net/dns/resolvconffile: unify three /etc/resolv.conf parsers into new package

Change-Id: I2120893ca802d12f1bd0407d49077d3672627d33
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/direct.go

@@ -5,7 +5,6 @@
 package dns
 
 import (
-	"bufio"
 	"bytes"
 	"context"
 	"crypto/rand"
@@ -20,6 +19,7 @@ import (
 	"time"
 
 	"inet.af/netaddr"
+	"tailscale.com/net/dns/resolvconffile"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/dnsname"
 )
@@ -30,64 +30,23 @@ const (
 )
 
 // writeResolvConf writes DNS configuration in resolv.conf format to the given writer.
-func writeResolvConf(w io.Writer, servers []netaddr.IP, domains []dnsname.FQDN) {
-	io.WriteString(w, "# resolv.conf(5) file generated by tailscale\n")
-	io.WriteString(w, "# DO NOT EDIT THIS FILE BY HAND -- CHANGES WILL BE OVERWRITTEN\n\n")
-	for _, ns := range servers {
-		io.WriteString(w, "nameserver ")
-		io.WriteString(w, ns.String())
-		io.WriteString(w, "\n")
-	}
-	if len(domains) > 0 {
-		io.WriteString(w, "search")
-		for _, domain := range domains {
-			io.WriteString(w, " ")
-			io.WriteString(w, domain.WithoutTrailingDot())
-		}
-		io.WriteString(w, "\n")
+func writeResolvConf(w io.Writer, servers []netaddr.IP, domains []dnsname.FQDN) error {
+	c := &resolvconffile.Config{
+		Nameservers:   servers,
+		SearchDomains: domains,
 	}
+	return c.Write(w)
 }
 
-func readResolv(r io.Reader) (config OSConfig, err error) {
-	scanner := bufio.NewScanner(r)
-	for scanner.Scan() {
-		line := strings.TrimSpace(scanner.Text())
-		i := strings.IndexByte(line, '#')
-		if i >= 0 {
-			line = line[:i]
-		}
-
-		if strings.HasPrefix(line, "nameserver") {
-			s := strings.TrimPrefix(line, "nameserver")
-			nameserver := strings.TrimSpace(s)
-			if len(nameserver) == len(s) {
-				return OSConfig{}, fmt.Errorf("missing space after \"nameserver\" in %q", line)
-			}
-			ip, err := netaddr.ParseIP(nameserver)
-			if err != nil {
-				return OSConfig{}, err
-			}
-			config.Nameservers = append(config.Nameservers, ip)
-			continue
-		}
-
-		if strings.HasPrefix(line, "search") {
-			s := strings.TrimPrefix(line, "search")
-			domain := strings.TrimSpace(s)
-			if len(domain) == len(s) {
-				// No leading space?!
-				return OSConfig{}, fmt.Errorf("missing space after \"domain\" in %q", line)
-			}
-			fqdn, err := dnsname.ToFQDN(domain)
-			if err != nil {
-				return OSConfig{}, fmt.Errorf("parsing search domains %q: %w", line, err)
-			}
-			config.SearchDomains = append(config.SearchDomains, fqdn)
-			continue
-		}
+func readResolv(r io.Reader) (OSConfig, error) {
+	c, err := resolvconffile.Parse(r)
+	if err != nil {
+		return OSConfig{}, err
 	}
-
-	return config, nil
+	return OSConfig{
+		Nameservers:   c.Nameservers,
+		SearchDomains: c.SearchDomains,
+	}, nil
 }
 
 // resolvOwner returns the apparent owner of the resolv.conf