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85 lines
2.5 KiB
Go
85 lines
2.5 KiB
Go
// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build linux freebsd
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package router
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import (
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"bufio"
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"bytes"
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"fmt"
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"os"
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"os/exec"
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)
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// resolvconfIsActive indicates whether the system appears to be using resolvconf.
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// If this is true, then dnsManualUp should be avoided:
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// resolvconf has exclusive ownership of /etc/resolv.conf.
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func resolvconfIsActive() bool {
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// Sanity-check first: if there is no resolvconf binary, then this is fruitless.
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//
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// However, this binary may be a shim like the one systemd-resolved provides.
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// Such a shim may not behave as expected: in particular, systemd-resolved
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// does not seem to respect the exclusive mode -x, saying:
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// -x Send DNS traffic preferably over this interface
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// whereas e.g. openresolv sends DNS traffix _exclusively_ over that interface,
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// or not at all (in case of another exclusive-mode request later in time).
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//
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// Moreover, resolvconf may be installed but unused, in which case we should
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// not use it either, lest we clobber existing configuration.
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//
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// To handle all the above correctly, we scan the comments in /etc/resolv.conf
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// to ensure that it was generated by a resolvconf implementation.
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_, err := exec.LookPath("resolvconf")
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if err != nil {
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return false
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}
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f, err := os.Open("/etc/resolv.conf")
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if err != nil {
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return false
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}
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defer f.Close()
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scanner := bufio.NewScanner(f)
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for scanner.Scan() {
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line := scanner.Bytes()
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// Look for the word "resolvconf" until comments end.
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if len(line) > 0 && line[0] != '#' {
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return false
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}
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if bytes.Contains(line, []byte("resolvconf")) {
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return true
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}
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}
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return false
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}
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// dnsResolvconfUp invokes the resolvconf binary to associate
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// the given DNS configuration the Tailscale interface.
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func dnsResolvconfUp(config DNSConfig, interfaceName string) error {
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stdin := new(bytes.Buffer)
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dnsWriteConfig(stdin, config.Nameservers, config.Domains) // dns_direct.go
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cmd := exec.Command("resolvconf", "-m", "0", "-x", "-a", interfaceName+".inet")
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cmd.Stdin = stdin
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out, err := cmd.CombinedOutput()
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if err != nil {
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return fmt.Errorf("running %s: %s", cmd, out)
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}
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return nil
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}
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// dnsResolvconfDown undoes the action of dnsResolvconfUp.
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func dnsResolvconfDown(interfaceName string) error {
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cmd := exec.Command("resolvconf", "-f", "-d", interfaceName+".inet")
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out, err := cmd.CombinedOutput()
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if err != nil {
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return fmt.Errorf("running %s: %s", cmd, out)
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}
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return nil
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}
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