net/dns: add tailscaled-on-macOS DNS OSConfigurator

This populates DNS suffixes ("ts.net", etc) in /etc/resolver/* files
to point to 100.100.100.100 so MagicDNS works.

It also sets search domains.

Updates #4276

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/manager_darwin.go

@@ -0,0 +1,127 @@
+// Copyright (c) 2022 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.
+
+package dns
+
+import (
+	"bytes"
+	"errors"
+	"os"
+
+	"go4.org/mem"
+	"tailscale.com/types/logger"
+	"tailscale.com/util/mak"
+)
+
+func NewOSConfigurator(logf logger.Logf, ifName string) (OSConfigurator, error) {
+	return &darwinConfigurator{logf: logf, ifName: ifName}, nil
+}
+
+// darwinConfigurator is the tailscaled-on-macOS DNS OS configurator that
+// maintains the Split DNS nameserver entries pointing MagicDNS DNS suffixes
+// to 100.100.100.100 using the macOS /etc/resolver/$SUFFIX files.
+type darwinConfigurator struct {
+	logf   logger.Logf
+	ifName string
+}
+
+func (c *darwinConfigurator) Close() error {
+	c.removeResolverFiles(func(domain string) bool { return true })
+	return nil
+}
+
+func (c *darwinConfigurator) SupportsSplitDNS() bool {
+	return true
+}
+
+func (c *darwinConfigurator) SetDNS(cfg OSConfig) error {
+	var buf bytes.Buffer
+	buf.WriteString(macResolverFileHeader)
+	for i, ip := range cfg.Nameservers {
+		if i == 0 {
+			buf.WriteString("nameserver ")
+		} else {
+			buf.WriteString(" ")
+		}
+		buf.WriteString(ip.String())
+	}
+	buf.WriteString("\n")
+
+	if err := os.MkdirAll("/etc/resolver", 0755); err != nil {
+		return err
+	}
+
+	var keep map[string]bool
+
+	// Add a dummy file to /etc/resolver with a "search ..." directive if we have
+	// search suffixes to add.
+	if len(cfg.SearchDomains) > 0 {
+		const searchFile = "search.tailscale" // fake DNS suffix+TLD to put our search
+		mak.Set(&keep, searchFile, true)
+		var sbuf bytes.Buffer
+		sbuf.WriteString(macResolverFileHeader)
+		sbuf.WriteString("search")
+		for _, d := range cfg.SearchDomains {
+			sbuf.WriteString(" ")
+			sbuf.WriteString(string(d.WithoutTrailingDot()))
+		}
+		sbuf.WriteString("\n")
+		if err := os.WriteFile("/etc/resolver/"+searchFile, sbuf.Bytes(), 0644); err != nil {
+			return err
+		}
+	}
+
+	for _, d := range cfg.MatchDomains {
+		fileBase := string(d.WithoutTrailingDot())
+		mak.Set(&keep, fileBase, true)
+		fullPath := "/etc/resolver/" + fileBase
+
+		if err := os.WriteFile(fullPath, buf.Bytes(), 0644); err != nil {
+			return err
+		}
+	}
+	return c.removeResolverFiles(func(domain string) bool { return !keep[domain] })
+}
+
+func (c *darwinConfigurator) GetBaseConfig() (OSConfig, error) {
+	return OSConfig{}, errors.New("[unexpected] unreachable")
+}
+
+const macResolverFileHeader = "# Added by tailscaled\n"
+
+// removeResolverFiles deletes all files in /etc/resolver for which the shouldDelete
+// func returns true.
+func (c *darwinConfigurator) removeResolverFiles(shouldDelete func(domain string) bool) error {
+	dents, err := os.ReadDir("/etc/resolver")
+	if os.IsNotExist(err) {
+		return nil
+	}
+	if err != nil {
+		return err
+	}
+	for _, de := range dents {
+		if !de.Type().IsRegular() {
+			continue
+		}
+		name := de.Name()
+		if !shouldDelete(name) {
+			continue
+		}
+		fullPath := "/etc/resolver/" + name
+		contents, err := os.ReadFile(fullPath)
+		if err != nil {
+			if os.IsNotExist(err) { // race?
+				continue
+			}
+			return err
+		}
+		if !mem.HasPrefix(mem.B(contents), mem.S(macResolverFileHeader)) {
+			continue
+		}
+		if err := os.Remove(fullPath); err != nil {
+			return err
+		}
+	}
+	return nil
+}

net/dns/manager_default.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !linux && !freebsd && !openbsd && !windows
+//go:build !linux && !freebsd && !openbsd && !windows && !darwin
-// +build !linux,!freebsd,!openbsd,!windows
+// +build !linux,!freebsd,!openbsd,!windows,!darwin
 
 package dns