tsdns: don't forward transient DNS errors

When a DNS server claims to be unable or unwilling to handle a request,
instead of passing that refusal along to the client, just treat it as
any other error trying to connect to the DNS server. This prevents DNS
requests from failing based on if a server can respond with a transient
error before another server is able to give an actual response. DNS
requests only failing *sometimes* is really hard to find the cause of
(#1033).

Signed-off-by: Smitty <me@smitop.com>

net/dns/resolver/forwarder_test.go

@@ -12,6 +12,7 @@ import (
 	"testing"
 	"time"
 
+	dns "golang.org/x/net/dns/dnsmessage"
 	"tailscale.com/types/dnstype"
 )
 
@@ -97,3 +98,45 @@ func TestResolversWithDelays(t *testing.T) {
 	}
 
 }
+
+func TestGetRCode(t *testing.T) {
+	tests := []struct {
+		name   string
+		packet []byte
+		want   dns.RCode
+	}{
+		{
+			name:   "empty",
+			packet: []byte{},
+			want:   dns.RCode(5),
+		},
+		{
+			name:   "too-short",
+			packet: []byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
+			want:   dns.RCode(5),
+		},
+		{
+			name:   "noerror",
+			packet: []byte{0xC4, 0xFE, 0x81, 0xA0, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00, 0x00, 0x01},
+			want:   dns.RCode(0),
+		},
+		{
+			name:   "refused",
+			packet: []byte{0xee, 0xa1, 0x81, 0x05, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01},
+			want:   dns.RCode(5),
+		},
+		{
+			name:   "nxdomain",
+			packet: []byte{0x34, 0xf4, 0x81, 0x83, 0x00, 0x01, 0x00, 0x00, 0x00, 0x01, 0x00, 0x01},
+			want:   dns.RCode(3),
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := getRCode(tt.packet)
+			if got != tt.want {
+				t.Errorf("got %d; want %d", got, tt.want)
+			}
+		})
+	}
+}