tailscale utn* interface, which seems promising. By examining the ifconfig source code, it turns out that this is done via a SIOCGIFDELEGATE ioctl syscall. Though this is a private API, it appears to have been around for a long time (e.g. it's in the 10.13 xnu release at https://opensource.apple.com/source/xnu/xnu-4570.41.2/bsd/net/if_types.h.auto.html) and thus is unlikely to go away. We can thus use this ioctl if the routing table says that a utun* interface is the default, and go back to the simpler mechanism that we had before #6566. Updates #7184 Updates #7188 Signed-off-by: Mihai Parparita <mihai@tailscale.com>
96 lines
2.8 KiB
Go
96 lines
2.8 KiB
Go
// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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package interfaces
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import (
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"net"
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"strings"
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"sync"
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"syscall"
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"unsafe"
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"golang.org/x/net/route"
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"golang.org/x/sys/unix"
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"tailscale.com/util/mak"
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)
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// fetchRoutingTable calls route.FetchRIB, fetching NET_RT_DUMP2.
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func fetchRoutingTable() (rib []byte, err error) {
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return route.FetchRIB(syscall.AF_UNSPEC, syscall.NET_RT_DUMP2, 0)
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}
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func parseRoutingTable(rib []byte) ([]route.Message, error) {
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return route.ParseRIB(syscall.NET_RT_IFLIST2, rib)
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}
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var ifNames struct {
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sync.Mutex
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m map[int]string // ifindex => name
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}
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// getDelegatedInterface returns the interface index of the underlying interface
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// for the given interface index. 0 is returned if the interface does not
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// delegate.
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func getDelegatedInterface(ifIndex int) (int, error) {
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ifNames.Lock()
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defer ifNames.Unlock()
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// To get the delegated interface, we do what ifconfig does and use the
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// SIOCGIFDELEGATE ioctl. It operates in term of a ifreq struct, which
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// has to be populated with a interface name. To avoid having to do a
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// interface index -> name lookup every time, we cache interface names
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// (since indexes and names are stable after boot).
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ifName, ok := ifNames.m[ifIndex]
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if !ok {
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iface, err := net.InterfaceByIndex(ifIndex)
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if err != nil {
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return 0, err
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}
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ifName = iface.Name
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mak.Set(&ifNames.m, ifIndex, ifName)
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}
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// Only tunnels (like Tailscale itself) have a delegated interface, avoid
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// the ioctl if we can.
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if !strings.HasPrefix(ifName, "utun") {
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return 0, nil
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}
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// We don't cache the result of the ioctl, since the delegated interface can
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// change, e.g. if the user changes the preferred service order in the
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// network preference pane.
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fd, err := unix.Socket(unix.AF_INET, unix.SOCK_DGRAM, 0)
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if err != nil {
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return 0, err
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}
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defer unix.Close(fd)
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// Match the ifreq struct/union from the bsd/net/if.h header in the Darwin
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// open source release.
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var ifr struct {
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ifr_name [unix.IFNAMSIZ]byte
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ifr_delegated uint32
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}
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copy(ifr.ifr_name[:], ifName)
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// SIOCGIFDELEGATE is not in the Go x/sys package or in the public macOS
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// <sys/sockio.h> headers. However, it is in the Darwin/xnu open source
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// release (and is used by ifconfig, see
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// https://github.com/apple-oss-distributions/network_cmds/blob/6ccdc225ad5aa0d23ea5e7d374956245d2462427/ifconfig.tproj/ifconfig.c#L2183-L2187).
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// We generate its value by evaluating the `_IOWR('i', 157, struct ifreq)`
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// macro, which is how it's defined in
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// https://github.com/apple/darwin-xnu/blob/2ff845c2e033bd0ff64b5b6aa6063a1f8f65aa32/bsd/sys/sockio.h#L264
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const SIOCGIFDELEGATE = 0xc020699d
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_, _, errno := syscall.Syscall(
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syscall.SYS_IOCTL,
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uintptr(fd),
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uintptr(SIOCGIFDELEGATE),
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uintptr(unsafe.Pointer(&ifr)))
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if errno != 0 {
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return 0, errno
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}
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return int(ifr.ifr_delegated), nil
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}
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