wgengine/router: dns: unify on *BSD, multimode on Linux, Magic DNS (#536)

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

wgengine/router/dns_resolvconf.go

@@ -0,0 +1,84 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build linux freebsd
+
+package router
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+// resolvconfIsActive indicates whether the system appears to be using resolvconf.
+// If this is true, then dnsManualUp should be avoided:
+// resolvconf has exclusive ownership of /etc/resolv.conf.
+func resolvconfIsActive() bool {
+	// Sanity-check first: if there is no resolvconf binary, then this is fruitless.
+	//
+	// However, this binary may be a shim like the one systemd-resolved provides.
+	// Such a shim may not behave as expected: in particular, systemd-resolved
+	// does not seem to respect the exclusive mode -x, saying:
+	//   -x            Send DNS traffic preferably over this interface
+	// whereas e.g. openresolv sends DNS traffix _exclusively_ over that interface,
+	// or not at all (in case of another exclusive-mode request later in time).
+	//
+	// Moreover, resolvconf may be installed but unused, in which case we should
+	// not use it either, lest we clobber existing configuration.
+	//
+	// To handle all the above correctly, we scan the comments in /etc/resolv.conf
+	// to ensure that it was generated by a resolvconf implementation.
+	_, err := exec.LookPath("resolvconf")
+	if err != nil {
+		return false
+	}
+
+	f, err := os.Open("/etc/resolv.conf")
+	if err != nil {
+		return false
+	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := scanner.Bytes()
+		// Look for the word "resolvconf" until comments end.
+		if len(line) > 0 && line[0] != '#' {
+			return false
+		}
+		if bytes.Contains(line, []byte("resolvconf")) {
+			return true
+		}
+	}
+	return false
+}
+
+// dnsResolvconfUp invokes the resolvconf binary to associate
+// the given DNS configuration the Tailscale interface.
+func dnsResolvconfUp(config DNSConfig, interfaceName string) error {
+	stdin := new(bytes.Buffer)
+	dnsWriteConfig(stdin, config.Nameservers, config.Domains) // dns_direct.go
+
+	cmd := exec.Command("resolvconf", "-m", "0", "-x", "-a", interfaceName+".inet")
+	cmd.Stdin = stdin
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("running %s: %s", cmd, out)
+	}
+
+	return nil
+}
+
+// dnsResolvconfDown undoes the action of dnsResolvconfUp.
+func dnsResolvconfDown(interfaceName string) error {
+	cmd := exec.Command("resolvconf", "-f", "-d", interfaceName+".inet")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("running %s: %s", cmd, out)
+	}
+	return nil
+}