net/dns/resolvd: store nameservers

Currently only search domains are stored. This was an oversight
(under?) on my part.

As things are now, when MagicDNS is on and "Override local DNS" is
off, the dns forwarder has to timeout before names resolve. This
introduces a pretty annoying lang that makes everything feel
extremely slow. You will also see an error: "upstream nameservers
not set".

I tested with "Override local DNS" on and off. In both situations
things seem to function as expected (and quickly).

Signed-off-by: Aaron Bieber <aaron@bolddaemon.com>
(cherry picked from commit 411c6c316c)

net/dns/resolvd.go

@@ -16,6 +16,7 @@ import (
 	"regexp"
 	"strings"
 
+	"inet.af/netaddr"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/dnsname"
 )
@@ -130,6 +131,25 @@ func (m resolvdManager) readResolvConf() (config OSConfig, err error) {
 			config.SearchDomains = append(config.SearchDomains, fqdn)
 			continue
 		}
+
+		if strings.HasPrefix(line, "nameserver") {
+			s := strings.TrimPrefix(line, "nameserver")
+			parts := strings.Split(s, " # ")
+			if len(parts) == 0 {
+				return OSConfig{}, err
+			}
+			nameserver := strings.TrimSpace(parts[0])
+			ip, err := netaddr.ParseIP(nameserver)
+			if err != nil {
+				return OSConfig{}, err
+			}
+			config.Nameservers = append(config.Nameservers, ip)
+			continue
+		}
+	}
+
+	if err = scanner.Err(); err != nil {
+		return OSConfig{}, err
 	}
 
 	return config, nil